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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
A practical and hands-on discussion of fog harvesting based on bioinspired materials
In Bioinspired Materials for Fog Harvesting: Principles, Design, Preparation, and Applications, a team of distinguished researchers delivers an up-to-date discussion of the principles, design, preparation, and development prospects of bioinspired fog harvesting materials. The book explores the phenomenon of fog collecting in nature and analyzes the mechanism behind it.
The authors offer a detailed overview of contemporary bioinspired fog collecting materials. They explain the preparation of modern fog collectors and the potential applications of functional materials in practice, as well as the latest developments in a broad range of bioinspired and biomimetic materials for fog harvesting.
Readers will also find:
- A thorough introduction to natural prototypes for bionic fog collection
- Comprehensive explorations of the basic theory of fog harvesting and atmospheric water harvesting
- Practical discussions of the micro-structures of fog harvesting materials
- Complete treatments of the preparation of fog collecting materials and the sustainable use of fog collectors
Perfect for materials scientists, surface chemists, physical chemists, and bioengineers, Bioinspired Materials for Fog Harvesting will also benefit biotechnologists, biochemists, and environmental chemists.
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Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Introduction
1.1 Overview of Research on Atmospheric Water Collection
1.2 Development of Biomimetic Fog Collection
1.3 Biomimetic Inspiration for Collecting Fog
1.4 Biomimetic Fog Collection Performance Evaluation
References
2 Natural Prototype of the Bionic Fog Collection
2.1 Fog Collection of Bionic Prototype Plants
2.2 Fog Collection of Bionic Prototype Animals
References
3 Basic Theory
3.1 The Theory of Fog Harvesting
3.2 The Theory of Atmospheric Water Harvesting
References
4 Factors of Fog Harvesting
4.1 Mesh Characteristic
4.2 Collector Design
4.3 Wettability of the Collector
4.4 Collector Durability
4.5 Environmental Factors
References
5 Micro‐Structures of Fog Harvesting Materials
5.1 Microscopic Water‐Collecting Structures in Desert Beetles
5.2 Microscopic Water‐Collecting Structures in Cacti
5.3 Inspired by the Spindle Knot Structure of Spider Silk
5.4 Fog Collection Material Inspired by Lotus Leaves
5.5 Microstructure of Other Biomimetic Fog‐Water Harvesting Materials
References
6 Surface Wettability of Fog‐Harvesting Materials
6.1 SHB–SHL Fog Harvesting Materials
6.2 SLIPS Fog Harvesting Materials
6.3 Fog and Water Collection Materials Based on Gradient Wetting
6.4 Janus Effect Fog Collection Material
References
7 Preparation of Fog‐Collecting Materials
7.1 Preparation of 1‐Dimensional (1D) Fog‐Collecting Materials
7.2 2D Fog Collection Material Design
7.3 Three‐Dimensional (3D) Preparation of Fog‐Collecting Materials
References
8 Sustainable Use of Fog Collectors
8.1 Mechanical Stability
8.2 Chemical Stability
8.3 Antibacterial
References
9 Non‐phase Fog Collection
9.1 Overview of the Development of Non‐phase Bionic Fog Collection
9.2 Surface Wettability Design
9.3 Design of Surface Geometry
9.4 Intelligent Design for Bionic Surface Fog‐Harvesting Material
9.5 Summary and Outlook
References
10 Adsorption‐Based Atmospheric Water Harvesting
10.1 Introduction
10.2 Evaporation Phenomenon and Materials for Solar Water Evaporation
10.3 Progress in Sorbents for SAWH
10.4 Composite Adsorbents in SAWH
10.5 Reflections and Perspectives in SAWH
References
11 Applications of Fog Harvesting
11.1 Collection of Water Resources
11.2 Other Energy Collection
References
12 Challenges of Fog Harvesting
12.1 Fog Volume and Humidity
12.2 Collector Surface Characterization
12.3 Cost‐Effectiveness
12.4 Water Quality Treatment
12.5 Sustainability and Environmental Impacts
13 Challenges and Prospects
13.1 Research Status and Challenges
13.2 Future Expectations
References
Index
End User License Agreement
List of Tables
Chapter 10
Table 10.1 A summary of the evaporation rates for some reported re...
List of Illustrations
Chapter 1
Figure 1.1 Global scarcity of water. Distribution of large cities ...
Figure 1.2 (a) Surface wettability. (b) Surface tension. (c) Wenze...
Figure 1.3 Various superwetting surfaces found in nature.
Figure 1.4 Surface wettability of fish scales and surface wettabil...
Chapter 2
Figure 2.1 A wide variety of biological surfaces with special wett...
Figure 2.2 (a–c) Optical image of cactus plants with tapered spine...
Figure 2.3 Summary of the fog collection mechanism of cactus thorn...
Figure 2.4 Schematic of the production of the cactus‐inspired, ord...
Figure 2.5 (a) Optical photo of the pitcher cage; (b) cross‐sectio...
Figure 2.6 Relative images of the slippery zone of Nepenthes. (a) ...
Figure 2.7 [(a) and (b)] Iron orchid in the desert; (c) surface vi...
Figure 2.8 Plants of the fog collection phenomenon in nature. (a) ...
Figure 2.9 Multifunctional hierarchical water catchment mechanism ...
Figure 2.10 (a) hydrophilic protrusions and hydrophobic groove st...
Figure 2.11 Study on the application of bionic composite surface ...
Figure 2.12 SEM images of the spider silk structure. (a) The peri...
Figure 2.13 Mechanism of directional collection of water droplets...
Figure 2.14 Some representative research of bioinspired fibers in...
Figure 2.15 (a) Schematic diagram of surface scales of desert liz...
Figure 2.16 Water management properties and preparation methods o...
Figure 2.17 Surface design of mussel‐inspired, highly efficient f...
Figure 2.18 A fibrous trichome with special wetting properties pr...
Figure 2.19 Changes in the wetting state of the different wing ve...
Figure 2.20 Bionic directional liquid delivery switch imitating w...
Chapter 3
Figure 3.1 (a) The contact angle of a droplet at equilibrium on an...
Figure 3.2 (a) Digital images of the hierarchical structure of the...
Figure 3.3 (a,b) The solid–liquid–gas interface between two cylind...
Figure 3.4 (a) Schematic illustration of light‐activated binary dr...
Figure 3.5 (a) Schematic diagram of the biomimetic hydrophobic–hyd...
Figure 3.6 (a) Loosely bound water in the center of the pores is r...
Figure 3.7 The mechanism of water absorption and desorption: (a) T...
