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- Sprache: Englisch
Comprehensive resource covering the latest development of surface engineering inspired by nature with a special focus on wetting control
Drawing from the natural abilities of plants and animals around the world, Controlled Surface Wetting takes a deep dive into wetting-controlled systems of biological surfaces with information on mechanisms, theory, surface design, fabrication, and effects. This book guides readers to design better engineering surfaces for applications in self-cleaning, water harvesting and repellency, anti-icing, liquid-transport, and beyond.
Exploring the latest literature, this book introduces bioinspired techniques and methods to design wetting-controlled surfaces by using organic or inorganic materials, including those with high/low surface energy, regular/irregular, ordered/disordered, or rough/smooth surfaces, or endless arrangements and combinations of micro- and nanostructures of various styles.
This book begins by introducing biological surfaces such as plant leaves and duck feathers, butterfly wings, and spider silks, as well as their functions, including superhydrophobic properties, water repellency, and capturing tiny water droplets, respectively, progressing through to more advanced topics such as dually-mobile super-repellency, multi-liquid repellency, and switchable repellency in both air and liquid.
Controlled Surface Wetting includes discussion on:
- Fundamental wetting theories, extension and theoretical models, wetting dynamics and kinetics, physics of wetting, wetting adhesion, and wetting chemistry
- Static and dynamic gradients, texture gradients such as gradient polymers, wedge- and helical-induced gradients, and synergism of multi-gradients
- Formation, control, and instability of Rayleigh instability, microfluidics, fluid-coating, electrospinning, fluid diffusion, and laser techniques
- Coalesced-droplet vertical transport, the hierarchical droplet size-effect, atmospheric water harvesting, and energy harvesting
- Artificial skins and sensors, including artificial skin vision, and medical applications, including directional-controllable drug delivery
Controlled Surface Wetting is an up-to-date and completely comprehensive resource for students and researchers in chemistry, physics, and materials science seeking to learn about the design of smart and advanced materials for engineering applications.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Dedication
Preface
Acknowledgments
1 Wetting‐Controlled Systems of Biological Surfaces
1.1 Introduction
1.2 Wetting Features of Biological Surfaces
1.3 Antiwetting Features of Biological Surfaces
1.4 Biological Patterns on Micro‐ and Nanoscale Structures
1.5 Wetting‐Controlled Effects
References
2 Mechanism and Theory of Wetting‐Controlled Surfaces
2.1 Concept of Wetting‐Controlled Effects
2.2 Wetting Theory of Surfaces
2.3 Physics of Wetting
2.4 Surface Chemistry and Structures
2.5 Bioinspired Wetting‐Controlled Mechanism
2.6 Self‐Propelling Effects of Surfaces
2.7 Capillary Regime
2.8 Liquid Infused Surfaces
References
3 Design on Surfaces with Wetting‐Controlled Effects
3.1 Concept of Gradients
3.2 Chemistry Gradient
3.3 Texture Gradients
3.4 Geometry Gradient
3.5 Synergism of Multi‐gradients
3.6 Surface Tension Gradient
References
4 Development of Bioinspired Fabrication and Methods
4.1 Rayleigh Instability
4.2 Microfluidics
4.3 Fluid‐Coating
4.4 Electrospinning
4.5 Electrochemistry
4.6 Fluid Diffusion for Gradient
4.7 Laser Techniques
4.8 Printing Techniques
4.9 Nanotechnology
4.10 Plasma Techniques
References
5 Wetting‐Controlled Effects for Functions and Applications
5.1 Condensate Droplet Transport
5.2 Fog Droplet Harvesting
5.3 Atmospheric Water Harvesting
5.4 Anti‐icing
5.5 Liquid Repellency
5.6 Energy Harvesting
5.7 Heat Transfer
5.8 Artificial Skin and Sensor
5.9 Medical Application
References
Summary
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 Laser techniques for fabrication of structures and functions for a...
Chapter 5
Table 5.1 Compare of fog‐harvesting rate (FHR) by different materials device...
Table 5.2 Biological and bioinspired thermal energy regulation and utilizati...
List of Illustrations
Chapter 1
Figure 1.1 The groundbreaking findings on wetting‐controlling from biologica...
Figure 1.2 Super‐hydrophilic and hydrophilic surfaces in nature.
Figure 1.3 Spider silk with hanging the large water drops.
Figure 1.4 (a–c) ESEM images of capture silk of cribellate spider silk inclu...
Figure 1.5 (a–d) Optical images of capture silk of cribellate spider silk ch...
Figure 1.6 (a–d) The directional water transport on the surface of the “peri...
Figure 1.7 (a–d) The ratchet capillary structures of Araucaria leaf for liqu...
Figure 1.8 (a–i) Appearance and surface structures of the cactus and fogdrop...
Figure 1.9 The illustration of (a) shark and (b) microstructures of shark sk...
Figure 1.10 Antiwetting features of biological surfaces.
Figure 1.11 (a–c) The microstructures of duck and switching wetting/dewettin...
Figure 1.12 (a–c) Photograph of leaf surface with air‐layer and topography o...
Figure 1.13 (a–e) The ESEM images of the nonwetting feature on the surface o...
Figure 1.14 (a–c) Water strider leg with multilevel structures that can trap...
Figure 1.15 Superhydrophobic properties of butterfly wings ((a)
M. agea
; (b)...
Figure 1.16 (a–c) The anisotropic water repellency property on
M. agea
butte...
Figure 1.17 (a–c) The mosquito's compound eyes with micro‐ and nanostructure...
Figure 1.18 (a–d) The rice leaf with an anisotropic structured surface.
Figure 1.19 (a–c) Beetle captures fog water by means of alternative hydrophi...
Figure 1.20 (a–g) The rough‐curvature microstructure on wetted spider silk a...
Figure 1.21 (a–d) Multilevel structures of cactus and cooperative effects....
Figure 1.22 (a–d) Liquid directional steering of Araucaria leaf.
Figure 1.23 (a, b) Illustration of the beetle back effect via the dynamic pr...
Figure 1.24 (a–e) Gecko skin with multilevel micro‐ and nanostructuring for ...
Figure 1.25 (a–e) Self‐propelling effect for water droplets condensing on th...
Figure 1.26 (a, b) Wetting features of tethered MpdAFPs on PDA and GOPTS sur...
Figure 1.27 Observation of the icing delay time for supercooled droplets on ...
Chapter 2
Scheme 2.1 Significant breakthroughs and advances in wetting investigations....
Figure 2.1 Different wetting states for liquid droplets on solid surfaces, i...
Figure 2.2 (a–e) Biological surfaces with superhydrophobic (SHB) properties ...
Figure 2.3 (a–e) Hydrophilic properties on biological surfaces in the wettin...
Scheme 2.2 Biological surfaces with wetting systems for promising applicatio...
Figure 2.4 Wetting models: Young (a); Wenzel (b); and Cassie–Baxter (c).
Figure 2.5 Feature of the three‐phase contact line. (a) A sessile drop is pl...
Figure 2.6 (a–c) The Illustration of advancing CA and receding CA in wetting...
Figure 2.7 (a–b) The roughness enhances the wettability with hydrophobic to ...
Figure 2.8 (a–d) Illustration of fabricating a heterogeneous wettability sub...
