Biomimetic functional surfaces with tailored wettability for water harvesting and anti icing applications

xi Figure 1.2 Cactus spines Donald Erickson, Spider web Alberto Ghizzi Panizza and Stenocara beetle Wikimedia commons...…5 Figure 1.3 Dew harvesting system in Morocco Fadel Senna/AFP ph

February 2019

Biomimetic functional surfaces with

tailored wettability for water harvesting

and anti-icing applications

Nguyen Thanh Binh

A Dissertation Submitted in Partial Fulfillment of Requirements

For the Degree of Doctor of Philosophy / Master

We hereby approve the Ph.D

thesis of Nguyen Thanh Binh.

Thesis Committee Member / Supervisor

UNIVERSITY OF SCIENCE AND TECHNOLOGY

i

ACKNOWLEDGEMENT

This study is the result of my PhD thesis carried out at Nature-Inspired Nanoconvergence Systems Department – Nano - Convergence Mechanical Systems Research Division – Korea Institute of Machinery and Materials, Korea with tremendous amount of support

First, I would like to express my sincere gratitude to my advisor, Professor Hyuneui Lim, for giving me the opportunity to become her student at Nano-mechatronics Department (UST), for giving me all the support, encouragement and advice over past six and half years, and for spending long hours editing this thesis Her insightful guidance will be great inspiration for my future work in my university afterwards

I would like to thank Dr Wandoo Kim for his valuable advice and encouragement during my Ph.D’s degree I would like to convey my great gratefulness to the members of my dissertation defense committee, Dr Changdae Park, Dr Junhee Lee, Dr Hyoungsoo Kim, Dr Youngdo Jung for giving me all valuable comments and suggestions Specifically, I would like to thank Dr Seungchul Park for his honest advice, valuable support and

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encouragement, who served as Chairman of my thesis committee

I am also thankful to Dr Duckgyu Lee for assistance regarding experiment process and theoretical support when I started my PhD’s degree A special thank should be given to all my laboratory members, Dr Sunjong Oh, Dr Cholong Jung,

Dr Seonggi Kim, Cheonji Lee, Gyuhyeon Han for their enthusiastic support Finally, I dedicate this thesis to my parents, my wife and my daughter for their sincere love, outstanding support, for always beside and encouragement during my PhD’s degree This would have been impossible without them

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ABSTRACT

bio-problems: water harvesting and anti-icing

Water condensation is a phenomenon which refers to the changing physical

state of a matter from gaseous into liquid phase The simplest process can be imagined is water condensation on objects near earth’s surfaces such as: fog, dew, frost, etc In this work, we will focus on optimizing suitable surface morphology for durable and high efficiency water harvesting performance Several geometries and surface energies will have been conducted on Aluminum (Al) plates in order

to maximize harvesting efficiency

On the other hand, icing phenomenon refers to a process when liquid

transferring its physical state to solid phase Ice accumulation on functional surfaces had illustrated many bizarre effects and disadvantages in aviation, industry and human activities Several passive approaches including water

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repellency, Slippery Liquid-Infused Porous Surfaces (SLIPS) and unique design structure in order to optimize anti-icing performance will be introduced throughout this study

Totally, we propose different physicochemical processes which arm to manipulate surface wettability towards solving specific problems including water condensation and anti-icing The understanding about mechanism and fabrication process is useful for designing water harvesting system and icephobic applications

_

*A thesis submitted to committee of the University of Science and Technology in a partial fulfillment

of the requirement for the degree of Doctor of Science conferred in February, 2019.

초록

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자연모사 기능성 표면을 이용한 맞춤형 수분수집 및

방빙 응용 연구

자연모사 및 자연모방은 이미 자연의 진화에 의하여 증명된 문제해결법을 이 용하여 지속 가능한 인류의 도전과제를 해결하는 접근방식입니다 본 연구는 표면의 젖음성을 제어할 수 있는 자연모사 기능성 표면의 제작 및 이에 대한 구체적인 적용인 수분수집 및 방빙의 응용을 제안합니다

수분 응축이란 공기중의 수분이 기체 상태에서 액체 상태로 변화하는 현상을 말합니다 이는 안개, 이슬, 서리 등과 같이 지구 표면에서 물이 응축되는 것