Chapter 4
Figure 4.1 Images of Cotula fallax in South African (a) in dry con...
Figure 4.2 Diagrams of different geometric grid structures.
Figure 4.3 The “harp” fog collector used for fog collection.
Figure 4.4 (a) A depiction of a 3D‐printed harp mesh design inspir...
Figure 4.5 (a) Schematic diagram of placing a water‐repellent nano...
Figure 4.6 The micromorphology of the Cu mesh with different wetti...
Figure 4.7 Mechanism of the different fog collection efficiency on...
Figure 4.8 (a) Three views of the fog collector. (b–d) The nucleat...
Figure 4.9 ESEM images of droplet condensation on perforated Al pl...
Figure 4.10 (a) The fog collection system at 0, 8, and 15 min. (b...
Figure 4.11 (a) Schematic diagram of the preparation process of a...
Figure 4.12 Direct micropatterning of superhydrophilicity on supe...
Figure 4.13 (a) Two designs of periodically patterned samples for...
Figure 4.14 (a) An overview of the whole fog harvesting process o...
Figure 4.15 (a) The expansion of the wave‐shaped bionic structure...
Figure 4.16 (a) Photograph of the pilot facility representing the...
Figure 4.17 The magnetic responses of the cactus‐inspired array u...
Chapter 5
Figure 5.1 (a1) The Namib Desert beetle. (a2) SEM image on the dor...
Figure 5.2 (a) Schematic diagram of the desert beetle sector and m...
Figure 5.3 (a1) Deposition efficiency of the target surface with a...
Figure 5.4 (a1) The physical characteristics of the cactus. (a2) S...
Figure 5.5 (a) A perspective view presenting three such structures...
Figure 5.6 (a) A schematic illustration depicting a surface featur...
Figure 5.7 (a) Structures of dry capture silk. (b) In the fog, the...
Figure 5.8 (a) Structure of wet‐rebuilt spider silk.(b–e) Bion...
Figure 5.9 (a) Scanning Electron Microscopy (SEM) images and Energ...
Figure 5.10 (a,b) The design of the kirigami structure for effect...
Figure 5.11 (a) The process of fog harvesting consists of three k...
Chapter 6
Figure 6.1 (a) Preparation diagram of fog collector. (b–f) are the...
Figure 6.2 Schematic illustration of the preparation of hydrophili...
Figure 6.3 (a) Comparison of drop motion trails on pattern less an...
Figure 6.4 Pushing the limits of the water harvesting rate in atmo...
Figure 6.5 Biomimetic water collection structure: (a) natural cact...
Figure 6.6 (a) The fog harvesting behaviors on a horizontal hydrop...
Figure 6.7 The transportation behaviors of different integrated me...
Chapter 7
Figure 7.1 (a) The structure of dried spider silk; (b) SEM images...
Figure 7.2 (a,b) Design of cactus cutouts, and optical images of t...
Figure 7.3 (a) Optical images depicting the mouth margin (left) al...
Figure 7.4 (a) A schematic illustration depicting the method for c...
Figure 7.5 (a) Kirigami structure based efficient fog collector....
Chapter 8
Figure 8.1 (a) This figure illustrates a strategy to enhance the m...
Figure 8.2 (a) Comparative SEM micrographs illustrating surface mo...
Chapter 9
Figure 9.1 (a–d) SEM images of different wetted surfaces and corre...
Figure 9.2 (a–b) Trend of droplet motion on horizontal and incline...
Figure 9.3 (a) The SEM images and fog collection images of hydroph...
Figure 9.4 (a) Optical photographs of the fog‐harvesting progress ...
Figure 9.5 Schematic illustration of the preparation process of Cu...
Figure 9.6 The preparation procedure for the hybrid superhydrophob...
Figure 9.7 (a) Schematic illustration of the preparation of hydrop...
Figure 9.8 (a) The preparation diagram of wettability patterned me...
Figure 9.9 Schematic illustration of the preparation process of po...
Figure 9.10 (a,b) Schematic illustration of the fabrication proce...
Figure 9.11 (a,b) Schematics of (a) the sample with a single tria...
Figure 9.12 (a–d) Schematic illustration of the fabrication proce...
Figure 9.13 (a) Schematic illustration of the method used to quan...
Figure 9.14 Design of a venation‐like patterned surface with hybr...
Figure 9.15 Schematic diagram of the preparation process of water...
Figure 9.16 Radiative cooling layer boosting hydrophilic‐hydropho...
Figure 9.17 Schematic diagram for the preparation process of the ...
Figure 9.18 Design of multiple‐biomimetic fog harvester and its c...
Figure 9.19 (a) Schematic diagram of fog collection of WCJM‐1 and...
Figure 9.20 (a) Schematic of the hybrid hydrophobic‐hydrophilic s...
Figure 9.21 The fog‐collecting performances of the HB − HL + JCF....
Figure 9.22 The fog‐collecting performances of the raw copper foa...
Figure 9.23 (a–c) Schematics of the Janus and sandwiched fog coll...
Figure 9.24 (a) Schematic diagram of an efficient fog‐trapping co...
Figure 9.25 Fabrication of ultrathin planar metal‐mesh Janus memb...
Figure 9.26 Inspired by pitaya, the bionic fog collector is used ...
Figure 9.27 (a) Surface properties of periodic roughness‐gradient...
Figure 9.28 (a) Scheme of fabricating bioinspired fiber with hump...
Figure 9.29 Design and characterization of NMGS and optical photo...
Figure 9.30 (a) Laser fabrication process of Janus membrane. (b) ...
Figure 9.31 (a, b) Optical microscope images of column arrays in ...
Figure 9.32 (a–c) Reversible response behavior of PNIPAAm cotton ...
Figure 9.33 (a–c) Schematic diagram of the fabrication process of...
Figure 9.34 (a) Schematic design of the bionic surface of the des...
Figure 9.35 (a), (d), and (g) are front views of Sample 1, Sample...
Figure 9.36 Sample structure; (a), (b), and (c) are CAD sketches ...
Figure 9.37 (a) 3D cactus turned into 2D paper cutout material an...
Figure 9.38 (a) BNF preparation process; (b–d) digital photograph...
Figure 9.39 (a) Preparation of smooth alumina microneedle structu...
Figure 9.40 (a) Hybrid membrane design scheme with wettability an...
Figure 9.41 (a) Schematic diagram and fog collection capacity of ...
Figure 9.42 (a) A graphic illustration of asymmetrical fibers der...
Figure 9.43 (a) Preparation process of Janus membrane with hetero...
Figure 9.44 (a) Flowchart of preparation of NF/MN Cu mesh. (b–d) ...