Figure 2.9 (a, b) Various wetting states on a rough surface.
Figure 2.10 Illustration of small‐molecule supramolecular adhesive materials...
Figure 2.11 (a–c) Illustration of the different models of adhesion.
Figure 2.12 (a–c) Illustration of lowly wet adhesion and highly wet adhesion...
Figure 2.13 Creatures depend on the microstructured pattern to control the a...
Figure 2.14 (a, b) Illustration of the shear hydrophobicity and tensile hydr...
Figure 2.15 (a–d) Illustration of characterization of liquid‐air interface b...
Figure 2.16 (a, b) Illustration of electrochemical‐based methods including D...
Figure 2.17 (a–e) Dynamic droplet behavior and surface nonwetting state.
Figure 2.18 Changes of liquid surface tensions with temperatures.
Figure 2.19 (a–d) Chemistry composites influence the wetting states.
Figure 2.20 Wetting of cell aggregates on microdisk structures of different ...
Figure 2.21 Illustration of the photoelectrochemical (PEC) enhancement mecha...
Figure 2.22 (a–c) Surface with chemical heterogeneity pattern for controllin...
Figure 2.23 (a–e) Janus materials with hydrophilic/hydrophobicity for contro...
Figure 2.24 (a, b) Fabrication of gradient‐Janus wetting via liquid confined...
Figure 2.25 (a–f) Fogwater harvester and electric energy generator by GJW ar...
Figure 2.26 (a–e) Illustration of the mechanism of blood repellency based on...
Figure 2.27 The mechanism of driving force for directional transport of liqu...
Figure 2.28 Gradient acts on a liquid droplet for transport in a direction a...
Figure 2.29 Phylogenetic tree of organisms that produce ice‐binding proteins...
Figure 2.30 (a–c) Illustration of bioinspired anti‐icing mechanism of PDMS‐g...
Figure 2.31 (a–f) Water harvesting mechanism with fog capture and water tran...
Figure 2.32 (a–j) Natural self‐propelling system and mechanism.
Figure 2.33 (a–j) Various gradients for self‐propelling effects.
Figure 2.34 (a–g) The self‐propelling effect on structure surfaces [113].
Figure 2.35 Illustration of self‐propulsion movement of condensate droplet a...
Figure 2.36 (a–d) Time‐lapse snapshots for condensed‐droplet coalescence mod...
Figure 2.37 (a–h) Bioinspired structures of butterfly wing for wetting self‐...
Figure 2.38 (a, b) Illustration of capillary force formation and capillary r...
Figure 2.39 (a–e) Bioinspired structured surfaces to control the capillary r...
Figure 2.40 (a, b) Various styles of interstructures in tubes for controllin...
Figure 2.41 (a–c) Defining the equilibrium rise heights in rough capillary r...
Figure 2.42 Illustration of inspired separation membranes for efficient dye/...
Figure 2.43 (a, b) Illustration of the mechanism of capillary effect and dye...
Figure 2.44 (a, b) Illustration of spider microfluidic device.(a) and bi...
Figure 2.45 (a–e) Bioinspired helical‐groove spindle‐knot microfibers and ca...
Figure 2.46 Illustration of capillary water flow and CPF evaporator array an...
Figure 2.47 Illustration of the mechanical forces analysis of capillary wate...
Figure 2.48 (a, b) Illustration of the design of dual‐photothermal freshwate...
Figure 2.49 (a–e) Illustration of the biological liquid‐infused surface (LIS...
Figure 2.50 (a–e) Illustration of five criteria, which are identified from s...
Figure 2.51 (a–d) The multisubstance transport with inspired lubricated slip...
Figure 2.52 (a–d) Object transport mechanism of the LS‐MMA surface.
Figure 2.53 Illustration of the design and response mechanism of SPIPS.
Figure 2.54 (a, b) Illustration of fabrication and the bifunctional and swit...
Figure 2.55 (a–c) Illustration of the slippery porous‐liquid‐infused porous ...
Figure 2.56 Illustration of slippery porous‐liquid‐infused porous surface (S...
Chapter 3
Figure 3.1 Illustration of various gradients, including chemical gradient (a...
Figure 3.2 (a–e) Illustration of bioinspired dynamic gradients with stimuli‐...
Figure 3.3 (a–c) The chemistry gradients that have been fabricated on the su...
Figure 3.4 (a, b) The gradient with tethered polydimethylsiloxane molecule....
Figure 3.5 The change in temperature gradient modulates the moving direction...
Figure 3.6 Illustration of multibioinspired Janus asymmetric metallic foam w...
Figure 3.7 (a, b) As‐designed water‐splitting device and gas bubble generati...
Figure 3.8 (a–d) Bioinspired design of gradient covalent organic framework (...
Figure 3.9 (a, b) Polymer brushes in gradients.
Figure 3.10 (a–e) Bionic three‐dimensional (3D) Efficient Fog Collector.
Figure 3.11 (a, b) The oil–water separation membrane inspired by desert beet...
Figure 3.12 (a–e) The mechanism of switchable wetting thin films.
Figure 3.13 (a, b) A surface chemistry‐based anti‐icing gradient polymer coa...
Figure 3.14 (a–c) Illustration of bioinspired spine with backward microbarbs...
Figure 3.15 (a–d) The SBHC‐based spider‐web‐like fog collector.
Figure 3.16 (a–f) Controllable liquid transport on various wedge‐shape patte...
Figure 3.17 (a–c) The analysis of the liquid–solid interaction.
Figure 3.18 (a, b) Air bubble transport on horizontal and vertical cones....
Figure 3.19 (a–c) Demonstration of liquid transport trajectory and controlla...
Figure 3.20 (a–i) Illustration of design and the superwettable microspine (S...
Figure 3.21 Illustration of fluorescence detection of PSA on the SMS chip....
Figure 3.22 (a–d) Illustration of fabrication of the ASS photocatalyst for c...
Figure 3.23 (a–c) The design and fabrication of the HDSSF with capillary cha...
Figure 3.24 (a, b) Illustration of the mechanism of fog harvesting by HDSSF....
Figure 3.25 (a–e) Illustration of design of trilayered fibrous dressing (TFD...
Figure 3.26 (a–c) Illustration of the mechanism of liquid transport.
Figure 3.27 (a–d) Illustration of the heat dissipation and sweat release pro...
Figure 3.28 (a, b) Illustration of the water moisture transportation of WGID...
Figure 3.29 (a–c) Illustration of fabrication and application of PIL membran...
Figure 3.30 (a, b) Illustration of surface tension gradient that formed in l...
Figure 3.31 (a–d) The mechanism of surface tension transport by asymmetric s...
Figure 3.32 (a–d) Illustration of deformation scenarios for a soft liquid‐in...
Figure 3.33 (a–d) Programming the curvature gradients.
Figure 3.34 (a–d) Illustration of design of CSMP‐DSF membrane.
Figure 3.35 (a–d) Illustration of bioinspired liquid transport and low bacte...
Chapter 4
Figure 4.1 (a–c) Several typical PRI phenomena.
Figure 4.2 The illustration of broken‐up liquid film into droplets on fiber....
Figure 4.3 (a, b) Controlling of Rayleigh instability.
