ElectrowettingElectrowetting

Drop profileWetting defects

Wetting transitions

Marguerite Bienia, Catherine Quilliet, Marcel ValladeLaboratoire de Spectrométrie Physique

Grenoble, France

2substrate

liquid

solid/liquid

solid/air

air

From wetting...From wetting...

3

reduction of the contact angle of water on the insulator

……to electrowettingto electrowetting

2// 2

1CVsolidwater

effectivesolidwater

Insulating solid

Counter electrode

V

- - - - - - - - - - - - - - - - - - - -

+ + + + + + + + + + + +

V

2

0 2

1coscos

CV

Ew equation

4

iso

r

e

V

2coscos

20

0

An alternative geometryAn alternative geometry

2-fluid EW (brine/oil) : by matching densities, capillary length capillary forces dominate

5

Berge et al, 1999Berge et al, 1999Variable focus lens

Cho et al, 2002Cho et al, 2002splitting and merging of droplets

Examples of applicationsExamples of applications

Blake et al, 2000Blake et al, 2000Coating assist

Hayes et al, Nature Hayes et al, Nature Sept 18 2003Sept 18 2003Passive display device

6

OutlineOutline

Introduction

1)Fundamental issue :

study of drop shape under electrical fieldstudy of drop shape under electrical field

2)Electrowetting as an experimental solution for fundamental study of classical wetting :

wetting defectswetting defectswetting transitionswetting transitions

7

1. Drop Profiles1. Drop ProfilesAbt.Angewandte Physik, Ulm, Pr Herminghaus, F. Mugele, EURODOCAbt.Angewandte Physik, Ulm, Pr Herminghaus, F. Mugele, EURODOC

Problem : instability and drop expulsion what is the shape of the drop when an electrical field is applied?

Total width : 1,3mm

water on silanized 0.7mm glassV=1088V

8

Idea : electrostatic pressure compensated by an excess of capillary pressure

electrode

V

Pel

Pcap

insulatorV~

9

Numerical resultsNumerical results

Buehrle et al, 2003Buehrle et al, 2003interface profiles for increasing electrical field

The range of the variation is proportional to e/R

1

12.02

1 2

R

CV

10

Direct measurementsDirect measurements

•EW on 150, 300,450µms glass coated with ~100A Teflon AF1600V range 1000-1500V range 9550°•liquid : BMIM

•EW on 160, 500µm teflonV range 1000-1500V range 100-50°•liquid : brine

=7, e>200µm

Video cameraV

and

=7, e>200µm

V

Video camera

11

ExperimentsExperiments

Typical picture, 0V, vol=3.2µL symmetry plane

1.3mm

12

Profile extraction (3)Profile extraction (3)

13

Curvature calculationCurvature calculation

2/122/32 1

11

1 rrr

r

Cylindrical symmetry r(z)

Successive derivatives very noisy results!

14

Master curveMaster curve

•Dots : experiment, =2

•solid line : theory

Relative heightRe

H

C-C

0 (

µm

-1)

15

PerspectivesPerspectives

•Profile extraction is impossible close to the triple line

very thick insulators needed

very high voltage required

•Theoretical work still running

16

2. Hysteresis2. Hysteresis

ideal case :

real case : a>r

17

Wetting defectsWetting 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

18

Electrowetting defectsElectrowetting defects

Characteristic (and drawbacks!) of classical Characteristic (and drawbacks!) of classical wetting defects :wetting defects : 1)wetting contrast is fixed!2)defects are sometimes both chemical and physical

Electrowetting may bring experimental solutions: Electrowetting may bring experimental solutions: 1)allow tunable wetting contrast for a given geometry of defects2)no surface alteration, the defects are virtualvirtual

study of a rectangular defectstudy of a rectangular defect

19

Principle of a bi-layered defectPrinciple of a bi-layered defect

2

1

22

21

0

2

1

C

C

V

V

VCVC

defect

back

defectback

for

+ + +

+ + + +

_ _ _ _ _ _ _

_ _ _

+ + +

1H

2H

backV defectV

insulator

Wettability contrast

20

Experimental setupExperimental setup

ITO (500Å =>transparent)