Chapter 10
Figure 10.1 Distribution of the world's saltwater resources and f...
Figure 10.2 The natural water cycle diagram mainly involves the e...
Figure 10.3 (a) Three‐phase transformation (liquid, solid and gas...
Figure 10.4 (a) Evaporation process in an open system. (b) Evapor...
Figure 10.5 (a–c) Conceptual drawings of the design of the AWH sy...
Figure 10.6 Progress in adsorbent materials, including (a–e) phys...
Figure 10.7 Illustrate the Silica gel adsorption capacity test sy...
Figure 10.8 Various MOFs adsorbents for atmospheric water harvest...
Figure 10.9 The structure of MOF‐801 consists of three symmetry‐i...
Figure 10.10 (a) Crystal structure of MOF‐303 constructed from i...
Figure 10.11 Schematic structure of MIL‐100(Fe)MOF.
Figure 10.12 (a) Top and side views of MIL‐160(Al) (1 × 1 × 2) s...
Figure 10.13 (a) Pufferfish change shape by absorbing and releas...
Figure 10.14 (a) Schematic diagram of the water evaporation test...
Figure 10.15 (a) Synthesis and morphological characteristics of ...
Figure 10.16 Schematic diagram of the in situ confined growth me...
Figure 10.17 Synthesis and morphological characterization of LiC...
Figure 10.18 Design of the nanocomposite MIL‐101/MCM‐41 and its ...
Figure 10.19 Characterizations of MIL‐101(Cr)/MCM‐41. (a–c) Scan...
Figure 10.20 (a) Synthesis scheme of MOF‐derived nanoporous carb...
Figure 10.21 Adsorption/desorption properties of PAM‐LiCl. (a) a...
Figure 10.22 (a) Schematic of the skeleton, porous structure, an...
Figure 10.23 (a) Absorption isotherm at 25 °C, (b) static absorp...
Figure 10.24 (a) Schematic illustration of the fabrication of SA...
Figure 10.25 (a) Schematic of the synthesis route. The GO (black...
Figure 10.26 (a) Schematics illustrating the structure and worki...
Figure 10.27 (a) Tillandsia absorbs water from the air for its v...
Chapter 11
Figure 11.1 (a) Conceptual representation of the bionic hydrogel ...
Figure 11.2 (a) Schematic diagram of fog harvesting by a desert b...
Figure 11.3 (a) The schematic diagram shows how a TiO
2
nanocompos...
Figure 11.4 (a) Schematic illustration of microneedle array archi...
Figure 11.5 (a) Schematic diagram of fog collection for AWF‐6. (b...
Figure 11.6 (a) Schematic of the atmospheric water irrigation bas...
Figure 11.7 (a) Schematic illustration of hygroscopic, phototherm...
Figure 11.8 (a) Schematic diagram of the device that combines a l...
Figure 11.9 (a) Schematic diagram of the hydrovoltaic nanogenerat...
Figure 11.10 (a) Schematic illustration of hydrated channels aro...
Figure 11.11 (a) Static hydroelectric power generation and tunab...
Figure 11.12 (a) Schematic diagram showing the construction of t...
Figure 11.13 Schematic diagram of humidity splitting mechanism....
Figure 11.14 Basic flow sheet of the intensified heat and mass i...
Chapter 13
Figure 13.1 Engineering biomimetic materials. for fog collecting....
Figure 13.2 (a) Distribution of water scarcity on a global scale,...
Figure 13.3 (a) Geographical distribution of the two extreme cond...
Figure 13.4 (a) Three different ways of collecting water.(b) ...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Bioinspired Materials for Fog Harvesting
Principles, Design, Preparation, and Applications
Zhiguang Guo and Weimin Liu
Authors
Prof. Zhiguang Guo Hubei University 430062 Wuhan China
Prof. Weimin Liu Lanzhou Institute of Chemical Physics Chinese Academy of Sciences 730000 Lanzhou China
Cover Design: WileyCover Images: © Zhiguang Guo, © Weimin Liu
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Print ISBN: 978‐3‐527‐35400‐9ePDF ISBN: 978‐3‐527‐84745‐7ePub ISBN: 978‐3‐527‐84744‐0oBook ISBN: 978‐3‐527‐84746‐4
© 2026 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
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Preface
This is my third time writing a preface for one of my upcoming books. Looking back, I began my journey in scientific research over twenty years ago as a PhD student, which makes us realize that time flies. Over the past two decades, a lot has changed both in my personal life and in the world. The only thing that remains constant is my love for and dedication to scientific research.
The field of fog harvesting has become a hot topic of research in the past decade, as freshwater resources are becoming increasingly scarce around the world. When a new field emerges and grows quickly, as judged by the number of publications, presentations, and patents. In the past decade, more than 8000 papers have been published. It is well known that the fog harvesting of a surface depends mainly on three factors: fog capture, fog droplet coalescence, and droplet transport. Discovering the relationship among them in enhancing the efficiency of fog harvesting is very important for us.
To this end, the idea for this book, titled Bioinspired Materials for Fog Harvesting (BMFH), dates back to 2014, when I published the first paper on fog harvesting, and my research group had just exceeded twenty members. More scientists paid their attention to this field, and the growing number of papers, preprints, presentations, and word‐of‐mouth discussions, the more important this field will be. Till now, I have published more than 100 papers in this field. It is time for me to summarize the progress and challenges in this field.
I am pleased to co‐author this book with Prof Weimin Liu, a full professor at the Lanzhou Institute of Chemical Physics and an Academician of the Chinese Academy of Sciences. He also was my PhD supervisor during 2004–2007. BEMFH presents the recent progress, ideas, and research from my group and many of the world's foremost scientists working in the field of fog‐harvesting materials. We discuss the theory, fundamentals, fabrications, properties, applications, and uses of these very unique materials inspired by nature.
BMFH covers 13 aspects of the fog‐harvesting story, as follows:
Chapter 1
analyzes the current state of global water resources and discusses the necessity of fog harvesting and the development of innovative fog harvesting materials. It also emphasizes the bionic design principles inspired by natural organisms and outlines the methods for evaluating the performance of these engineering materials.
Chapter 2
identifies natural prototypes for fog collection, detailing the collection strategies employed by these biomimetic models, such as cacti, iron orchids, pitcher plants, desert beetles, spider silk, and desert lizards.
Chapter 3
introduces the fundamental principles of fog harvesting, including the nucleation mechanism of water molecules, droplet growth theory, and droplet transport mechanisms. It also presents a theoretical model for atmospheric water collection based on adsorption.