Figure 4.4 Spindle‐knots distribution by drawing the nylon fibers out of the...
Figure 4.5 Increase of research interest on the topic of “microfluidics,” “o...
Figure 4.6 (a, b) Define of closed and open microfluidic technology.
Figure 4.7 (a–e) Illustration of the microfluidic design for fabrication of ...
Figure 4.8 (a–e) Illustration of the microfluidic method and water collectio...
Figure 4.9 (a–i) Illustration of fabrication of bioinspired microfiber via m...
Figure 4.10 (a, b) Illustration of design and fabrication of bioinspired fib...
Figure 4.11 (a–c) Fluid‐coating for fabrication of bioinspired fiber.
Figure 4.12 (a–d) Velocity‐regulated fluid‐coating for fabrication of gradie...
Figure 4.13 (a–f) Fabrication on bioinspired spider silk fiber.
Figure 4.14 (a–e) The electrospinning system for fabrication of fibers.
Figure 4.15 (a–d) The lotus leaf‐like structures by electrospinning method....
Figure 4.16 (a–d) Capture silk of ecribellate spiders and fabrication by ele...
Figure 4.17 (a–c) Illustration of the multiscale structured membrane with ro...
Figure 4.18 (a–g) Bioinspired spider‐silk‐inspired wet adhesive by electrosp...
Figure 4.19 (a–e) Design and fabrication of the omniadhesive fibers by elect...
Figure 4.20 (a–g) Electrospinning method for fabrication of wet‐built nanofi...
Figure 4.21 (a, b) Wettable patterns and gradient surface by mask‐based grad...
Figure 4.22 (a, b) Bioinspired 3D wettable gradient by needle‐face anodic ox...
Figure 4.23 (a–e) Bidirectional microchannel‐connected pattern by electroche...
Figure 4.24 (a–d) Capillarity‐induced oxidation with an integrative gradient...
Figure 4.25 (a, b) Illustration of differences between conventional electrol...
Figure 4.26 (a–c) The gradient chemical components formation by bipolar elec...
Figure 4.27 (a–c) Janus gradient fabrication liquid confined modification (L...
Figure 4.28 (a–c) Ethanol‐infused wire cuts the droplet.
Figure 4.29 (a–c) Multiinspired structured surface by laser fabrication.
Figure 4.30 (a–d) Heterogeneous wettability‐patterned surface (UPS) by Laser...
Figure 4.31 Illustration of printing techniques for a wetting‐controlling pa...
Figure 4.32 (a–f) The foundation of printing techniques for the fabrication ...
Figure 4.33 (a–d) Wet‐controlled heterostructure patterning on a printed mat...
Figure 4.34 (a–d) Illustration of visualizable multiplex detection of exosom...
Figure 4.35 (a–c) Illustration of the design of SPHN.
Figure 4.36 (a–e) Dual‐nanoscaled coating with photothermal deicing effect....
Figure 4.37 (a–g) Fabrication and characterization of MOF‐based photothermal...
Figure 4.38 (a–d) Illustration of fabrication and characterization of PMNT....
Figure 4.39 (a–d) Illustration for the surface functionalities on TiO
2
nanot...
Figure 4.40 (a, b) Plasma treatment for the surface patterns via wetting‐ena...
Chapter 5
Figure 5.1 (a–c) The design of structured surface for condensate droplet jum...
Figure 5.2 (a–d) Hierarchical condensation on as‐designed copper oxide micro...
Figure 5.3 (a–c) Effects of coalescence‐induced self‐propelled jumping on hi...
Figure 5.4 Fog‐harvesting devices (FHDs) from natural biological system.
Figure 5.5 (a–c) The design of fogwater harvesting for applications.
Figure 5.6 (a–d) Bioinspired needle array and Janus membrance system for fog...
Figure 5.7 (a–c) MOF‐composite nanofiber textures with high‐efficient water ...
Figure 5.8 (a, b) MOF‐based polymer composite for water harvesting at low hu...
Figure 5.9 (a–f) Super‐hydrophilic photothermic nanocapsule with excellent w...
Figure 5.10 (a–g) Excellent anti‐icing performance for wind turbine blades....
Figure 5.11 (a–e) The photothermal icephobic surface of multi‐inspired struc...
Figure 5.12 Development chronology of chemical ice‐inhibition molecules for ...
Figure 5.13 (a–c) The dually‐mobile super‐repellent surfaces.
Figure 5.14 (a, b) Bio‐based coatings for multiliquid repellency.
Figure 5.15 (a–d) Switchable repellency in both air and liquid on bioinspire...
Figure 5.16 (a–e) Composite material with energy storage, electrothermal, an...
Figure 5.17 (a–c) Reversed electrodialysis (RED) approach for the salinity‐g...
Figure 5.18 (a, b) Illustration of a three‐dimensional (3D) gel interface sy...
Figure 5.19 (a–g) The electrical output of the vertical aerogel‐on‐water‐col...
Figure 5.20 (a–k) The biological and bioinspired thermal energy regulation a...
Figure 5.21 (a–g) Bioinspired fabrics design with Janus personal thermal man...
Figure 5.22 (a–k) Bioinspired structured passive radiative cooling ceramic w...
Figure 5.23 Skin‐inspired wearable devices and systems.
Figure 5.24 (a–c) Bioinspired skin vision from design to application.
Figure 5.25 (a–j) FIL−CNC nanostructured film with structure color and bend‐...
Figure 5.26 (a, b) Bioinspired hydrogel‐functionalized bandage with excellen...
Figure 5.27 Illustration of wet adhesion types for drug delivery toward targ...
Figure 5.28 (a, b) Bioinspired active injection drug delivery and mechanism ...
Guide
Cover
Table of Contents
Title Page
Copyright
Dedication
Preface
Acknowledgments
Begin Reading
Summary
Index
End User License Agreement
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Controlled Surface Wetting
From Bioinspiration to Applications
Yongmei Zheng
Author
Prof. Yongmei ZhengKey Laboratory of Bio‐Inspired Smart Interfacial Science and Technology of Ministry of EducationSchool of ChemistryBeihang UniversityBuilding 7, No.9 South Third StHigher Education ParkChangping DistrictBeijing 100191China
Cover Image: © KRIACHKO OLEKSII/Shutterstock
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All rights reserved (including those of translation into other languages, text and data mining and training of artificial technologies or similar technologies). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐35289‐0ePDF ISBN: 978‐3‐527‐84430‐2ePub ISBN: 978‐3‐527‐84431‐9oBook ISBN: 978‐3‐527‐84432‐6
To my beloved family: To my mom and dad, who are no longer with us but whose love and guidance continue to inspire me every day. To my husband, Hongtao, and my son, Fei, for your endless love, patience, and encouragement. To my granddaughter, Beier, for your endless happiness and love. This book is dedicated to you with all my love and gratitude.
Preface
Creatures and animals have achieved the unique wetting‐controlled systems with the evolvement of biological surfaces via thousands of years in nature environment. These natural biological properties have been paid more attentions from foundation researches to application investigations with developments of scientific technology, especially, nanotechnology.
