Electrowetting Drop profile Wetting defects Wetting transitions Marguerite Bienia, Catherine Quilliet, Marcel Vallade Laboratoire de Spectrométrie Physique Grenoble, France

From wetting... air liquid   solid/liquid solid/air substrate

- - - - - - - - - - - - - - - - - - - - …to electrowetting V - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + V Insulating solid Counter electrode Ew equation reduction of the contact angle of water on the insulator

An alternative geometry 2-fluid EW (brine/oil) : by matching densities, capillary length  capillary forces dominate

Examples of applications Hayes et al, Nature Sept 18 2003 Passive display device Blake et al, 2000 Coating assist Cho et al, 2002 splitting and merging of droplets Berge et al, 1999 Variable focus lens

Outline Introduction 1)Fundamental issue : study of drop shape under electrical field 2)Electrowetting as an experimental solution for fundamental study of classical wetting : wetting defects wetting transitions

1. Drop Profiles Abt. Angewandte Physik, Ulm, Pr Herminghaus, F 1. Drop Profiles Abt.Angewandte Physik, Ulm, Pr Herminghaus, F. Mugele, EURODOC water on silanized 0.7mm glass V=1088V Total width : 1,3mm Problem : instability and drop expulsion  what is the shape of the drop when an electrical field is applied?

Idea : electrostatic pressure compensated by an excess of capillary pressure V Pel electrode Pcap insulator V~

Numerical results Buehrle et al, 2003 interface profiles for increasing electrical field The range of the variation is proportional to e/R

Direct measurements =7, e>200µm Video camera V and EW on 150, 300,450µms glass coated with ~100A Teflon AF1600 V range 1000-1500V  range 9550° liquid : BMIM EW on 160, 500µm teflon V range 1000-1500V  range 100-50° liquid : brine

Experiments 1.3mm Typical picture, 0V, vol=3.2µL symmetry plane

Profile extraction (3)

Curvature calculation Cylindrical symmetry r(z) Successive derivatives  very noisy results!

Master curve C-C0 (µm-1) Dots : experiment, =2 solid line : theory Relative height

Perspectives Profile extraction is impossible close to the triple line very thick insulators needed very high voltage required Theoretical work still running

2. Hysteresis ideal case :  real case : a>r

Wetting defects Joanny et al, 1984 : a model for contact angle hysteresis Robbins et al, 1987 : hysteresis on random surfaces Raphael et al, 1989 : single defect study De Jonghe et al, 1995 : experimental physical and chemical defects on SiO matrix Tanguy et al, 1998 : from individual to collective pinning: effect of long-range elastic interactions

Electrowetting defects Characteristic (and drawbacks!) of classical wetting defects : 1)wetting contrast is fixed! 2)defects are sometimes both chemical and physical Electrowetting may bring experimental solutions: 1)allow tunable wetting contrast for a given geometry of defects 2)no surface alteration, the defects are virtual study of a rectangular defect

Principle of a bi-layered defect + + + + + + + _ _ _ _ _ _ _ _ _ _ + + + insulator Wettability contrast

Experimental setup ground PTFE 25 microns Vb ITO, with an glass 1mm Vd ground PTFE 25 microns water oil Vb glass 0.17mm hydrophilic ring etched using Tetra-Etch (Gore) ITO, with an etched defect ITO (500Å =>transparent)

Results 25x30mm Reference electrowetting curve (increasing/decreasing voltage) obtained with a cancelled defect

Wetting contrasts 2mm

Wetting contrasts Wetting and non-wetting defects left : experiment with an oil drop in water right : simulations with Surface Evolver, for theoretical contact angles A : Vb=305V,Vd=400V b=107°, d=64° C : Vb=93V, Vd=696V b=45°, d=106° scale : 5mm

Sharp edge effect Attraction between water and electrode wetting is favoured along the edge of the defect oil water system insulator insulator

Conclusion Feasibility of e-wetting defects is proved  Bienia et al, Langmuir Wetting contrast is tunable, with (De Jonghe, 1995 : =+71°) Theoretical model : the precision of the defect is of the order of magnitude of the thickness of the insulating layer Perspective : other defect geometries

3. Electrically induced wetting transitions Motivation : Induce wetting transitions through electrowetting water in air or oil : partial to complete wetting impossible (EW saturation) what kind of transition is possible?

Effective interface potential oil insulator water e effective interface potential P(e) : water-insulator interactions through oil P(e) energy per unit surface e=0 : long range (a few nm): Hamaker constant A ( for e) short and intermediate range: no universal model

Wetting regimes attraction repulsion P(e) P(e) S P(e) e S e S eeq e S>0, A>0 pseudo-partial wetting S<0, partial wetting S>0, A<0 : complete wetting

Transition Effective potential without electrical field P(e) water electrode insulator d,d water oil e, V eeq P(e) e resulting potential Initial state : oil wets the insulator completely By applying voltage : transition towards pseudo-partial wetting electrostatic energy Quilliet et al, 2002

Hamaker constant : A=-6.2.10-21 J.m-2 <0, repulsive System Brine and bromododecane on parylene, in the defect setup Hamaker constant : A=-6.2.10-21 J.m-2 <0, repulsive Same experiment on parylene+teflon AF1600

Ellipsometry multilayer ellipsometer made by Patrice Ballet Oil thickness after the transition : eeq<10nm for 20V

Setup ccd detector laser V ~ ground water 5mm oil gold 1000Å Parylene Silicon wafer Teflon cell

Multilayer ellipsometry on the detector Multilayer with thick and thin layers the water layer can be neglected air water (5mm) Multilayer system (oil + parylene) substrate

Signal detection and treatment Ellipsometric parameters (,) : We consider the second spot : Preliminary results :

Cross defect Profile extraction Appendices Cross defect Profile extraction

Artefacts water on glass

Profile extraction (1) intensity profile on a line

Profile extraction (2) intensity

An example : cross defect V~ Idea : cross shaped electrode Increasing V