Chapter 4
provides a comprehensive summary of the factors influencing the fog‐harvesting process, discussing the relationship between each factor's impact and its efficiency.
Chapter 5
reviews a range of microscopic materials utilized in fog harvesting, in which how variations in microstructure lead to differing wettability were emphasized.
Chapter 6
categorizes the fog‐harvesting materials introduced in the previous chapters based on their microstructural characteristics.
Chapter 7
explores how the mechanical properties of fog‐harvesting materials influence their efficiency.
Chapter 8
outlines various surface preparation methods and highlights how they yield different micro‐ and nano‐scale surface characteristics.
Chapters 9
and
10
, present two types of fog‐harvesting systems (non‐phase change and phase change).
Chapter 11
assesses the feasibility and potential applications of fog collection in current and future texts.
Chapters 12
and
13
summarize the challenges in the field of fog harvesting and propose future research directions.
The book is aimed at the following readers:
Advanced students and instructors in the fields of science and engineering.
Professional scientists and engineers, who may be trained in more traditional disciplines but who need to learn about this emerging area.
Policymakers and management experts looking for an understanding of scientific challenges, prospective uses, and emerging markets for fog harvesting.
The overall goal is to capture the multidisciplinary and multifunctional flavor of biomimetic engineering materials for fog harvesting, while providing in‐depth discussions of select areas.
Importantly, I would like to express my sincere appreciations to the contributors for their effort and dedication in preparing the chapters, to Hubei University, and to my colleagues and my PhD students: Dr Shangzhen Xie, Dr Fuchao Yang, Dr Xia Li, Dr Xian Zhang, Dr Ke Pei, Dr Shulun Ai, Mr Yuxuan He, Mr Guangyi Tian, Mr Pengyu Zhang, Miss Huayang Zhang, Miss Danyan Zhan, Miss Huimin Zhang, Miss Yidan Zhang, and Miss Anhui Yu for their continuous assistance and hard work in drawing the figures, in compliling the chapters, and for their corrected proof for this book. I also thank Dr Lifen Yang of Wiley for giving me the idea of starting a book at the beginning of this year, and her publication team for their continuous help and support. Finally, I am indebted to my wife, Yan Xin, and my two daughters, Zhehan and Zheyuan, for their kind and endless enthusiasm and support since we have experienced the difficult moment of the COVID‐19 outbreak in Wuhan from 2020 to 2022, when I completed the first draft of this book with their help.
This work was supported by the National Natural Science Foundation of China (Grant No. 51735013).
December 2024
Zhiguang Guo Wuhan
1Introduction
Water is the source of terrestrial life and the necessary condition for reproduction and the continuation of life. Despite oceans cover 65% of the Earth's surface and contain 97.2% of global water, large portions are saline and undrinkable. Human use is limited to fresh water from freshwater lakes, rivers, and groundwater systems, accounting for only 2.53% of the world's water. Of this, about 70% is locked in polar and alpine glaciers, which are difficult to extract. Currently, our primary access to water includes rivers, freshwater lakes, and shallow groundwater – about 0.3% of global fresh water and 0.0007% of total world water. Although the ocean accounts for a significant proportion of global water resources (mostly saline), most available water comes from ice caps, glaciers, groundwater, inland seas, and rivers: 1.8%, 0.9%, 0.02%, and 0.001%, respectively. Regionally, nine countries, including Brazil, Russia, Canada, China, the United States, Indonesia, India, Colombia, and Congo, hold more than 60% of the world's fresh water, but at the same time, nearly a fourth (about 80 countries) of the global population faces severe shortages. Globally, about 1.5 billion people, or one sixth of the planet, suffer from inadequate supplies of water, with an additional third of countries amount to a billion people experiencing this crisis.
Harvesting water from ambient air holds potential for freshwater provision. Given all indicators, 3.8 billion people experienced varying levels of water scarcity in 2005 [1]. This is expected to persist until population growth peaks in 2050 [2]. Surface rivers constitute approximately 40% of precipitation across global land [3]. Nevertheless, WHO's report forecasts a 20–30% surge in demand for water by 2050 [4]. Urbanization and climate change compound this issue, with half of the urban populace projected to encounter water shortages by 2050 (Figure 1.1) [5].
Researchers analyze the scarcity of water resources across two dimensions, supply vs. consumption, while also appraising the quality of the supplied water in correlation to factors such as demographics, agricultural productivity, and topographical layout. Of the 304 cities examined, an astonishing 273 (about 89.8%) were confronted with water security risks in 2015. Among these, 262 municipalities (or roughly 86.1%) faced quality risk, another 85 entities (about 27.9%) ventured into scarce risk, and an additional 74 municipal governments (about 24.3%) encountered dual risks (Figure 1.1). Moreover, 69 cities (around 22.7%) were marked as severely exposed to extreme water quality risk, and another 48 (or almost 15.8%) being severely encumbered by severe shortage of water scarcity risk, while 7 (about 2.3%) were grappling with both extremely high‐water quality risk and severe water scarcity risk.
Figure 1.1 Global scarcity of water. Distribution of large cities in hydric stress regions (capitals exceeding 10 million inhabitants in 2016).
Source: [5] / Springer Nature / CC BY 4.0.
Strangely enough, among the evaluated eighteen megacities (i.e. those with population exceeding 10 million), an overwhelming 25 (around 92.6%) were affected by water security risks. Among these, 23 cities (about 85.2%) grappled with quality risks, an additional 10 (about 37.0%) were involved in scarce risk, and 8 (or around 29.6%) had to contend with dual risks. Notably, eleven of these major cities including São Paulo, Mumbai, and Dhaka were severely under threat of highly risky water quality, whereas five major cities (Delhi, Beijing, Los Angeles, Moscow, and Bangalore) were severely burdened by severe water scarcity risk, with Bangalore bearing the brunt of dual risks on both ends [6].
In terms of technology, desalination is recognized as an effective method to alleviate freshwater scarcity crisis. Broadly, desalination can be categorized into thermally driven (distillation) and membrane‐based processes. Thermal seawater desalination remains the predominant method in the Middle East region [7]. However, membrane desalination methods exemplified by reverse osmosis (RO) have rapidly grown since the 1960s, and more seawater desalination plants opt for RO technology [8]. Through analysis and comparison, it can be found that one of the paramount challenges associated with RO technology is high energy consumption, which may escalate greenhouse gas emissions [9]. Despite extensive work researchers undertaking in integrating seawater desalination with renewable energy to address energy consumption issues, seawater desalination to obtain freshwater is impractical in inland regions distant from the ocean. Furthermore, even at small scales, seawater desalination technology not only necessitates accessible brackish water sources but also competent operators and maintenance personnel, which significantly restricts the application scope of seawater desalination. However, the geographical environment and climatic conditions of remote or inland areas, as well as issues concerning cost, energy consumption, secondary pollution, etc., impose limitations on the development of the technology [10]. Consequently, there is an urgent need to develop a promising technology to tackle situations in remote areas or areas where economic levels do not advocate centralized treatment and distribution networks [11].
