2009 trans nelson sliding resistance ewod droplet

8/6/2019 2009 Trans Nelson Sliding Resistance EWOD Droplet

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RESISTANCE OF

Wyatt NelsMechanic

University o

ABSTRACTWe report an experimental investi

resistive forces imparted on dropletselectrowetting-on-dielectric (EWOD).

advancing contact angle is always larger thof a droplet moving on a solid surface undactuation means, EWOD actuation is uniqadvancing angle is smaller than the recedinhigh-speed videos of sliding droplets undeto elucidate this seemingly paradoxicalhysteresis. The results indicate a transitio

10-3, after which the dominant resistancestrongly speed- and voltage-dependent.

KEYWORDSElectrowetting; Electrowetting-on-diele

contact angle; dynamic contact angle; dropl

INTRODUCTION

Electrowetting-on-dielectricThe driving mechanism electrowetti

(EWOD) is an effective way to movelength droplets at very high rates, e.g. 10mode works by electrostatically pulling th

polar liquid towards a potential. EWO

implemented on chip via planar substratethin-film electrodes that lie beneath a hydr dielectric layer, on which grounded dropletto fill spaces over highest fields [1]. Patformed and reconfigured electronically, onegligible compared with that of dropletlimiting factor of overall system speed.

We have performed an experimeresistance force as it varies with sliding sp

voltage. The results are of practical ioptimizing chip design and accurate nume

Resistance under EWOD actuation hasexperimentally for droplets immersed in oil

on a thin film of oil [3]. Our work repr observations of electrowetted droplets slihydrophobic solid with air as the surrou

We tested water over a speed range frommm/s, or capillary number (Ca = ην / γ) ran

to 10-2. The maximum test voltage of 60to the onset of contact angle saturation.

During steady transport, the voltage-deforce, concentrated at the advancing c

DROPLETS SLIDING BY EWOD ACTUA

n, Prosenjit Sen, and Chang-Jin “CJ” Kiml and Aerospace Engineering DepartmentCalifornia, Los Angeles (UCLA), CA, USA

gation of the

actuated byWhile the

an the recedinger conventionalue because the. We recordedelectrowettingcontact angle point at Ca ≈

force becomes

ctric (EWOD);et resistance

g-on-dielectric

sub-millimeter-0 mm/s. Thismeniscus of a

D is typically

s with arrayedophobic-coateds slide in order ways are thus

n a time scalemovement, the

tal study of ed and EWOD

terest towardsrical modeling.

been studied[2] and sliding

esents a set of ding on a dryding medium.

1 mm/s to 500e of about 10-5

corresponded

endent EWODontact line, is

balanced by an overall resistancworking fluid, medium, and con surface characteristics anddrag component, called contactindependent and holds the droplforce is higher than a criticadynamic drag component limspeed and has been attributed

molecular phenomena at the con

Experimental conceptInstead of observing a mo

shape of a stationary droplet sli

This scheme enables high-smeniscus shape under variousconditions. The test setup wasEWOD configuration, in whicinto a disk-like shape by a hycover. In real devices, squeesplitting [1] and slows evaporasubstrate is hydrophilic, servielectrode but also to hold thecamera field of view, while th

Figure 1 depicts side viewscondition and (b) the ty

configuration.

Figure 1: Side view schematiccondition of a stationary droplet s

and held in place by a hydrophilic

two- late EWOD actuation of

stationary substrates. Upper an

labeled for each case; subscripts

and receding contact angles, respeattached to angles affected by EWO

ION

e force, which, for a givenip configuration, dependssliding speed. The staticangle hysteresis, is speed-et in place until the drivingl value. The additionalits the maximum slidingto both viscosity [4] and

tact line [5,6].

ing droplet, we track theing on a moving substrate.

eed video recording of steady speed and voltagemodeled after a two-plate

the droplet is squeezedrophobic-coated groundedzing the droplet enablestion. In our case, the topg not only as a groundroplet in place, i.e. in thee bottom substrate slides.

of: (a) the experimentalical two-plate EWOD

of: (a) the experimental liding on a moving substrate

top plate, and (b) the typical

a droplet sliding between

d lower contact angles are

‘a’ and ‘r’ denote advancing

tively, and superscript ‘V’ is D voltage.