을 통해 손쉽게 확인이 가능합니다 본 연구에서는 고내구성 및 고효율의 수 분수집을 위한 표면 구조 최적화에 집중을 하였습니다 수분수집 효율 극대화

를 위한 다양한 형상 및 다양한 표면에너지를 가지는 알루미늄 기판을 제작하 였습니다

또한, 빙결현상은 물이 액체 상태에서 고체상태로 변화하는 과정을 말합니다 기능성 표면에 얼음이 쌓이는 현상의 경우 항공, 산업 및 사람들의 활동에 많

은 문제를 야기합니다 본 연구에서는 발수특성 유도, 미끄러운 유체가 주입된 다공성 표면 (SLIPS) 및 독특한 구조 등 다양한 수동적인 접근방식을 연구하 였습니다

따라서, 본 연구에서는 수분응축 및 방빙 등의 구체적인 현안을 해결하기 위

한 표면 젖음성 제어에 기반한 다양한 물리화학적 공정을 제안합니다 이러한 메커니즘 및 공정과정에 대한 기반지식은 수분수집 및 방빙 관련 응용이 가능 합니다

TABLE OF CONTENTS

ACKNOWLEDGMENTS……… i

vii

ABSTRACT……… …… iii

ABSTRACT (KOREAN)……… …… v

TABLE OF CONTENTS….……… ………vi

LIST OF FIGURES……… ……… x

LIST OF TABLE……… … xvii

1 INTRODUCTION …… ……… … ………… 1

1.1 Bio-Inspired Surfaces.……….… ….……….1

1.2 Water Harvesting……….….… ….……… … 4

1.3 Icing and Anti-icing ……… ………… 8

2 BASIC THEORY………… ……… ……… …… 12

2.1 Wettability ……… ……… ………… 12

2.1.1 Surface Tension……… ……… …….13

2.1.2 Superhydrophobic Surface………….… ………… 16

2.1.2.1 Wenzel State………….……… …… 17

2.1.2.2 Cassie-Baxter State……… …….…20

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2.1.3 Transition from Wenzel to Cassie-Baxter state… …21

2.2 Nucleation Phenomenon… … …… ……….23

2.2.1 Homogeneous Condensation…… ……… ……… 25

2.2.2 Heterogeneous Condensation…….……….27

2.3 Water Condensation……… ……… 30

2.4 Icing and Anti-icing……….……… … 33

2.4.1 Freezing Time……… ………… 37

2.4.2 Adhesion Strength……….………… 38

3 RESEARCH ON WATER HARVESTING……….……… 42

3.1 Current Research ……… ………….……… 42

3.2 Experimental methods……… … 52

3.3 Results and Discussion……… …… …….……… 55

3.4 Conclusion 70

4 RESEARCH ON ANTI-ICING……… ………… …………71

4.1 Current Research……….………71

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4.2 Effects of Morphology Parameters on Anti-icing Performance

of SH Surfaces………82

4.2.1 Experimental methods……….82

4.2.2 Results and Discussion………86

4.2.3 Conclusion……… …………96

4.3 Anti-icing on Slippery Liquid-Infused Porous Surface (SLIPs)……… 97

4.3.1 Experimental methods……….97

4.3.2 Results and Discussion……… ………103

4.3.3 Conclusion……….……… 114

4.4 Unique Structure for Multi-Functional Surface……… ….115

4.4.1 Experimental methods……… ……… …….116

4.4.2 Results and Discussion……… … …………119

4.4.3 Conclusion……… …… …………131

5 CONCLUSION.……… ……… …… …… ….133

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Figure 1.2 Cactus spines (Donald Erickson), Spider web (Alberto Ghizzi Panizza)

and Stenocara beetle (Wikimedia commons) …5

Figure 1.3 Dew harvesting system in Morocco (Fadel Senna/AFP photo) and Dehumidifier (LG.com)……… ……… 6 Figure 1.4 Water condensation behaviors on Bare and Hybrid Al………….…7

Figure 1.5 Ice accumulation on aircraft (aircraft.sewaro.us) and De-icing in process (aviationtroubleshooting.blogspot.com) ……… ……….…8

Figure 1.6 Superhydrophobic surface for anti-icing……… 10

Figure 1.7 Penguin feather (Steve Gschmeissner), pitcher plant (Britannica.com) and Inspired Slippery Liquid – Infused Porous surface ……….11