As an immense renewable reservoir, the atmosphere contains approximately 12 900 km3 of water [12], sufficient to meet part of the water needs of households, agriculture, and industry. Even in remote regions and arid deserts, abundant water vapor is present; the water in the atmosphere is deemed an enormous renewable reservoir. Capturing water vapor from air not only saves significant labor and energy costs but is universally viable across vast regions of Earth. The water vapor in the air is relatively free of impurities and bacteria, and the water harvested from air can reach potable water standards without elaborate purification and sterilization processes. Unlike seawater desalination, when water is extracted from air, it hardly disrupts the water cycle and does not have any detrimental impact on nearby crucial water resources after transportation. Even in the driest desert regions, water vapor is widely dispersed in the form of molecules in the atmosphere. Atmospheric water harvesting (AWH) produces fresh water through the collection of moisture in the air, enabling sustainable water transport without substantial infrastructure and geography or hydrology restrictions [13]. A viable water heating technology should meet the following primary criteria: robust water harvesting capabilities, low energy consumption, affordability, stability, and negligible constraints imposed by the environment and climate [14]. Therefore, the development of efficient AWH technology paves the way to harness this portion of the atmospheric water resource and holds promise to address the freshwater shortage issue, particularly in geographically distant or rural regions where water transportation is costly [15].
1.1 Overview of Research on Atmospheric Water Collection
Traditional fog collectors are a simple yet sustainable system, implemented when a gauze material is exposed to cloud banks. Some droplets collide and accumulate on the gauze material, aggregating to form larger fog droplets, which are subsequently attracted by gravity and channeled into drainage pipes, ultimately reaching water tanks or distribution systems. They can be categorized as standard fog collectors (SFCs) and large fog collectors (LFCs) . SFCs typically find use in exploratory research for assessing the potential quantity of fog accumulated under specific conditions. LFCs primarily perform actual fog collection operations. If the performance of an SFC surpasses a certain threshold, it transitions seamlessly into an LFC. Since the mid‐twentieth century, conventional fog collecting initiatives have enjoyed considerable success in Chile, subsequently being deployed in several parts across the world. These locales feature climates and geographies conducive to natural fog gathering predominantly arid tropical and subtropical regions with high altitude. In most nations, the prevalent fog collector preferred involves placing a Raschel grid vertically between two poles to harness moisture from fog [16]. The grid features a trihedral pattern, comprising fine filaments and pores at the millimeter level. Woven polyolefin Raschel nets are a favored capturing medium, treated with UV protection and possessing a shading coefficient of 35%. Eiffel, an innovative 3D fog collector, comprises two layers of Raschel meshes and ten meshes. It optimizes the collection of windblown fog parallel to a heat exchanger, achieving a collection efficiency of 281.2 L day−1 – approximately 10 times that of standard full‐sized heat exchangers. However, this ideal scenario for collecting fog is subject to strict constraints. Consequently, the emergence of “Harp” and “Diagonal Harp” fog processors has made significant strides beyond these limitations. Comprising a series of closely spaced vertical stainless steel wires, close to a fog net lacking horizontal lines, they capture fog originating from all directions, enhancing the precipitation rate of liquid droplets, averting blockages, thereby enhancing the performance of the collector [17].
However, some of traditional fog collectors merely harvest a portion of fog droplets, exhibiting a relatively modest collection efficiency. A portion of droplets bypass the mesh without colliding with its fibers, while some may bounce off the resilient fibers back into airflow. Furthermore, much of the current efforts geared toward refining such systems focus primarily on investigating the mesh topology of conventional fog collectors. In comparison, there remains insufficient investment in functionally modifying the mesh surfaces and synthesizing enduring functional surfaces given the constraints imposed by wind velocity, atmospheric humidity, droplet size, and other geographical environmental factors [18].
In the comprehensive process of studying the design concept of a biomimetic fog water collector, we have innovated and advanced the fog water collector by thoroughly absorbing the abundant inspiration from biological structures. Using the natural biological water collection mechanism as a guide, we meticulously explored this concept and have consequently designed a series of efficient fog water collectors and systems. Currently, insects such as desert coleopteran, spider silk, cacti, pitcher plants, and toothedge mosses that possess potentiality for fog water gathering have been widely recognized. An integrated water collection process is principally composed of three segments: fog water capture, fog water distribution, and water resource transportation. The most critical aspect is the capture of fog, and the structural attributes and moist degree of the capture surface significantly influence its efficiency. While hydrophilic surfaces have the advantage of swiftness in capturing moisture in fog, their formidable water drop binding force can easily lead to decreased heat transfer efficiency and reduced fog water capture positions when larger droplets or films rapidly form on the surface. Conversely, hydrophobic surfaces are incapable of immediately collecting surrounding fog drops; when minute droplets vertically collide with the surface of hydrophobic materials, a small amount of liquid is repelled, posing an impediment to an efficient fog water capture process.
Simultaneously, the removal of collected droplets facilitates the initial coalescence and further deposition of new droplets in the capture position, thereby forming an efficient and continuous collection cycle. The smooth operation of this cycle relies on the surface possessing superior moisture transportation capabilities. Therefore, how to establish an optimal capture interface for achieving the highest efficient fog water collection efficiency has emerged as a significant challenge. By seeking inspiration from nature and conducting thorough research into organisms with exceptional fog collection abilities, we observed that these organisms developed a sophisticated water flow system for the collection, accumulation, and removal of minute droplets after a long evolutionary journey. For example, natural organisms such as spider silk and cacti also have their unique and intricate fog water capturing, transporting, and collection systems. The periodic spindle and joint structure formed by spider silk in humid air not only can effectively capture a large number of droplets, but its elaborate curvature gradient and surface energy gradient also lay the groundwork for droplet directional transmission. Through the analysis of the structure of cacti, for the purpose of efficiently gathering fog, its needle‐like substructure progressively diverges, forming multilayered and functionalized forms. This specialized form enables the centralized direction of captured, merged liquids and ultimately finalizes the work of end collection in the root hair body of the spines. In stark contrast, desert beetles living in the desert, also utilizing a hydrophilic back ridge to construct a hydrophobic periphery, accomplish the task of capturing fog and separating it effectively.