The researches are revealed that natural creatures display the biological wet‐controlled functions, for examples, plants leaves and duck feather for superhydrophobic property, butterfly wings for directional water repellency, spider silks for capturing tiny water droplets from air, tree frog toes with wet‐adhesion, beetle with thermal‐moisture management, and so on. In recent years, more and more biological surfaces with wetting‐controlled abilities have been discovered, which have inspired researches and scientists to develop novel systems of surfaces with self‐cleaning, water repellency, super‐slipper, water collecting, wet‐adhesive drug delivery, e‐skin for sensor, etc.
The bioinspired wet‐controlled surfaces and materials have being opened much more promising applications for engineering, agriculture, industry, building, medical realms. The book can be described from the five chapters to reveal the bioinspired wet‐controlled effects, i.e., in Chapter 1, the topic is focused on wetting‐controlled systems of biological surfaces, the section contents are included in wetting features of biological surfaces, anti‐wetting features of biological surfaces, biological patterns on micro‐ and nanoscale structures, wetting‐controlled effects. In Chapter 2, the topic is focused on mechanism and theory of wetting‐controlled surfaces, the section contents are included in concept of wetting‐controlled effects, wetting theory of surfaces, physics of wetting, surface chemistry and structures, bioinspired wetting‐controlled mechanism, self‐propelling effects of surfaces, capillary force, and liquid‐infused surfaces. In Chapter 3, the topic is focused on design on surfaces with wetting‐controlled effects, the section contents are included in the concept of gradient, chemistry gradient, texture gradient, geometrical gradient, synergism of multi‐gradients, and surface tension gradient. In Chapter 4, the topic is focused on development on bioinspired fabrication and methods, the section contents are included in Rayleigh instability, microfluidics, fluid‐coatings, electrospinning, electrochemisty, nanotechnology, fluid diffusion in gradient, green printing technology, laser etching, laser treatment, plasma activation, plasma‐based coating technology, other dry etching. In Chapter 5, the topic is focused on wetting‐controlled effects for functions and applications, the section contents are included in condensate droplet transport, fogdroplet harvesting, atmospheric water harvesting, anti‐icing, liquid repellency, energy harvesting, heat transfer, artificial skin and sensor, drug delivery, etc.
In this book, we introduce the biological surfaces with unique controllable surface wetting properties, including the theories and mechanisms in physical chemistry and smart micro‐ and nanostructure effects. We also introduce the bioinspired fabrication techniques and methods on designs of wetting‐controlled surfaces by using organic or inorganic materials, including those with high/low surface energy or regular/irregular, ordered/disordered, or rough/smooth surfaces, in addition to being endlessly arranged and combined with micro‐ and nanostructures of various styles. The bioinspired methods and technologies have been developed, e.g., soft lithography, nanocrystalline growth, dip coating, electrodynamics, fluid coating, self‐assembly, and nanocomposite techniques. These can become significant for designing or fabricating novel controllable wetting surfaces that can be extended to many applications.
This book collect a larger amount of literature that are published in recent years, which can be greatly an interesting issue based on the bioinspired wetting‐controlled effects related to applications. This book is highly suitable for readers such as scientific PhD students and researchers to learn about the design of smart and advanced materials for engineering applications.
Acknowledgments
I thank the authors for a large of literature with published excellent works and my students for collecting literature. I also thank Editor Dr. Alice Qian for offering me the opportunity to publish a book on my favorite subject. I also thank Editor Kavipriya R. for managing manuscript of book and Editor Adaline Jebamalar J. for checking and editing on manuscript.
I also acknowledge the support of the National Natural Science Foundation of China (grant nos. 22275008) for enabling the preparation of this book.
June 6, 2024, Beijing
Yongmei Zheng
1Wetting‐Controlled Systems of Biological Surfaces
1.1 Introduction
Creatures and animals have achieved the unique wetting‐controlled system through the evolvement of biological surfaces over thousands of years in a natural environment. These natural biological properties have taken much attention: for instance, plant leaves and duck feathers for superhydrophobic property, butterfly wings for directional water repellency, spider silks for capturing tiny water droplets from air, and so on. In recent years, more biological surfaces with wetting‐controlled abilities have been discovered, which have inspired researchers and scientists to develop novel systems of surfaces with self‐cleaning, water repellency, super‐slipper, water collecting, etc.
The wetting phenomenon can be easily explained. Wenzel explained the state of droplets on plants in 1936 [1]; he proposed that the leaves' roughness raised the droplet's contact angle (CA). A rule of wetting contact is regulated by Wenzel, which is named as Wenzel equation. Meanwhile, in 1944, Cassie and Box suggested another rule about the air fraction on the rough surface of plants [2], explaining why droplets slide away from the surfaces of plants' leaves and animals. The air–solid fraction plays a role in raising the contact angle of water droplets on the rough surface of lotus leaves. It is attributed that the trapped‐air state appears on the surface of lotus leaves, accordingly reducing the solid interface contact with the droplets, resulting in the droplet's easy‐sliding‐off property. A rule can be set and named as Cassis and Box equation, having been extended into today, which is used to elucidate the properties of superhydrophobic surfaces. Following these equations and viewpoints based on surfaces, scientists and researchers have basically understood the wetting‐controlled properties of biological surfaces.
Based on the first discoveries of wetting properties on biological surfaces in years, the examples can be taken as shown in Figure 1.1.
Figure 1.1 The groundbreaking findings on wetting‐controlling from biological surfaces in years for promising future applications.
1.1.1 Duck Feather
In 1945, Cassie A. B. D. first reported the hydrophobicity of duck feather surfaces in his groundbreaking study of large CAs on animal and plant surfaces [3]. This seminal work laid the foundation for our understanding of the unique surface properties exhibited by various natural materials. Cassie's pioneering research revealed that duck feathers possessed an exceptional resistance to wetting by water droplets. He observed that when water droplets came into contact with the feather surfaces, they did not immediately wet the feathers but instead formed highly spherical beads that rested atop the surface. This phenomenon was attributed to the combined effects of the microstructure of the feather surface and the intrinsic hydrophobicity of the materials involved. As a result, water droplets could maintain their spherical shape, exhibiting high CAs, and could easily roll off the surface, leaving the feathers dry.
Cassie's findings not only advanced our understanding of the physical properties of natural surfaces but also inspired the development of biomimetic materials with hydrophobic characteristics. Researchers in various fields, from materials science to engineering and beyond, have drawn inspiration from the remarkable hydrophobicity of duck feathers to design surfaces and coatings with applications ranging from self‐cleaning materials to water‐resistant textiles and advanced adhesives.
Over the decades since Cassie's pioneering work, our comprehension of surface wetting and the underlying principles governing the behavior of liquids on solid surfaces has deepened.
1.1.2 Insect Wings
In 1996, the research conducted by Wagner [4] provided groundbreaking insights into the relationship between insect wing surface structures, wettability, and contamination resistance. This study focused on the surface structures found on insect wings and how these structures influence wettability and contamination resistance. It highlighted the role of these structures as vital adaptations in the insect world and sparked interest in biomimicry for developing advanced materials and technologies. The study highlighted that different insect species have evolved unique surface sculptures on their wings, leading to variations in wettability and contaminability.
The research has implications beyond entomology. It inspired biomimicry in materials science and engineering, where scientists and engineers aim to replicate these hydrophobic surface structures for various applications, such as self‐cleaning materials, antifouling coatings, and water‐repellent textiles.