Figure 2.1 Molecule at surface misses its half attractive inter-across actions and always tends to move inward……….13

Figure 2.2 Surface tension determines the formation of liquid (a) In air and (b) in contact with solid (glass) wall……… ……… ….…… …14

Figure 2.3 Schematic of liquid drop showing the quantities of Young equation 16

Figure 2.4 Wenzel wetting regime…… …18

Figure 2.5 Geometry when liquid droplet moves an unit area dA SL …………19

Figure 2.6 Cassie-Baxter wetting regime……… ……….21

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Figure 2.7 Apparent contact angle exhibits as a function of Young’s angle via

predicted theoretical……….….…………22

Figure 2.8 Cluster free energy of formation against cluster radius (r)……….26

Figure 2.9 Energy barriers for homogeneous and heterogeneous nucleation 29

Figure 2.10 Condensation process in term of heterogeneous nucleation…… 30

Figure 2.11 Normalized nucleation energy barrier and nucleation rate for water heterogeneous nucleation (Varanasi, 2010) ……… 32

Figure 2.12 Icing phenomenon……….……… ……….34

Figure 2.13 The freezing time definition……… 38

Figure 2.14 The difference between adhesion and cohesion……… …39

Figure 2.15 Cohesion and adhesion force in case of water and ice ……….40

Figure 2.16 Adhesion strength (in term of shear stress) in our experiment…… 41

Figure 3.1 Dew and Fog collector in Morocco (Neil Hall – dailymail.co.uk) and Warka Warka tower for water harvesting in India (Warka Water Inc.).……….43

Figure 3.2 (a) Surface of water capture behavior discovered by Parker (2001) and (b) Further investigation conducted by Thomas Norgaard (2010) ………45

Figure 3.3 Water collection on spider silk (Lei Jang, 2010) and Opuntia

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microdasys catus spines (Jie Ju, 2012)……….46

Figure 3.4 (a) Hybrid pattern investigated by Garrod(2007) and (b) water formation in Dorrer model(2008)……… …… ………48

Figure 3.5 Hierarchical structure for enhancing water collection performance using (a) cylinder micropillars (Chen C-H, 2007) and (b) pyramidal structure (Chen Xuemei, 2011)……… ……… …49

Figure 3.6 Water harvesting via dewing (Anna Lee, 2012)………51

Figure 3.7 Fabrication process of Al surface……… ……….53

Figure 3.8 Our design inspired from Stenocara beetle’s back morphology (a) and

shadow masks with different spot sizes used in our work………54

Figure 3.9 Experimental setup (a) and environmental chamber (b)…….…….55

Figure 3.10 Condensation process on different wettability by the time….……56

Figure 3.11 Prior evolution direction of new form nucleus on different wetting states……… 58

Figure 3.12 Condensation rate by the time on surfaces with different wettability……….………60

Figure 3.13 Water condensation performances on different hybrid samples… 63

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Figure 3.14 Water droplet formation on hybrid samples (a) and (b) our experiment

to determine receding and advancing contact angles …… ……….65

Figure 3.15 (a) Water droplet formation on hybrid samples and (b) our experiment

to determine receding and advancing contact angles …… ……… 67

Figure 3.16 Condensation rate with different pattern size ………… ………69

Figure 4.1 Model of water impact with initial dynamic energy conducted by

Bahadur (2010) and Mischenko (2011)……… ……… 73

Figure 4.2 Spontaneous jumping behavior as a removal method Experiments conducted by (a) Boreyko (2009) and Chen C-H (2009)……… …74

Figure 4.3 (a) Ice adhesion measurement custom-built equipment proposed by

Kulinich and (b) ice adhesion summarization conducted by Adam (2010)… 76

Figure 4.4 Freezing time delaying conducted by (a) Cao (2009) and (b) Li(2014)… ……….77

Figure 4.5 (a) Superhydrophobic may not always reduce adhesion strength (Jing Chen, 2012) and (b) extend freezing time (Stefan Jung, 2011)……… 79

Figure 4.6 (a) Nepenthes pitcher and peristome morphology (Holger F.Bohn 2004) and (b) design of SLIPs (Wong, 2011) ……… ….…… …80