In addition to the previously discussed sophisticated fog collection mechanism consisting of the typical organisms such as desert beetles, spider silk, and cacti, Nepenthes has consistently demonstrated exceptional capabilities in continual water supply and precise positioning delivery. In a humidity conducive environment, with its excellent absorption of moisture‐laden nectar and ingenious microstructure, Nepenthes is capable of creating a smooth and stable thin film of water molecules on the leaf surface, thereby significantly reducing fluid resistance within this region. This feature is cleverly harnessed to successfully capture and utilize insects as a food source. Through rigorous research analysis of Nepenthes' unique structure, human beings have progressively developed numerous irrigation surfaces inspired by water‐skid animals, which, due to their advantages of lower surface energy and higher nucleation density, can dramatically stimulate the swift coalescence of fog drops if integrated into the implementation process of fog capture. Alternatively, a layer of lubricating oil is applied to further enhance the fog liquefaction effect, transforming the entire surface into a durable flat surface that facilitates more easy agglomeration of water droplets. The resultant water droplets exhibit minimal contact angle retention phenomenon during descent and strong fluidity, which not only expedites water drop removal but also creates ample space for new droplet nucleation. With reference to the behavior pattern of drops in nature, we note that drops often follow the path with minimum surface energy when inscribed on an Araucaria surface, which has served as an inspiration for our fabricated simulation of Araucaria tree leaf surfaces achieving directed transfer of liquid drops in the direction opposite to gravity. There is no doubt that the forthcoming research process will provide us with additional insights from the rich ecological systems of nature, whether it is regarding fog collection, growth provisioning, or fluid transmission mechanisms and methods, all of which serve as invaluable resources to enhance fog collection efficiency and alleviate the universal issue of insufficient freshwater resources.
To tackle the critical problem of constructing an appropriate fog collection system structure and adjusting optimal chemical composition for designing efficient fog collection facilities, it is imperative that we fully explore and draw lessons from biological water accumulation strategies in nature and conduct innovative scientific research based on these. Owing to the tireless efforts of numerous scientists, significant and breakthrough achievements have been made in contemporary novel fog collection designs inspired by biology. Therefore, in subsequent sections, we will delve into and summarize the innovative ideas of bionic fog collection. With the rapid advancement of nanotechnology, various high‐performance bionic fog collection systems are emerging like mushrooms, and the detailed content will be thoroughly explained in subsequent chapters. We are confident that they will provide valuable enlightenment and technical reference for further production of bionic fog collectors. In the evolution of fog collection equipment, wettability theory holds an indispensable role, providing us with the essential knowledge for a more accurate understanding of the fog collection process and the design of more efficient collection devices. To provide a more comprehensive portrayal of the overall picture and development trend of the fog collection field, we have decided to provide relevant theoretical support in a chapter on the evolution process of incorporating the concept of bionic fog collection.
Upon collection of fog, this process can be effectively undertaken when the unique weather conditions that surround moisture‐laden, fogy areas (i.e. relative humidity [RH] approaching saturation) are present. However, this ideal environment is not present universally across Earth's regions, particularly in mountainous terrains that are steep, arid zones, and coastal locales. While these locations may typically yield air flow with high humidity, due to a lack of abundant water resources, implementing fog collection becomes an exceedingly challenging task. From an outlook of biomimetics, the diverse biological structures within nature provide substantial insights, offering us fertile resources for constructing corresponding equipment or systems. Nevertheless, current biomimetic fog collection technology remains in the experimental phase in laboratory high humidity environments (RH exceeding 60%). To genuinely apply this technology to daily life, significant hurdles remain. Consider wind direction shifts as an example; the fog collector may fail to exhibit its anticipated effects at such times. To address this issue, we require the unique ability of the fog collector to cope with any potential complex environmental changes. Crucially, the entire fog collection process must be conducted outdoors, where it may be exposed to high wind speeds, which can inflict irreversible damage on the equipment. Therefore, enhancing the durability and stability of fog collection equipment is crucial. Furthermore, a vast majority of biomimetic fog collection devices employ a sophisticated framework with precision reaching nanoscale levels. Any damage to these sophisticated structures could result in a significant reduction, if not complete loss, of fog collection efficiency. Also, most collection apparatus rapidly expel captured water into the atmosphere, which exacerbates the rapid evaporation and loss of water. To address this concern, designing and refining a closed loop system for fog collection become imperative tasks, including circumventing prolonged contact of droplets between the collector and atmosphere for optimal sealing and storage effects, etc., these are crucial problems that demand our immediate attention. Finally, the safety of harvested drinking water after collection demands undivided attention. Prolonged wetting of the sample surface can harbor extensive bacterial growth; similarly, functional coatings adhering to the droplet surface may delaminate during wear, posing a potential health risk in terms of water quality.
In the operational execution of fog collection, we are practically delving into and exercising a fundamental physical phenomenon of physics that clearly exhibits its physics principle essence – phase transition process, which lucidly illustrates this foundational principle. Specifically, in the dew drop formation system, we meticulously manage the ambient temperature, progressively imposing air humidity on saturated conditions, thereby elevating the water vapor pressure to the saturated steam pressure of water. This ensuing situation propels cohesion between water molecules, leading to the final crystallization of dew. Concerning the execution details of low‐temperature operations, we can either resort to passive cooling methodologies (such as the effective conversion of solar energy into the primary energy reservoir, or radiant cooling) or more aggressively employ active technological tactics (like heat pumps and chilled water units). However, it is crucial to highlight the previous studies predominantly focused on passive radiative condensers not dependent on additional energy support, whereas for those low‐humidity areas requiring greater quantities of dew collection, externally sourced active energy‐assisted cooling technologies are inherently needed [18]. Moreover, it is essential to acknowledge that under certain unique thermal and moisture conditions, although active cooling facilities are employed, still no feasible moisture retrieval can be attained due to excessively low dew point temperature. Similarly, considering all the aspects discussed, a meticulous analysis and contrast of conventional desalination technologies (such as RO) reveal that active refrigeration‐driven dew collection indeed necessitates extensive consumption of energy, with an average energy expenditure generated in this procedure estimated at about 3.5–4.5 kWh m−2. Nevertheless, given the procurement costs, maintenance costs, and scarce availability of high‐quality saline resources, the practical feasibility of distributed water sources can be assessed and anticipated. Even though dew collection is constrained by specific climatic and geographical conditions, it distinctly outperforms fog collection. Thus, a comprehensive evaluation of various factors may lead us toward proactively engaging people in the action of dew collection as a novel yet practical solution to alleviate freshwater scarcity issues.