1.1.3 Lotus Leaf
In 1997, Barthlott and Neinhuis [5] published a groundbreaking paper titled “Purity of the sacred lotus, or escape from contamination in biological surfaces” in the journal Planta. This paper is notable for presenting the first scientific exploration and documentation of the lotus leaf effect, a natural phenomenon characterized by the remarkable self‐cleaning and water‐repellent properties of lotus leaves and certain other plant surfaces. They identified and described the specific micro‐ and nanoscale structures found on lotus leaves' surfaces. These structures included tiny wax‐covered bumps and nanoscale wax crystals. They recognized these surface features as crucial contributors to the lotus leaf's hydrophobicity.
This work highlighted the potential applications of the lotus effect in various fields, such as materials science and engineering. It inspired scientists and engineers to mimic the lotus leaf's surface structures to create self‐cleaning materials and technologies.
1.1.4 Beetle Back
In dry and hot Namib Desert, there are several beetles showing superior water collections to feed themselves. In 2001, Parker A.R., et al. found the hydrophilic smooth “bumps” were distributed on superhydrophobic wax‐covered valleys [6]. The “bumps” (0.5 mm in diameter) are randomly distributed in an array with 0.5–1.5 mm apart, and the wax layer constructs the hydrophobic valley. Therefore, the integration of alternated hydrophilicity and hydrophobicity has endowed the beetles with efficient fog‐harvesting abilities.
During the nighttime, the wind containing dense drops that blow from the nearby ocean brings amounts of water to the creatures in the Namib Desert. Water in a foggy atmosphere is captured by the hydrophilic regions then coalesces into a bigger one, and finally transported to the hydrophobic area till to the maximal drop weight. It can be understood that water is captured on hydrophilic areas and transported by hydrophobic regions on the dorsal back of the Namib Desert beetle to achieve fog harvesting.
1.1.5 Rice Leaf
In 2002, a significant development in the field of superhydrophobic surfaces was made by Feng et al. [7], who made an intriguing discovery regarding the unique properties of rice leaves. The research unveiled an anisotropic dewing trend on the surface of rice leaves and identified striking similarities in microstructures and nanostructures between rice leaves and the renowned superhydrophobic lotus leaves.
The insights gained from the study of rice leaves, along with the principles outlined, have been instrumental in the design of superhydrophobic coatings, surfaces, and materials with applications in self‐cleaning, anti‐icing, and anticorrosion technologies, as well as in the development of water‐repellent textiles and countless other innovations across various industries.
1.1.6 Water Strider
Water striders, also known as pond skaters or Gerridae, are fascinating insects known for their ability to move effortlessly on the surface of the water. In 2004, a significant scientific breakthrough occurred when Gao and Jiang et al. [8] discovered and explained the underlying physics and mechanisms that allow water striders to perform this remarkable feat. This discovery enhanced our understanding of interfacial science and biomechanics.
This work not only provided a more comprehensive understanding of water strider locomotion but also offered insights into biomimicry and the design of small‐scale water‐walking robots. His research has continued to influence the field of fluid dynamics and the study of surface tension in biology and engineering.
1.1.7 Butterfly Wing
The existence of scales on the surface of butterflies makes their wing surfaces hydrophobic. In 2007, Zheng et al. [9] published an article revealing the influence of nanostructures on the surface wettability of butterflies. This groundbreaking research shed light on the intricate mechanisms underlying the hydrophobicity of butterfly wings.
One of the key findings of the study was the directional adhesion exhibited by butterfly wings. This directional adhesion is essential not only for repelling water but also for self‐cleaning. Furthermore, the study highlighted the potential for biomimicry in engineering superhydrophobic surfaces. Researchers and engineers have since drawn inspiration from butterfly wings to develop new materials and coatings with enhanced water‐repellent properties. These innovations have found applications in a wide range of industries, from self‐cleaning surfaces to anti‐icing coatings on aircraft and ships.
1.1.8 Mosquito Eye
In 2007, a significant breakthrough in the study of antifogging properties was made by Gao et al. [10], who discovered that the eyes of mosquitoes exhibit remarkable resistance to fogging. This discovery opened up new avenues for understanding and harnessing antifogging mechanisms in both biological and synthetic materials. This research shed light on the connection between this phenomenon and the superhydrophobic effect, which is based on the unique micro‐ and nanostructures found on the surfaces of superhydrophobic materials.
Today, the principles discovered by Jiang L continue to play a crucial role in the design of antifog materials for a wide range of applications, including eyewear, camera lenses, and various optical devices, where maintaining clear vision in humid or foggy conditions is essential.
1.1.9 Rose Petal
The research conducted by Feng et al. [11] in 2008 was the first to systematically investigate the surface hydrophobicity of rose petals and introduce the concept of the “petal effect.” It unveiled the intricate micro‐ and nanoscale structures on rose petals that enable superhydrophobicity and high adhesive force, opening up opportunities for biomimetic materials design and applications in a wide range of industries.
The research highlighted the potential for biomimicry, where the surface structures of rose petals could inspire the design of advanced materials with superhydrophobic and adhesive properties. Such materials have applications in self‐cleaning surfaces, antifogging coatings, and water‐harvesting technologies. The study suggested that the ability to control wettability and adhesion on surfaces, as demonstrated by rose petals, could have implications for the development of functional materials in various industries, including textiles, electronics, and medical devices.
1.1.10 Fish Scale
In 2009, Liu et al. [12] made a significant contribution to the field of biomimicry and surface science by drawing inspiration from the remarkable ability of fish to maintain surface cleanliness in oil‐contaminated water. Liu MJ's research focused on the wetting behavior of oil droplets on the surface of fish in water, leading to the discovery of the superhydrophobic properties of fish surfaces, primarily originating from their micro‐ and nanohierarchical structures in the water phase.
The study underscored the value of biomimicry in materials science, demonstrating how nature's solutions to complex challenges could serve as a wellspring of inspiration for the design of advanced materials and technologies. Researchers have since drawn from these insights to develop a wide range of innovative solutions in both environmental and industrial contexts, all with the aim of achieving more effective and sustainable means of managing oil‐contaminated water.
1.1.11 Cicada Wing
Sun et al. [13] published a study on the hydrophobic structure of cicada wings in 2009. This research represents a significant milestone in the exploration of natural adaptations and their potential applications in materials science and engineering. In their pioneering study, the researchers meticulously examined the wings of cicadas, a group of insects known for their distinctive songs and fascinating adaptations. What they uncovered were intricate nanostructures covering the wing surfaces, a discovery that would revolutionize our understanding of superhydrophobicity in the natural world.
The nanostructures on cicada wings inspired the design of materials with superhydrophobic properties, leading to the development of self‐cleaning surfaces, antifog coatings, and water‐repellent technologies.
1.1.12 Spider Silk
In 2010, a significant scientific breakthrough emerged when Zheng et al. [14] and her colleagues published an academic cover paper in the prestigious journal Nature. This groundbreaking study unveiled, for the first time, the remarkable water‐harvesting effect of spider silk and proposed a multisynergistic driving mechanism for liquid droplets on these extraordinary natural fibers. This research was focused on the water‐interaction properties of spider silk, which had long intrigued scientists due to its unique surface characteristics.