Figure 4.7 (a) Fabrication process of quartz nanopillars and (b) SEM images of

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different top size with corresponded contact angles……… 83

Figure 4.8 Experimental setup for adhesion strength and freezing time measurement……….84

Figure 4.9 Schematic of nanopillar morphology and areal fraction f… 86

Figure 4.10 Adhesion strength and areal fraction ….……… …90

Figure 4.11 Icing evolution on surfaces with different morphologies… 92

Figure 4.12 Adhesion strength and areal fraction… ……….…… …93

Figure 4.13 Height effects in anti-icing performance……… … 94

Figure 4.14 Schematic of parameters contributing on anti-icing performance in term of contact area and height………95

Figure 4.15 Schematic of fabrication process ……… … 98

Figure 4.16 (a) Surface roughness and 3D mapping images of samples before and after lubricant coating using a confocal microscope and (b) SEM images of sides and top view of etched Al……… 100

Figure 4.17 Interfacial surface tension measurement using tensiometer (a) and

phenomena description (b)……… 102

Figure 4.18 Experimental setup for measuring ice adhesion strength ……… 102

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Figure 4.19 Individual effect of coating layers (a) and initial wettability

contribution (b)……….104

Figure 4.20 Relationships between adhesion strength versus density (a), surface

tension (b) and viscosity (c).……… ……… … 108

Figure 4.21 Relation between adhesion strength and interfacial surface tension………111

Figure 4.22 Young’s model for spreading droplet on low and high spreading

coefficient ……… 112

Figure 4.23 Relation between adhesion strength and spreading coefficient 113

Figure 4.24 Our design for multi-functional surface…… …….……….… 115

Figure 4.25 Fabrication process (a) and surface morphology (b)…… …117

Figure 4.26 Wettability of fabricated samples ……… …… 118

Figure 4.27 Adhesion strength and freezing time on different treated samples 120

Figure 4.28 Contribution of height in anti-icing performance ……… … 123

Figure 4.29 Contribution of coating thickness in anti-icing performance…… 124

Figure 4.30 Heat transfer model for icing process….……….……… 125

Figure 4.31 Thermal resistance and heat transfer rate of different paraffin layer

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thickness (a) and different pillar’s height (b) Open symbols denote the thermal

resistance corresponding to the left……… 127

Figure 4.32 Ice accumulations on (a) Bare coated, (b) SH and (c) SHP samples

after freezing test……… 128

Figure 4.33 Transmittance spectra on prepared surfaces with (a) different paraffin layer thickness and (b) different pillar’s height ……… 130

Figure 4.34 Mechanical durability test of SHP surface………….……….131

LIST OF TABLES

Table 2.1 Surface tension of a few common liquids (20oC) and interfacial tension

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of water-lubricants………….……….……….……… ……16

Table 3.1 Critical droplet size and corresponding volume at 90o tilted angle ….60

Table 3.2 Critical droplet size and corresponding volume at hydrophilic-S.Phobic barrier………68

Table 4.1 Sample morphology information, areal fraction and contact angle….87

Table 4.2 Lubricant characteristics and interfacial surface tension ………… 99

Table 4.3 Contact and sliding angles of samples… 101

Table 4.4 Temperature and time until freezing……… 121

1 INTRODUCTION

1.1 Bio-inspired surfaces

Nature evolution endows the creature numerous unique and precious

functions that are prerequisite for survival in harsh wilderness Lotus leaf presents

the water-repellent property in order to elevate above water level for sunlight

absorption Namib Desert beetles climb to dune sand’s top and expose their body

toward humid air flow to collect water using their extraordinary back’s

morphology Pitchers plants in tropical rainforest are granted a weird body shape

like a trap which contains viscoelastic fluid inside to drown and occupy the

prey…Sharks can move easily in the ocean owing to its dynamic shape of

micro-scale structures which are arranged orderly on their fins

Biomimetic or Biomimicry refers to an approach that imitates nature’s proved models, elements and strategies to solve sustainability human challenges The process consists of four consecutive steps starting from observation of phenomena, understanding the mechanism, mimicking their function through technique and finally applying to real applications Base on the typical application, bio-inspired function surfaces are categorized into several groups: optical enhancement, mechanical enhancement and tailored wettability Moth eye structure consists of nano-pillar array introduces as a gradient of reflective index for reducing reflection and applied for optical equipment such as: camera lens, windshield, eye-glass Iridescence of beetle’s back and butterfly’s wing are resulted from photonic crystals or periodical structural with parameter scale is comparable to visible wavelengths The sheer abundance of nanostructure on

time-gecko feet can be imitated to make time-gecko tape (enhance adhesive strength); unique shape of shark skin is the great inspiration for designing aerodynamic systems

