Seminar on

Superhydrophobic Surface Coating

Guided by

Prof. S.A.Desai

Presented by

Kininge Madhuri M.M.E 1 (Chem. Engg.)

2014-2015

Introduction:-

Superhydrophobic surfaces have a lot of interest both in academia and in industry because of the self-cleaning properties.

This seminar begins with a brief discussion about

What is superhydrophobic

application of superhydrophobicity

characterization of superhydrophobic surfaces.

methods superhydrofobic surfaces.

History:-

superhydrophobicity have been published.

R .N . Wenzel (1949) J. T. Woodward and H . Gwin

& D . K.Schwartz.(2000) B . Kong & X . Yang (2006) M. L. Ma & R .M . Hill (2006) P . Roach &N.J.Shirtcliffe (2008) C. H . Xue (2010) B.Bhushan &Y.C. Jung (2010)

What is Superhyadrophobic ?

-Hydrophobic :-

-Superhyadrophobic:-

Superhydrophobicity is a property that governs the extreme water repellence and non wettability of a solid surface. On such surfaces water or any other liquid forms nearly spherical droplets and not continuous films.

-Example:-

Lotus leaves.

An environmental scanning

electron micrograph image of

the famed Lotus leaf.

-water

droplets adopt an almost

spherical shape on the waxy

protrusion of the leaf surface.

Characterization :-

1) Contact Angle (CA):

For a superhydrophobic surface, it must exhibit Contact Angle greater than150o and the contact angle hysterisis must also be low.

That contact angle measure by using instrument contact angle meter or goniometer.

2)Angle of Slide: -Another feature is that the surfaces must have a low angle of slide less than 10o , measured from the horizontal i.e. the water droplets should roll down the surface very easily,

similar to rolling of rigid spheres.

3) Self cleaning ability: -Superhydrophobic coatings or surfaces the adhesion between the water droplets and the dust particles is much more than that between the surface and the dust.

-Hence most of the dust is picked up by the rolling water drops and no residue is left behind.

-Hydrophilic dust particles therefore have a greater chance of removal.

Self cleaning Action:-

Water rolls off the surface taking

duct or contaminant along with.

4) Elastic collision of liquid droplets:

Water droplets which fall on a superhydrophobic surface with some velocity can bounce back, practically without any deformation after having suffered an almost elastic collision. This is also one reason why these surfaces remain dry even after coming in contact with some liquid.

How measure the angle of contact ?

1) For smooth surface - “Young Equation”:-

when a liquid drop is placed in contact with a solid, the equilibrium of the solid and liquid surfaces will be established at a certain angle called the static contact angle CA, θ0, given by the Young equation:-

where γSA, γSL and γLA are the surface energies of the solid against air, solid against liquid and liquid against air, respectively.

Rough surfaces-

1) Wenzel model:-He modified the Young equation as follows:

where, r is the non-dimensional surface roughness factor, defined by the equation , ratio of the actual area of a rough surface, ASL, to its flat projected area, Af.

2)Cassie-Baxter :-

model for heterogeneous surfaces composed of two fractions,

one with a fractional area f1 and contact angle θ1 and the other with f2 and θ2, with f1 + f2 = 1.

The contact angle is thus given by equation:-

cosθ=f1cosθ1+f2cosθ2

Various methods are used for the preparation of rough surfaces:-

1) layer-by-layer And Self assembly

2) sol–gel processing

3) Etching

4) Electro-spinning Method

5) electrochemical deposition.

6) chemical vapor deposition.

1) Layer-by-layer self-assembly:-

It is a simple and cheap method to construct thin-film coatings by depositing alternating layer

TiO2 and SiO2 coated steel surfaces containing micro- and nanoscale with different surface roughness were fabricated by means of a layer by layer deposition process .

That such modified surfaces have a strong repulsive force to water droplets.

Their static contact angles exceed 165°, receding angle >160°, advanced angles >170° and slide angle <1°.

SEM images of different layers of TiO2 nanoparticles in steel surface: (a) Water droplets on the treated steel surface. (b) TiO2 one layer (TiO2) 1. (c) TiO∗ 2 two layers (TiO2) 2 and (d) TiO∗ 2 three layers (TiO2) 3 ∗

2) Etching:-

Etching is another simple and efficient method to produce superhydrophobic coatings with rough surfaces.

Different etching techniques, such as chemical, plasma and laser etching.

Chemical Etching:- A superhydrophobic surface was fabricated by chemical

etching (HCl) on aluminum alloy A static water contact angle of more than 150° was

achieved , when the surface chemical etching time was more than 3 min.

1)Sol–gel process:-

 -The roughness of the surface obtained with this process can be easily modified simply by changing the composition of the reaction mixture.

Sol–gel processes can produce rough surfaces on a variety of oxides such as silica, alumina, and titania.

The process can form a flat surface coating, xerogel coating or aerogel coating depending on the process conditions; both xerogel and aerogel show rough or fractal surfaces.  

Sol-Gel Process Scheme:-Sol

vvC

Powers

Xerogel Film

Dense Ceramic Film

SolventEvaporation

Gel

Heat

Xerogle Aerogel

Dense Glass Film

Extraction

Sol-Gel method:-

Silicone mould

Gel from

Si(OMe)4

drop diameter 0.2 mm

Hydrophobization by a

monolayer

Coating materialsinorganic (ceramic materials)inorganic-organic hybrid materials

Which materialscan be coated

Glasses Ceramics Metals Polymers

Coating techniquesDippingSpraying bladingPaintingRollingFlow coating

Applications of superhydrophobic surfaces:-

Anti-adhesion and self-cleaning.

-Self cleaning glasses.

-Self cleaning textile.

Anti-biofouling applications.

Corrosion inhibition.

Drag reduction.

Technological challenges:-

for textile-

The first and probably the most important point to be taken care of is that self-cleaning textiles take a long time to clean themselves.

Titania (TiO2 ) based textiles will also the additional problem that the high oxidation power of the catalyst will not only degrade the stains but will also adversely affect the fibers themselves.

The catalyst is also a skin irritant.

Presently many of the coatings are damaged during machine washing.

For glass-

Most of the methods involve very specialized and delicate techniques that are suitable for laboratory scale studies. On a large scale these methods are not very feasible.

Most of the self-cleaning glasses that are currently available have the ability to destroy organic dirt and grime but they are not really effective towards the inorganic impurities. Hence this is also an aspect that needs further investigation.

Conclusion

Surface roughness increases hydrophobicity

Superhydrophobic if contact angle > 150°

Superhydrophobicity leads to self-cleansing

Several methods reported to synthesize artificial superhydrophobic surfaces, but yet it is only in lab scale.

References :-1. Li et.al., Chemical Society Reviews, 2007.

2. J. Bravo et. al., Langmui.r, 2007.

3. Adel M.A. Mohamed , Aboubakr M. Abdullah , Nathalie A. Younan “Corrosion behavior of superhydrophobic surfaces” 2014

4. Journal of Surface Engineered Materials and Advanced Technology, 2012, “Recent Progress in Preparation of Superhydrophobic Surfaces. ”

5) C. Jeffrey Brinker, Sandia Fellow Sandia National Laboratories, “ Superhyadrophobic Surface coating”

Thank YouThank You