The research had implications for bioinspired technologies, inspiring scientists and engineers to explore the potential applications of spider‐silk‐inspired materials for water harvesting, moisture management, and other areas.
1.1.13 Salvinia
The floating‐leaved water fern of the Robinia genus, commonly known as Salvinia, has long intrigued scientists due to its exceptional ability to retain air under water. In 2010, a groundbreaking study led by Barthlott et al. [15] and his colleagues revealed the precise surface design of Salvinia, shedding light on the remarkable phenomenon of air retention under water. Salvinia's ability to trap and retain air for extended periods while submerged in water is often referred to as the “Salvinia paradox.” This unique adaptation is critical for the plant's survival in aquatic environments, as it allows the fern to access atmospheric oxygen and maintain buoyancy.
The research had profound implications for biomimicry, inspiring scientists and engineers to replicate Salvinia's surface design for various applications. These applications include the development of underwater devices with enhanced buoyancy and reduced drag.
1.1.14 Cacti
Cacti are renowned survival experts in arid desert environments, characterized by their succulent inner stems and needle‐like leaves, which serve as adaptations for maximizing water storage. In 2012, a groundbreaking study conducted by Ju et al. [16] brought to light the extraordinary droplet collection behavior of cacti in fog. This research not only elucidated the mechanism behind cactus cone‐shaped needles and hydrophilic fluff in droplet capture and liquid transport but also proposed a novel mechanism for air–water extraction by these remarkable desert plants.
This research inspired biomimicry and sparked interest in developing technologies inspired by cacti's fog collection mechanisms. These technologies have applications in water‐harvesting systems for arid and water‐scarce regions.
1.1.15 Gecko Skin
Watson et al. [17] in 2015, delved into a comprehensive investigation of the lesser explored interfacial properties of gecko skin, offering a holistic perspective on the remarkable attributes of this reptile's integument. While much research has been devoted to understanding the exceptional adhesive capabilities of gecko feet, he expanded the scope of the investigation to encompass the entire gecko skin, acknowledging that geckos possess remarkable properties beyond just their adhesive abilities. It highlighted the intricate micro‐ and nanostructure present on gecko skin. This structure contributes to a multitude of beneficial surface properties, including low adhesion, superhydrophobicity, antiwetting characteristics, and self‐cleaning capabilities. The research underscores the biocompatibility of the gecko skin surface, making it suitable for applications in the field of biomaterials.
1.1.16 Nepenthes
Nepenthes, a tropical carnivorous plant, boasts a remarkable adaptation in the form of its insect‐trapping organ, often referred to as the “pitcher.” This organ is cylindrical in shape, with a slight enlargement in the lower half and a lid covering the opening, which gives it a resemblance to a pig cage. The pitcher is a critical structure for the plant's nutrient absorption as it captures and digests insects. The research on the directional transport of water droplets inside the pitcher plants took a significant leap forward, thanks to the pioneering work of Chen, H.W.'s research group [18]. Their groundbreaking findings revealed a fascinating mechanism related to water transport within the pitcher of Nepenthes.
1.2 Wetting Features of Biological Surfaces
Some biological surfaces with wetting features evolved as facing the natural environment, such as spider silk, beetle back, Nepenthes pitcher plant, etc. These wetting properties can play an essential role in biological activities and their functions. The wetting features can be characterized with super‐hydrophilic and hydrophilic properties, as shown in Figure 1.2. The liquid droplets on the surface take on different shells, along with an angle θ. The θ < 5° can be termed as superhydrophilicity, and 5° < θ < 90° can be called as hydrophilicity. In nature, such a phenomenon generally exists. Interestingly, biological creatures use such unique properties to complete their activities.
Figure 1.2 Super‐hydrophilic and hydrophilic surfaces in nature.
Here are some typical examples to elucidate the wetting features; for instance, spider silk for water harvesting, beetle back for fog water collecting, and cactus spines for liquid transport are of different wetting styles based on structures of surfaces and surface chemistry.
1.2.1 Wet‐Rebuilt Spindle‐Knot with Nanofibrils on Spider Silk
The marvel of spider silk extends far beyond its legendary strength and elasticity. One of its most intriguing characteristics is its unique interaction with water, a product of nature's meticulous design shaped by eons of evolutionary fine‐tuning. Upon close inspection, spider silk reveals a mesmerizing landscape of varying structures. The silk is punctuated by spindle‐knot‐like formations, interspersed with slender connecting sections. This variation in diameter, coupled with its nanoscale roughness, plays a pivotal role in its interaction with water. Water droplets are instinctively drawn to these spindle‐knots, congregating around them due to the interplay of surface tension and capillary forces. The result is a string adorned with water droplets at regular intervals, shimmering like jewels in the morning sun.
Adding to this complexity is the spider silk's unique chemical composition. Woven from a tapestry of proteins, the silk contains regions with distinct affinities for water. Some segments are hydrophilic, drawing water toward them, while others are hydrophobic, repelling it. This intricate balance allows water droplets to not only cling to the silk but also move along its length, guided by the delicate dance of attraction and repulsion. Some spiders, especially those in regions where morning dew is a frequent visitor, harness this water‐collecting prowess of their silk. They craft dense webs that trap moisture from the air, channeling it along the strands. As droplets merge and grow, the spider gains access to a crucial source of hydration, especially vital in arid environments. There is also an evolutionary angle to this story. In areas where fog or morning dew is commonplace, the water‐collecting capacity of spider webs might offer a survival advantage. Not only does it provide the spider with a drink, but a moisture‐laden web could also be more adept at ensnaring unsuspecting prey.
Spider silk, which is composed of humidity‐sensitive hydrophilic flagelliform proteins, enjoys a high reputation as a fiber with excellent mechanical properties. A recent report reveals another intriguing feature, i.e., its ability to collect water from humid air, where the pearly droplets brighten under sunlight on the web of spider silks. This phenomenon can be observed further by an optical microscopy camera (Figure 1.3).
Figure 1.3 Spider silk with hanging the large water drops.
The environmental scanning electronic microscopy (ESEM) images of the cribellate spider silk in Figure 1.4 illustrate its structure. Puffs composed of nanofibrils are spaced along two main‐axis fibers with a periodicity of ∼86 μm (Figure 1.4a). The puffs' diameter is ∼130 μm, and they are separated by joints with a diameter of ∼42 μm (Figure 1.4b). The zoomed‐in image in Figure 1.4c shows the puffs to be composed of random nanofibrils (∼20–30 nm in diameter). These highly hydrophilic nanofibrils enhance the wettability of spider silk, which is favorable for condensing water drops.
Figure 1.4 (a–c) ESEM images of capture silk of cribellate spider silk including periodic puff composed of nanofibrils.
Interestingly, when dry capture silk of spider silk is placed in fog, its structure changes as water starts to condense and form drops that move along the silk fiber (Figure 1.5). At the initial stage, tiny water drops (black dots indicated by arrows in Figure 1.5a) condense on the semitransparent puffs. As water condensation continues, the puffs shrink into opaque bumps (Figure 1.5b) and finally form periodic spindle‐knots (Figure 1.5c). The impact of fog on the mechanical properties of spider silk has been noted before; interestingly, the observation indicates that in the case of cribellate spider silk, the material changes its fiber structure as a result of wetting. After this “structural wet‐rebuilding,” directional water collection starts (Figure 1.5d), i.e., it is shown that the pearly droplets form on spider silk.