Concerning the severe climatic conditions and geographic challenges faced when harvesting fog and condensate, a highly promising method of atmospheric moisture collection is attracting increasing attention from researchers. This approach utilizes an adsorbent to process the intricate steps of water molecules in the air, thus effectively capturing moisture. Liquid droplets accumulate progressively within the absorbing material, ultimately forming liquid droplets, facilitating further collection. This is essentially the common approach to decrease the surface energy structure and absorb the aggregation of water molecules, and commonly utilized adsorbents include traditional adsorption materials, metal–organic framework (MOF) materials, polymer gel‐based materials, and porous carbon, four categories of adsorbents. In the forthcoming fifth chapter, we will thoroughly analyze the mechanism of water vapor collection in the atmospheric environment, covering key aspects such as fundamental theory, material design, collection and release of phase change condensate, and future development directions. Despite the high value and rarity of freshwater resources available globally, current operational atmospheric condensate systems still remain scarce. Therefore, the commercial operation of atmospheric condensate technology must adhere to the following five core criteria: efficiency, resource conservation, adjustable size, exceptional adaptability, and long‐term stability to ensure continuous smooth operation or at least maintain the normal function for an entire monsoon cycle annually. While existing commercial atmospheric water generators have been introduced, there are currently no products capable of comprehensively meeting these ideals.
Presently, various types of specialized atmospheric condensate collectors are being researched for various atmospheric condensate technologies, with the expectation of gradually mass production and market them. Such special collection equipment not only coordinates closely with the various reaction bodies that need water supply, but once an atmospheric environmental condensate system based on the adsorption principle is implemented, it can purposefully acquire water resources from the atmosphere, holding the promise to become the guarantor for providing stable, reliable, and superior quality water sources for miniature reaction systems such as power generation and chemical reactions. Therefore, in the sixth chapter of this book, we will delve into some actual cases that are currently underway for development and application, demonstrating the tremendous potential and considerable role of water collection in miniature reaction systems.
1.2 Development of Biomimetic Fog Collection
The earlier roots of superwetting bionic surfaces can be traced to studies on the lotus leaf. In recent years, research has advanced tremendously from considering complex structures like desert beetles' exoskeletons, pitcher plants' pitchers, cactus spines, and spider silk. Fundamental wettability theory is critically instrumental in guiding the evolution of bioinspired fog collectors during their development process. Thus, our discourse will commence with wettability theory by commencing with a theoretical dissection of static wetting models and dynamic motion theory models followed by an extensive exploration of the evolution of bioinspired fog collectors.
The static wetting model originates from the smooth interface of an ideal surface. Given the asymmetric attractions surrounding molecules or atoms on a perfect surface, particles at the outermost interface (such as atoms and molecules) have increased energy levels. When a droplet attaches to the surface, it tends to exist in its lowest energetic state, which forms the contact angle, i.e. the angle between the external boundary of the droplet and the solid surface. As it can be seen in Figure 1.2, the contact angle serves as the primary indicator for assessing the wettability of a rigid surface, and it can be vividly depicted through Young's equation that defines static contact angle as follows:
In particular, the components represent the surface tension for the solid–gas, solid–liquid, and liquid–gas interfaces, respectively. θ denotes the apparent contact angle where a hydrophilic interface is identified if θ is below 90°, an intermediate hydrophobic one (90° < θ < 150°), and finally a superhydrophobic state when θ surpasses 150°. It should be highlighted that Young's equation primarily serves to depict ideal smooth surfaces, rendering it inapplicable to rough interfaces. Consequently, in actual scenarios, describing static contact angles necessitates the account of nonideal rough surfaces, further evolving the science behind surface studies.
Figure 1.2 (a) Surface wettability. (b) Surface tension. (c) Wenzel model. (d) Cassie model. (e) Contact angle hysteresis. (f) Surface energy gradient. (g) Laplace pressure.
Source: [19] / The Royal Society of Chemistry.
(h) Superposition scheme; (h1) represents the real case; (h2) represents a mesh where all the flow is forced to pass through it, and (h3) represents a non‐permeable screen or plate.
Source: [20] / with permission of Elsevier B.V.
In recognition of the presence of nonideal rough surfaces, researchers have developed two excellent theoretical models to elucidate how surface texture influences the contact angle of liquid droplets: Wenzel's model and Cassie–Baxter's model. In 1936, Wenzel introduced roughness into static wetting theory for the first time, fostering a revolutionary theoretical framework, as depicted in Figure 1.2c [21]. Wenzel's model may be expressed as follows: When the droplet is in full contact with the interface without interruption, it will be influenced by the surface's irregularities; subsequently, this influence will cause the droplet to completely cover the area where these features are present [22]:
Wenzel introduced the concept of roughness (r, r > 1) into Young's equation. This ratio signifying the disparity between the actual contact area of an aqueous droplet with a solid surface and its relative surface area has been unequivocally defined as roughness. Hence, when the contact angle θ escalates beyond 90°, the roughness amplifies; conversely, if θ diminishes below 90°, the roughness diminishes. However, the limitation of Wenzel's model lies in its inability to depict interfaces composed of liquid–solid–gas under conditions that deviate from uniform wetting. When drops fail to permeate into concavities on a rough surface, they form a state of “locked” gas on the solid surface, thereby leading to a three‐phase wetting condition comprising liquid–solid–gas, which can be delineated through Cassie–Baxter models,
As shown in Figure 1.2d, these models can be classified into two distinct scenarios: (i) liquid–solid phase interface and (ii) liquid–gas phase interface. Consequently, Eq. (1.3) could be further simplified,
Actually, both the Wenzel model and Cassie–Baxter model effectively describe two distinct wetting scenarios. Consequently, Marmur et al. have proposed a more realistic hybrid model where certain droplets infiltrate into the recesses of the rough structure, while some gas molecules are “imprisoned” therein [23],
Among them, rf represents the proportion of the contact surface area between the liquid and solid to the total surface area.
Overall, droplets under the Wenzel model show a highly adhesive nature, while those under the Cassie–Baxter model show a tendency to roll off. Under external disturbances such as vibration and shock waves, the Cassie–Baxter state can effortlessly transition to the Wenzel state.
