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

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

3. - - - - - - - - - - - - - - - - - - - -
…to electrowetting V V
Insulating solid
Counter electrode
Ew equation
reduction of the contact angle of water on the insulator

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

5. Examples of applications
Hayes et al, Nature Sept
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

6. 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

7. 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?

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

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

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

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

12. Profile extraction (3)

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

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

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

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

17. 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

18. 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

19. Principle of a bi-layered defect
_ _ _ _
_ _ _
_ _ _
insulator
Wettability contrast

20. 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)

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

22. Wetting contrasts
2mm

23. 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

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

25. 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

26. 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?

27. 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

28. 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

29. 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

30. 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= J.m-2 <0, repulsive
Same experiment on parylene+teflon AF1600

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

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

33. 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

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

35. Cross defect Profile extraction
Appendices
Cross defect
Profile extraction

36. Artefacts
water on glass

37. Profile extraction (1)
intensity profile on a line

38. Profile extraction (2)
intensity

39. An example : cross defectV~
Idea : cross shaped electrode
Increasing V