Figure 1.5 (a–d) Optical images of capture silk of cribellate spider silk change its structures after wetting.
These exciting features open the inspiration of scientists and researchers to design materials for water collecting and water capture for promisingly practical applications in arid places.
1.2.2 Slippery in Multiorder Ridges on Peristome Surface of Nepenthes
Nepenthes, commonly known as the pitcher plant, is an astonishing testament to the ingenuity of nature's designs. This carnivorous plant has perfected an elegant trap over millions of years, ensuring its survival in nutrient‐poor soils by luring, capturing, and digesting unsuspecting prey. Central to this mechanism is the sophisticated wetting feature of the pitcher's lip, also known as the peristome.
The peristome's wetting characteristics are an intricate dance between structure and function. On a microscopic level, the surface of this lip reveals a dense network of ridges and grooves. These structural intricacies, combined with the plant's natural secretions, result in a surface that is strikingly hydrophilic, meaning it readily attracts and spreads water. When the pitcher plant is exposed to the ambient humidity or after a bout of rain, the peristome becomes slick with a thin layer of water. This watery film drastically reduces the friction between the surface and any creature that happens to tread upon it. Insects, attracted by the plant's vibrant colors and enticing nectar, often find themselves on this treacherously slippery slope. The unique hydrophilic properties ensure that their feet cannot find purchase, causing them to slide uncontrollably into the pitcher's deep cavity. Once inside, escape becomes a near impossibility. The inner walls of the pitcher are equally crafty, with hydrophobic properties that prevent climbing. Moreover, the digestive enzymes waiting at the bottom of the pitcher ensure that the entrapped prey meets its demise, providing the plant with essential nutrients.
Beyond its role in prey capture, the wetting properties of the pitcher plant have another subtle yet crucial function. By maintaining a consistent layer of water on the peristome, the plant ensures that the inner walls of the pitcher remain humid. This humidity is vital, as it aids in the digestion of prey and prevents the trapped insects from dehydrating too quickly, which would make them less nutritionally valuable. The Nepenthes provides a vivid illustration of how nature, through the crucible of evolution, devises intricate solutions to life's challenges. Its wetting features underscore the elegance with which nature can intertwine form and function, turning a simple leaf into a sophisticated trap that ensures the plant's survival in some of the world's most challenging habits.
The peristome surface of the Nepenthes pitcher plant is featured with a relatively regular microstructure, where straight rows of epidermal cells constructed first‐ and second‐order radial ridges. When rainwater wetted peristome in a slippery state, a glossy surface is obtained without any individual droplets. The intermediary liquid is locked in microtextures on the pitcher, forming a repellent layer. The surface energies between liquid and solid are well matched, which cooperated with microtextural roughness to create a highly stable state and a continuous overlying film is obtained. So, amounts of ants are quickly captured and moved down to digestive fluid. Recently, it is found that the continuous, directional water transport happens [18] on the surface of the “peristome” – the rim of the pitcher (Figure 1.6a–c) – because of its multiscale structure (Figure 1.6d), which optimizes and enhances capillary rise [20, 21] in the transport direction, and prevents backflow by pinning in place any waterfront that is moving in the reverse direction (from inner side to outer side). This feature inspires researchers to design structured surfaces that control liquid transport in different directions [19].
Figure 1.6 (a–d) The directional water transport on the surface of the “peristome” along the rim of the pitcher with multiorder ridges.
Source: Li et al. [19]/National Academy of Sciences.
1.2.3 Selectively Directional Ratchet Transport
Recently, there is an unexpected liquid transport behavior that happens on the Araucaria leaf, which consists of three‐dimensional (3D) ratchets with transverse and longitudinal reentrant curvatures that are characterized by the low surface‐tension liquids selecting a pathway along the ratchet‐tilting direction, whereas high surface‐tension liquids select an opposite.
As shown in Figure 1.7, the Araucaria leaf consists of periodically arranged ratchets tilting toward the leaf tip (Figure 1.7a). Such ratchets exhibit dual‐reentrant topography with a transverse curvature (radius R1, Figure 1.7b, the top) and a longitudinal curvature (radius R2, Figure 1.7b, the bottom) [22]. There are water and ethanol CAs of ∼59° and ∼21° on the Araucaria leaf, respectively, along with a typical hydrophilic property. When continuously infusing water and ethanol (flow rate of 3 ml s−1) on the Araucaria leaf, the ethanol spreads along the ratchet‐tilting direction (Figure 1.7c), whereas water propagates in the opposite direction (Figure 1.7d).
Figure 1.7 (a–d) The ratchet capillary structures of Araucaria leaf for liquid directional steering.
Source: Feng et al. [22]/American Association for the Advancement of Science ‐ AAAS.
The spreading direction of liquids with different surface tensions can be tailored by designing 3D capillary ratchets that create an asymmetric and 3D spreading profile both in and out of the surface plane direction. Different from the conventional microstructures with directional liquid transport in the two‐dimensional (2D) domain [23,27], the Araucaria leaves have 3D ratchets that enable liquid wicking both in and out of the surface plane. Moreover, the transverse and longitudinal reentrant curvatures impart asymmetric contact line pinning, which enables the directional steering and rapid transport of liquids with different surface tensions in a well‐controlled manner. This feature inspires researchers to design novel surface structures for selective fluid‐controlling [22].
1.2.4 Multilevel Structured System of Cacti
In the harshest sun‐drenched deserts, cacti emerge as nature's marvels, having perfected the art of water collection and conservation. Every aspect of their design serves the singular mission of capturing, directing, and preserving every possible droplet in their arid homes.
The surface of cacti, upon closer examination, reveals a complex network of grooves and channels. These are not merely decorative quirks; they are nature's aqueducts, guiding water from the sparse rain or morning dew down toward the plant's base. Even the spines, which may seem like mere defensive structures at first glance, have a dual role. They cast shadows, reducing evaporation from the plant's surface and guiding water droplets toward the plant, acting like mini catchment systems. Adding to this is the skin of the cacti, which leans toward being hydrophilic, meaning it attracts water. In a world where every drop counts, ensuring that water clings to you rather than evaporating or being whisked away by the breeze is invaluable. This affinity for moisture comes into particular play during the cool desert nights. As humidity rises, cacti capitalize on it, capturing and condensing moisture on their surfaces, which is then absorbed.
However, the magic is not just on the outside. Within their green walls, cacti have evolved a unique method of photosynthesis. Cacti predominantly do so at night, unlike most plants that open their stomata during the day. This strategy, called CAM photosynthesis, is a masterstroke to prevent water loss from the intense daytime heat. Further aiding in their water retention efforts is a thick waxy coating that many cacti sport. This is not just about giving them a unique sheen. The wax acts as a shield, reflecting some of the sun's intense rays and forming a barrier that locks moisture in, keeping the internal tissues hydrated. Finally, beneath the sand and pebbles, the cacti extend their reach with an impressive root system. These roots, both wide‐spreading and deep‐penetrating, are always ready to absorb any available moisture, be it from a rare shower or hidden underground reserves.