Figure 1.1 Summarization of bio-inspired functional surfaces and our approaches in this study

On the other hand, some creatures tailor their body wettability in order to adapt specific living environments Lotus leaf and Salvinia plant induce strong water repellency owing to hierarchical structure which consists of papillae, wax cluster and wax tubules Penguin feather, pitcher plants are typical examples of a

slippery-based function in order to prevent ice accumulation in cold climate condition or occupying prey Spider web, cactus’s spine, desert beetles can capture dew water from ambient air through their singular morphologies such as: wetting states combination (desert beetle, spider web), gradient wettability (cactus’s spine)…

Motivated from such interesting behaviors, researchers have tried to imitate their concepts and consecutively applied for specific purposes By controlling the wettability, numerous desired applications have been introduced in self-cleaning (water repellency property), anti-icing (slippery and water repellency), anti-bio fouling (water repellency), water condensation (hybrid wetting)

Among them, water harvesting and anti-icing performance have recently attracted much attention owing to the severe water shortage (especially in poor and arid areas) and bizarre icing problems In addition, recent researches have also illustrated the close relationship between water harvesting and anti-icing efficiency with wetting property of exposed systems Therefore, in this study we are going to

optimize the performance of two main topics: water harvesting (in term of dew collection) and anti-icing by tailoring surface wettability with the ideas are inspired from creatures: Desert beetle (water harvesting), Pitcher plant and Lotus leaf (anti-

icing)

1.2 Water harvesting

For several decades until now, water shortage still outlines as a serious problem for human life and agriculture, especially in the arid, seclude areas or

developing countries despites the reaction of governments and international organizations1–6 Irrigation has been considered at some regions however, it has proved the infeasibility already owing to long-time construction, high cost and low efficiency The reality questioned us an innovative, environmentally appropriate and economically viable solution for human demands and other purposes

Recently, an interesting approach involves a low cost alternative - generally referred to as "water harvesting" has been introduced and attracted much attention due to its unique mechanism and possibility7–16 In principle, water is captured from the air through various sources: rain, dew, fog, etc… using a system which is pertinently designed for water collection process According to the form of airborne water, the water harvesting phenomena can be categorized into three different terms: rain collection, fog collection and dew water collection All harvesting approaches should satisfy several criteria such as: low cost, efficient, scalable, wide enough and high durability for long time using Among them, dew water collection has been believed as innovative candidate owing to minimally affected by climatic and geographical constraint compare with fog collection Dew collection system can be divided into two passive and active systems, which may refer to the requirement of external energy or not

Figure 1.2 Cactus spines (Donald Erickson), Spider web (Alberto Ghizzi

Panizza) and Stenocara beetle (Wikimedia commons)

To enhance dew collection performance, several factors have been brought into order such as: ambient condition, materials, surface temperature, setting position… Among them, surface morphology modification has been named as the easiest and most effective approach to enhance collection volume A pertinent wettability or appropriate combination can ensure a good efficiency in water condensation This concept was inspired from fog basking behavior of Namib Dessert’s beetles17–19, spider silk20,21 or cactus spines22 Parket et al had revealed

the morphology of Dessert beetle’s back which consisted of alternative

hydrophobic, wax-coated and hydrophilic, non-coated regions18 This combination was reported as useful property for water collection due to the clear assignment for condensation (hydrophilic regions) and water rolling effect (hydrophobic regions)

Figure 1.3 Dew harvesting system in Morocco (Fadel Senna/AFP Photo) and Dehumidifier (LG.com)

In fact, several water condensation systems have been built and started operating in Mediterranean islands and Asia recently11,23,24 Such a harvesting

system usually contains a condensing device which aims to capture water and connect to a container to store harvested water for long-term purposes Condensing

part could be a single panel or a system consists of same or different materials A passive harvesting system should meet several requirements such as: economical, easy setup, environmentally appropriate and durable in high humidity