ITO, with an etched defect

hydrophilic ring etched using Tetra-Etch (Gore)

glass 1mm

Vd

ground

PTFE 25 microns

water

oil

Vb

glass 0.17mm

21

ResultsResults

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250 300

V(volts)

thet

a (d

egre

es)

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

25x30mm

22

Wetting contrastsWetting contrasts

2mm

23

Wetting contrastsWetting 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=400Vb=107°, d=64°C : Vb=93V, Vd=696Vb=45°, d=106°

scale : 5mm

24

Sharp edge effectSharp edge effect

oil

water

Attraction between water and electrode

wetting is favoured along the edge of the defect

insulator

insulator

-3,5

-3

-2,5

-2

-1,5

-1

-0,5

0

-0,1 0 0,1 0,2 0,3 0,4 0,5

X/H

delt

a ga

mm

a /g

amm

asystem

25

ConclusionConclusion

•Feasibility of e-wetting defects is proved Bienia et al, LangmuirBienia 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

4361

.4043 1

mmN

3. Electrically induced wetting 3. Electrically induced wetting transitionstransitions

Motivation :

Induce wetting transitions through electrowetting

•water in air or oil : partial to complete wetting impossible (EW saturationEW saturation)

what kind of transition is possible?

27

Effective interface potentialEffective interface potential

P(e) energy per unit surface

•e=0e=0 :

•long rangelong range (a few nm): Hamaker constant A

( for e)

•short and intermediate range:short and intermediate range: no universal model

wateroiloilsolidwatersolidSP )0(

effective interface potential P(e) :

water-insulator interactions through oil

212)(

e

AeP

oil

insulator

water

e

28

Wetting regimesWetting regimes

S>0, A<0 : complete wetting

S

S

S

S<0, partial wetting

S>0, A>0 pseudo-partial wetting

P(e)

e

P(e)

e

P(e)e

repulsionattraction

eeq

29

TransitionTransition

•Initial stateInitial state : oil wets the insulator completely

•By applying voltageBy applying voltage : transition towards pseudo-partial wetting

electrodeinsulator d,d

water

oil e,

V

eeq

Effective potential without electrical field

electrostatic energy

resulting potential

P(e)

e

Quilliet et al, 2002Quilliet et al, 2002

30

SystemSystem

Brine and bromododecane on parylene, in the defect setup

Same experiment on parylene+teflon AF1600

Hamaker constant : A=-6.2.10Hamaker constant : A=-6.2.10-21-21 J.m J.m-2-2 <0, repulsive <0, repulsive

31

EllipsometryEllipsometry

multilayer ellipsometer made by Patrice Ballet

Oil thickness after the transition : eeq<10nm for 20V

32

water 5mm

SetupSetup

Teflon cell

gold 1000Å

Silicon wafer

ground

V ~

oil

ccd

detectorlaser

Parylene2µm

33

Multilayer ellipsometryMultilayer ellipsometryon the detector

air

water (5mm)

Multilayer system (oil + parylene) substrate

Multilayer with thick and thin layersthe water layer can be neglected

34

Signal detection and Signal detection and treatmenttreatment

We consider the second spot :

Ellipsometric parameters (,) : s

p

R

Ri )exp(tan

Preliminary results :

80,178

25.36

89,178

59.33

theo

theo

mes

mes

multilayers

p

airwaters

waterairs

airwaterp

waterairp

refls

reflp

R

R

tt

tt

E

Ei

)(

)()exp(tan

//

//

AppendicesAppendices

Cross defect

Profile extraction

36

ArtefactsArtefacts

water on glass

37

Profile extraction (1)Profile extraction (1)

intensity profile on a line

38

)(exp1 DxC

BAy

inte

nsity

Profile extraction (2)Profile extraction (2)

39

An example : cross defectAn example : cross defect

Idea : cross shaped electrode

Increasing V

V~