Consider the example [16] as shown in Figure 1.8. The optical image shows a plant of Opuntia microdasys stem covered with well‐distributed clusters of spines and trichomes (Figure 1.8a). Magnified optical images reveal a single cluster with spines growing from the trichomes (Figure 1.8b, c). Scanning electron microscopy (SEM) reveals that a single spine can be divided into three regions: the tip with an apex angle (2α) and oriented barbs (Figure 1.8e), the middle with gradient grooves, and the base with belt‐structured trichomes (Figure 1.8f, g). The microgrooves near the base are wider and sparser than those near the tip. There are barbs with an apex angle (2β) covering the tip of the spine (Figure 1.8h). Such a spine system achieves the directional collection of the water drops, as shown in Figure 1.8i, where the deposited drop (1) and the coalesced drops (2–4) combine together, moving directionally along the spine (black arrows) to form a large drop (1 + 2 + 3 + 4 + 5). Finally, the as‐combined drops were absorbed immediately through the trichomes for the water‐harvesting task.
Figure 1.8 (a–i) Appearance and surface structures of the cactus and fogdrop harvesting.
Source: Ju et al. [16]/Springer Nature.
In the world of plants, cacti are the ultimate survivalists. [28] In addition, there is also a spine of plant similar to cacti for water harvesting [29]. Their entire being, from the spiny tip to the deepest root, is a testament to evolution's power to innovate, turning challenges into opportunities. Through their many adaptations, cacti offer lessons in resilience, efficiency, and the elegance of nature's engineering, which inspire scientists to design novel materials.
1.2.5 Overlapping Arrangement of Fish Scale
Fish scales are a marvel of nature's design, seamlessly integrating protection with fluid dynamics to help these aquatic creatures glide effortlessly through the water. While, at first, they might appear as simple protective elements, a closer look reveals the multifaceted roles they play.
The texture and topography of fish scales are meticulously designed. Their overlapping arrangement, reminiscent of roof shingles, ensures that water flows smoothly over the fish's body. This overlapping design channels water in a laminar flow, minimizing turbulent wake behind the fish and consequently reducing drag. Essentially, each scale serves as a tiny deflector, guiding water along the contours of the fish. Beyond the physical architecture, fish scales have a unique relationship with water due to their protective mucus layer. This slimy coating might be off‐putting to us, but it is a crucial component of a fish's hydrodynamic toolkit. Being hydrophilic or water‐attracting, this mucus ensures that the interaction between the fish and its environment remains smooth and frictionless. The lubrication it provides allows fish to swim with reduced resistance, conserving energy and increasing agility.
This mucus layer has implications beyond just reducing friction. It plays a vital role in the dynamics of the boundary layer, which is the thin layer of water directly in contact with the moving fish. By keeping this layer stable and smooth, the scales and their mucus coat help diminish drag forces acting on the fish, allowing it to move faster and more efficiently. Protection is another feather in the cap of the mucus‐coated scales. They form the first line of defense against potential pathogens, ensuring harmful microorganisms find it challenging to take hold. The mucus can even contain compounds that aid in wound healing, showcasing its multifunctional nature.
Interestingly, the wetting properties of fish scales can also impact their appearance. In some species, the combination of the mucus, the inherent structure of the scales, and the way they interact with light can produce stunning visual effects, such as iridescence. This is not just about esthetics; these optical properties can be essential for camouflage, communication, or mate attraction. In essence, fish scales epitomize the elegance of nature's solutions through their wetting features and design. They harmoniously balance protection with hydrodynamic efficiency, demonstrating how evolution has fine‐tuned organisms for optimal survival. For us, they offer a window into understanding fluid dynamics in nature and inspire innovations in areas as diverse as swimwear technology and naval engineering.
Sharks, known for their swift swimming capabilities and resistance to fouling organisms, owe much of these abilities to the microsized dermal denticles on their skin. These denticles have riblet geometries, tiny grooves that reduce friction by encouraging the flow of water to slide in the direction of the shark's movement, thus resulting in reduced drag and a surface less likely to be fouled by marine life.
Figure 1.9 shows the environmental scanning electron microscope (ESEM) images of the bonnethead shark (Sphyrna tiburo) skin surface at different body locations, including an illustration of the shark (Figure 1.9a). The different positions can be marked with letters A (the head), B (the leading edge dorsal fin), and C (the anal fin). As observed by ESEM (Figure 1.9b), the head position has the normal oriented structure array, the leading edge fin has the irregular structures, and the anal fin has the sharp oriented structure array, where the overlapping scales are shaped with normal, complex, and distinct ridges, respectively. These structured features can play a role. When the shark is swimming, the natural flow direction across the denticle surface is from lower left to upper right, from denticle base to tip [30]. The structured feature has inspired researchers to develop a method for the fabrication of biomimetic surfaces for applications [31, 32].
Figure 1.9 The illustration of (a) shark and (b) microstructures of shark skin.
Source: Wen et al. [30]/The Company of Biologists Ltd.
1.3 Antiwetting Features of Biological Surfaces
Antiwetting features (Figure 1.10) of biological surfaces can be taken with some examples, such as duck feathers, cicada wings, lotus leaves, strider legs, butterfly wing, etc., which is significant to develop the materials of anti‐icing, waterproof, water repellency, superhydrophobicity that would have application in the building construction and electron devices manufacturing industries. By understanding the biological features, the bioinspired researches are opened with a new angle for the design of new materials.
Figure 1.10 Antiwetting features of biological surfaces.
Source: moccabunny/Adobe Stock Photos.
1.3.1 Multilevel Wetting‐Controlling on Duck Feather
Duck feathers, nature's marvel of water repellency, showcase a blend of intricate structures and biological processes that collectively deter water absorption. When you look closely at a duck feather, you do not see a simple, flat surface. Instead, there's a central shaft with myriad barbs branching out. These barbs further break down into tinier structures called barbules. This complex design does more than just add beauty; it amplifies the feather's surface area, creating a multitude of air pockets that actively repel water.
Zooming in even further, these barbs and barbules reveal minuscule nanostructures. These tiny formations serve as air traps, bolstering the feather's inherent ability to fend off water. Think of it as nature's version of a high‐tech water‐repellent fabric on a microscopic scale. However, it is not just about structure. Nature equips ducks with a fascinating adaptation – a specialized gland located near their tail base, known as the uropygial gland. This gland secretes a waterproofing oil. If you have ever observed a duck closely, you might have noticed it frequently running its beak over its feathers. It is not merely grooming; the duck is meticulously spreading this oil, thereby enhancing its feathers' resistance to water. This natural oil ensures that water does not soak in but rather forms beads that gracefully roll off. Consider the layered armor ducks possess: on the exterior, contour feathers lay closely packed, forming the first line of defense against water. Right beneath, the down feathers act as insulating agents, trapping warmth. This layered defense means that the inner sanctum remains untouched and dry even if a droplet trespasses the outer layer. Additionally, the tight arrangement of the barbs coupled with the preen oil disrupts the capillary action. This phenomenon, in most contexts, would pull water into narrow spaces. Here, it is effectively neutralized, ensuring moisture does not seep deep into the feather.