Figure 1.4 Water condensation presents on Bare and Hybrid Al

In this work, we proposed the investigation on water harvesting performance on different wettability and morphology fabricated on Aluminum plates The condensation evolution was observed in detail and showing the great contribution of wettability on collection mass We also evaluated the condensation process on hybrid sample which refers to the combination of hydrophilic spots surrounded by hydrophobic area (Figure 1.4) Several combinations of morphology and geology were introduced and showing the advantage when compared with single wettability surfaces, demonstrating a potential approaches for water harvesting system and other applications Al plates were used for water harvesting experiments due to its wide application, good thermal conductivity, cheap, and facile in fabrication

1.3 Icing and Anti-icing

Icing phenomenon refers to a process when liquid transferring its physical state homogeneously or heterogeneously to solid phase The homogeneous nucleation corresponds to solidified process occurs inside the parent liquid phase while heterogeneous nucleation refers to process happens at the surface of foreign solid objects

Figure 1.5 Ice accumulation on aircraft wing (aircraft.sewaro.us) and icing in process (aviationtroubleshooting.blogspot.com)

De-Ices can be found easily around us such as: cube ice for drinking, ice frost

in refrigerator, icebergs adhere on the road in cold climate regions Although there are certain benefits, almost spontaneous icing phenomenon express unexpected

effects and disadvantages in aviation, energy industry, transportation, agriculture and human beings Ice accumulation on aircraft might change the wing’s shape, disturbing the air-flow for supporting lifting force, increasing the drag-force and cause control problems25,26 In addition, bulk ices can be ingested into engine or collide with airframe and lead to disastrous failures

Typically, formed ice on airplane was separated into 2 types: glaze and rime which related to the air temperature, liquid water content and droplets size Glaze refers to transparent and quite large super-cooled water droplets (SLW), occurs at higher temperature while rime usually associated with opaque and smaller SLW, occur at lower temperature Glaze is more dangerous than rime since it tends to spread along the aircraft body before solidify while rime is quite brittle and freeze immediately upon contact Ice formed on aircraft individually in glaze or rime or mixed ice i.e combination of glaze and rime On the other hand, ice accretion on energy transmission system sometimes causes serious damage due to weight overloading and potentially endangering people and vehicles underneath Ice crystal found on transportation vehicles, ships, roads in cold climate regions could

be the main reason for massive destruction and eventually leads to unexpected accidents27–30 Freezing rain or drizzle occurs in winter weather can lead to agriculture crop failure, power outage or property damage31,32 In recent years, numerous studies have been conducted in order to inhibit or minimize icing problem, referred to “anti-icing” definition Generally, anti-icing strategies can be

divided into two main approaches: active and passive, which refers to the using of

external energy Among introduced approaches, superhydrophobic (SH) surface

has named as strongest candidate for passive anti-icing owing to its unique properties such as: high contact angle, extreme low sliding angle Recently, Slipper Liquid Infused Porous surface has been introduced as innovative solution for passive anti-icing due to its water repellency and humidity tolerance ( Figure 1.7)

Figure 1.6 Superhydrophobic surfaces for anti-icing

In this work, we demonstrate the useful of SHS for anti-icing field and morphology contribution on anti-icing performance when contact area plays an important role in determining adhesion strength The unique design based on SHS incorporates with original characteristic of materials can propose multi functions convergence in one samples including: high transparent, anti-icing and water repellent In addition, the anti-icing property of SLIPs surface also has been deeply investigated in order to optimize surface morphology and lubricant characteristic

for icephobic applications (Figure 1.7)

Figure 1.7 Penguin feather (Steve Gschmeissner), pitcher plant (Britannica.com) and Inspired Slippery Liquid – Infused Porous surface

2 BASIC THEORY

2.1 Wettability

Wettability or wetting refers to a phenomenon when a liquid comes in contact with a solid substrate or material Due to the physicochemical correlation between them, liquid might form a spherical-like status (water on lotus leaf) or film-like status (water on titanium dioxide under light irradiation) on solid surfaces Wetting state can be evaluated by determining the contact angle at three phases contact line or theoretically calculating spreading coefficient based on liquid and solid information In practical, by modification the properties of surface through physical (texturing surface) or chemical (manipulating surface energy) methods

we can optimize wettability for desired intentions The importance of wettability controlling can be seen through many applications in commercial and medical when people modifying surface wettability for desired purposes such as: water repellent wind shields, self-cleaning effect, semiconductor fabrication, body implants, biomaterials… Furthermore, the wetting behavior in micro and nanoscale also plays an important role in determination performance of many applications such as: composite materials, painting, cosmetics, pharmaceuticals, anti-icing and anti-fouling… The understanding about wettability mechanism is useful for designing various functional surfaces