The theoretical models of dynamic movement are encapsulated in gradient wetting displacement, rolling, sliding, superlubricity, and hydrophilic and hydrophobic structure dynamics, as well as hydrophilic conduit motion. Droplets suspended on a surface with graded wettability exhibit an inclination toward migration from hydrophobic to hydrophilic zones, as illustrated in Figure 1.2e. The use of gradient wetting for directing droplet transport is a primary method used to achieve directional control of droplets on horizontal surfaces. For inclined surfaces, due to the influence of gravity, different transport modes may be selected by the fluid. For example, on a wet surface dominated by the Wenzel model, the predominant mode of motion is the sliding mode; however, on a wet surface governed by the Cassie–Baxter model, due to the larger static contact angle and weaker droplet adhesion force, droplets migrate through the rolling mode. On injection surfaces, the difference in tension between the droplet and injected fluid results in unique structures such as wetting ridges, which facilitate an ultra‐sliding motion pattern characterized by low resistance and ease of slippage (Figure 1.3).
Figure 1.3 Various superwetting surfaces found in nature.
Source: Reproduced with permission from [24] / Elsevier.
The inception of trees and shrubs accumulating substantial volumes of fog spans back several centuries, or perhaps as far back as human habitation among fogy mountain summits [25]. Nonetheless, in arid landscapes bereft of greenery, fog dissipates into dry interior regions, uncollected. Early literature citations and localized experimental trials involving fog accumulation exist; however, it wasn't until 1987 that a large‐scale initiative was embarked upon, intended to provide sustainable water supply to a coastal village in Chile, home to approximately 330 residents. At Chile's elevated altitudes (780 m), the average droplet diameter falls within the vicinity of 8–12 μm, and the droplet density typically fluctuates between 100 and 400 cm−3. Fog droplets are captured by a straightforward impaction method. A vast polypropylene net is strategically positioned along the trajectory of the droplets. As the droplets neared the surface, some circumvented the object, while others impacted the surface. The initial triumph of the Chilean fog collection endeavor, the subsequent field collections during the summer monsoon seasons in 1989 and 1990 in the Hofar region of Oman, and a pragmatic scheme implemented in Peru in 1990, collectively indicate that fog collection could potentially be universally applicable across regions or nations with distinct climatic and geographic attributes [26]. Following the recognition of the water‐acquisition capabilities of flora and fauna in their natural habitats, various biomimetic fog collectors were devised. We shall now explore the progression of biomimetic fog collection, commencing with organisms such as the Namib Desert beetle and spider silk exhibiting water‐acquisition capabilities.
In 2001, Parker and Lawrence unveiled the survival mechanism of the Namib Desert beetle [27]. Drops from the foggy breeze accumulate on hydrophilic protuberances until they attain critical dimensions, subsequently rolling down the hydrophobic valleys toward the beetle's maw [28]. Since this revelation, surfaces possessing both hydrophilic and hydrophobic characteristics have attracted considerable interest, culminating in the creation of diverse water‐collection materials derived from substrates such as fabrics [29], cellulose films [30], composite nets [31], copper nets/sheets [31], and stainless steel nets [32]. To date, engineered hybrid surfaces of the Namib Desert beetle have been regulated in terms of both surface structure design and surface chefogry, yielding a plethora of invaluable research findings. It has been confirmed that surfaces featuring periodic peak concavities exhibit superior water collection efficacy due to the mitigation of the capture and aggregation processes of water, thereby diminishing the reevaporation of microdroplets. By scrutinizing the water‐collection proficiency of nanostructured surfaces with varying patterns and morphologies, encompassing pure superhydrophilic surfaces, superhydrophobic surfaces, circular patterns, and star patterns, it was discovered that pure superhydrophilic and superhydrophobic surfaces displayed relatively modest water‐collection efficiencies, while the star pattern exhibited the greatest collection potential [33]. Additional evidence indicates that the pentagram star pattern surpasses other point numbers of star patterns in terms of water‐collection efficiency. The collection efficiency also appears to be profoundly influenced by the actual dimension of the pattern. When the star shape and total hydrophilic area remain consistent, reducing the pattern size amplifies the effect considerably. Nevertheless, attaining a high fog collection efficiency may necessitate sophisticated processing of the surface pattern, which invariably results in an augmentation in production time and scale constraints.
The spiderwebs are ubiquitous in our daily encounters, functioning as tools for arachnids to secure their prey. On fogy mornings, immediately preceding sunrise, one may witness spiderwebs abundant with water droplets. These complex constructs efficiently absorb humidity from ambient air. The fibrous strands consist of periodically spaced spindles constructed of disordered nanoparticles and junctions emitting nanofibers organized in a semi‐ordered fashion [34]. Driven by the difference in roughness between spindles and junctions, not just a spatial structure gradient but also a surface energy gradient exists between them. This permits water droplets to spontaneously transition from the junction to the spindle due to these dual influences. Motivated by the water retention strategy of spider silk, scientists have engineered artificial spider silk. Currently, four methodologies can be utilized to fabricate biomimetic fog collectors: electrokinetics, impregnation, microfluidics, and flow coating [34].
Species belonging to the Cactaceae family indigenous to arid deserts display remarkable drought tolerance due to the cooperative interaction of conical structures on cactus thorns and wettability gradients. The synergy of surface energy gradients and Laplace forces underpins the capacity of cacti to exploit fog collection abilities [35]. Taking cues from cacti, several artificial conical structures have been devised via methods like magnetic particle‐assisted molding [36], electrospinning, 3D printing [37], etc. To maximize the efficacy of fog collection, the apex angle and surface wettability of the conical structure have been scrutinized and refined by diverse research groups. Studies indicate that an optimal apex angle of approximately 10° facilitates the directional movement of trapped droplets on the hydrophobic layer‐coated cone surface, thereby augmenting the water collection efficiency. Given that conical spikes exhibit proficient fog capturing capability and directed transport to the base of the cone‐shaped structure, the accumulation and preservation of these droplets become a pivotal concern. Consequently, a comprehensive fog capture system encompassing fog capture, coalescence, directional transportation, and collection necessitates development. Presented proposals incorporate Janus and porous membranes (comprising a hydrophobic surface and a hydrophilic surface), facilitating unidirectional droplet penetration prior to gravity causing droplets to descend into the collection receptacle. Alternatively, superhydrophilic sponges can be assembled at the base of the cone‐shaped spine for continual water storage [38].
Unidirectional fluid transportation devoid of energy expenditure is an indispensable attribute of water capture, applicable for spontaneous water accumulation at precise locations. In sustained water capture procedures, prompt transit accelerates the process of water acquisition and reduces moisture volatilization. Chen et al.'s research established that the pitcher plant (Nepenthes alata) exhibits the ability to unilaterally transport superficial moisture through multiple groove micronetworks on its superior leaf margin. Moreover, Chen's group also disclosed the extremely rapid water capture prowess and transport procedure of Sarracenia trichomes [39]