2.1.1 Surface Tension

Liquid corresponds to one of fundamental states of matter (the others being solid, gas and plasma) with unique characteristics compare to the others The attraction between molecules inside liquid is stronger than gas phase but weaker

than solid phase This interesting property allows it conform the shape of container when maintain the volume independent of pressure The distinctive property of

liquid state is surface tension which leading to wetting phenomena

their shape in order to expose the smallest possible surface area and surface tension

is responsibility for such phenomenon

Figure 2.2 Surface tension demonstrates the formation of liquid (a) In air and (b) in contact with glass wall

Surface tension is not the property of liquid alone, but the property of liquid’s interface with other medium We usually use surface tension in the meaning of liquid-air interface but actually we should consider also the liquid-medium (not air) surface tension term The water-air surface tension is usually greater than water-solid surface tension, that is the reason why a water droplet staying in the air without external force would be approximately spherical but performs as curves up when contact with solid (like water in glass cup – Figure

2.2) Indeed, the surface tension results from the imbalance between cohesion and

adhesion Cohesion or cohesive force is the attraction between same molecules

inside a material and adhesion or adhesive force is the attraction between two types

of molecules at the interface of two immiscible materials

There exist numerous techniques to measure surface tension such as: sessile

or pendant drop method33, Wilhelmy plate method34 or confocal microscopy35 A

first class of measurement techniques consists of analyzing drops formation in specific situations and in deducing surface tension values by adjusting the probe parameter using mathematical model

As mention above, surface tension corresponds to surface energy and it is the key parameter to determine wetting state of liquid droplet in contact with a solid at interface Denote solid, liquid and gas as S, L and G, respectively Equilibrium considerations allow us to calculate the wetting angle from the surface tension values using Young-Dupre’s equation35

𝛾𝐿𝐺cos 𝜃 = 𝛾𝑆𝐺− 𝛾𝑆𝐿 (Eq

2-1)

The Young – Dupré equation dictates trigonometric relations between contact angle and forces acting on the three phase contact line in mechanical equilibrium state Three values 𝛾𝑆𝐺, 𝛾𝑆𝐿, 𝛾𝐿𝐺 are surface tensions of Solid-Gas, Solid-Liquid and Liquid-Gas, respectively The relationship between surface tensions and contact angle has been expressed mathematically in several researches36–42

Table 2.1 Surface tension of a few common liquids (20 o C) and interfacial tension of water-lubricants

Liquid Si oil (KF-96) Glycerol Water Mercury

on the surface, meaning altering the surface morphology, which is inspired from nature functional structures

Controlling wettability of a functional surface is sufficient in employing to

commercial applications with various purposes: condensation system, fouling coating, anti-icing or self-cleaning surfaces Especially, a surface having surface roughness comparable to visible wavelength and pertinent textured can reduce reflections effect, which now is favorable applying to windshields or transparent building walls Indeed, wetting behavior of artificial textured surfaces

anti-bio-at submicron plays an important role in surface technology and the better understanding of wetting mechanisms on composite surfaces might propose a methodology to control the hydrophobicity for desired purposes In this section,

we evaluate the different of wetting regimes of liquid droplet depressed on textured super-hydrophobic surface, which were found widely in nature such as Lotus Leaves or Petal Effect43–45

2.1.2.1 Wenzel state

In practically, real surfaces are not ideal, which assumed to be flat, rigid and chemical homogeneously and zero in contact angle hysteresis Zero hysteresis implies that the advancing and receding contact angles are equal Contact angle hysteresis results from numerous thermodynamically stable contact angles introduced on non-ideal surfaces In this part, we will consider two cases: rough surface and chemical heterogeneously

Wenzel40 in 1936 and Cassie-Baxter41 in 1944 have initially described the relationship between physical structure and apparent contact angle on textured surfaces Both approaches have demonstrated the decisive contribution of