978-1-4244-4193-8/09/$25.00 ©2009 IEEE Transducers 2009, Denver, CO, USA, June 21-25, 20092014

W3P.067



Fig. 1(a) represents the field of view of thence we were able to record cross-sectshapes and extract apparent contact an

processing. A similar method was usedcoating for immersion lithography [7].regarding the deformation of a moving dro

transduction mechanism tracking resistancvaried with voltage and sliding speed.

METHODS

Substrate fabricationTop and bottom substrate fabricatio

evaporation of 200 nm gold onto siliconsubstrates received 350 nm PECVDfollowed by 200 nm spin-coated Teflontypical EWOD device surface. Wafersinto two-by-one inch strips, so that a fresused for each test. The top plate surface

unlike typical EWOD devices, to hold thewhile the bottom substrate slides. Wirestop and bottom plates prior to testing.

Measurement techniqueFigure 2 illustrates side view and c

vision of the experimental setup. An L

was positioned directly behind the droplet f The top substrate was mounted to a vertica

order to precisely adjust the plate gap, h in bottom substrate was mounted to a comlinear actuator. A high-speed camera (

captured 4000 frames per second.

Figure 2: Side view of the experimental s

micrometer adjusts plate gap, and a linear sliding speed. The dashed box shows the camand a LED is positioned behind the droplet for i

A plate gap of about 0.5 mm was found t backlighting for the stated frame rate. TheFigure 1) of the droplet was about 1 mmcompletely in the camera field of view and

resolution.

e camera, andional meniscusles via imageto study wafer

Informationlet served as a

behavior as it

n began withafers. Bottomsilicon nitride, similar to aere then dicedstrip could be

is hydrophilic,

roplet in placeere soldered to

amera field of D light source

or illumination.micrometer in

Fig. 1(a). Theuter-controlled

Phantom v7.2)

tup. A vertical

ctuator controlsra field of view,

llumination.

allow enoughdiameter (d of in order to fit

provide a high

From videos, in-house softwar frames to obtain a time series otime-average of a contact anframes) is represented as one dathis paper. The software usedtechnique combined with curv

droplet shape and contact angles

Force transductionWhen a droplet steadily

surface due to gravity, the menisenergy balance. More spec

interface stretches in the directidone to create new surface is th

overcome drag, neglecting heaSurface tension acts on a pl

surface, and for one-dimensionasliding on a flat substrate), onlyis of interest. When meniscus de

the contact line (at low Ca),contact angles can be used to

surface tension force F1 as follo

F1

= γ xd = γ cosθ

r

V(

The droplet diameter d is perpemotion. Subscripts ‘a’ and ‘receding moving contact line

attached to angles affected by EEquation 1 provides a rela

drag force on a droplet in the estationary droplet sliding on a

1(a). In this case, both advanangles are reduced due to electr only considers meniscus defor

line and assumes the top platewith equal advancing and recedi

In the case of a moving droinfluences the advancing coadditional resistance from thedifferent advancing and recedinsurface tension force on an EWin-plane component of surface t

F2

= γ x

d = γ cosθr

− cosθa

V

( This equation accounts for t

dependent advancing angle isangle during EWOD actuatiotransport because surface tensidirection of motion.

e analyzed the individualf the contact angles. Thele history (typically 500a point in all the graphs in

a sub-pixel edge detectionfitting to determine the

with high accuracy.

slides down an inclinedcus shape indicates the netifically, the liquid-vapor

n of motion, and the work e work done by gravity to

lost to the environment.ne tangent to the liquid

l translation (e.g. a dropletthe in-plane component γx formations are proximal to

advancing and recedingapproximate the in-plane

s:

− cosθa

V )⋅ d (1)

dicular to the direction of ’ denote advancing and, and superscript ‘V’ is

OD voltage.ion by which to estimate

perimental condition of amoving substrate, Figure

ing and receding contactwetting voltage. Also, F1 ations local to the contact

wetting line is stationaryng contact angles.

let, however, EWOD onlytact line, and there istop plate, indicated by

g angles. F2 estimates theD-actuated droplet via thension:

cosθr

top − cosθa

top

)⋅ d (2)

e fact that the voltage-

lower than the receding, making F2 < 0 duringn pulls the droplet in the

2015



RESULTS AND DISCUSSION

Dynamic contact anglesDynamic contact angle data are summ

3, with a separate vertical axis for eachversus Ca in logarithmic scale on the horiz

sliding test yielded about 500 frames froangles were extracted and averaged. We treceding and advancing angle for each Ca.

Figure 3: Measured dynamic contact angles

number. A receding and advancing angle is pl

tested. Every data point represents the avera frames, taken at 4000 fps.

From Figure 3 it is clear that static contactis dominant up to Ca ≈ 10-3, and above this

effects are evident and voltage dependereceding angles for 60 V are affected by sais the reason why measured advancing andare almost the same below Ca ≈ 10-3. DataCa tests was not plotted because stick-sliobserved. This oscillatory sliding behavior

investigation and will be presented elsewhe

To highlight the onset of the contadependence observed at Ca ≈ 10-3, Figure

rized in Figurevoltage plottedntal axis. Each

which contactherefore plot a

versus capillary

tted for each Ca

ge of about 500

ngle hysteresis point dynamic

nt. Note thatturation, whicheceding anglesfrom 60 V lowmotions were

is under further

e.

t angle speed4 shows F1/γd

versus Ca, covering a stage speIn the 0 V case, no stark transit

behavior was observed. For slothe force impeding droplet

evidenced by only slight increasCa reached 10-3, correspondi

speed of about 70 mm/s. Also,or no voltage dependence, sidrops under DC electrowettingused to calculate F1, because coaffected by saturation and stick-accurately represent the in-plane

Figure 4: Dimensionless force

experimental condition of a stati

moving substrate plotted versus

range of 1 to 500 mm/s.

For a droplet pulled byresistance force F2/γd, which a

plate, is plotted in Figure 5. Nand voltages for which transpor a chip with the same dielectricour experimental plates. For mm/s) transport is predicted for about 300 mm/s is predictedexpected for EWOD voltages le

Figure 5: Dimensionless force F

droplet sliding between to plates pl

d range of 1 to 500 mm/s.on from static to dynamic

w 20 V and 40 V sliding,otion is primarily static,

es in resistance force untilg to a transition sliding

at low speed there is littleilar to results for sessile[8]. 60 V data was not

ntact angles were stronglylips and as a result did notmeniscus stretching.

1 / γd on a droplet in the

onary droplet sliding on a

a, covering a stage speed

WOD, the dimensionlesscounts for drag at the top

egative F2 predicts the Ca by EWOD is possible onroperties and geometry asxample, low-speed (~ 1040 V, and a maximum of or 60 V. No motion iss than 40 V.

2 / γd on an EWOD-actuated

tted versus Ca.

2016



Figure 6. Meniscus profiles: (a) 0 V (solid) & 2

At high Ca dynamic dissipation mechanfluid viscosity significantly affect the resist

We observed during experiments in the higrange that the overall shape of the droplet

both voltage and speed, and therefore dangles were not sufficient to capture thedeformation. In fact, from Figure 4 we maEWOD force amplifies dynamic drag whdroplet slides over a moving substrate.droplet shape, however, may lead us to co

trend is reversed at higher voltages, when tassists sliding.

High sliding speed analysisFigure 6 shows meniscus profiles over

compare shape as it varied with voltagetested sliding speed of 500 mm/s. These p

how wetting via voltage affects in-planesurface tension. In the first comparison,

electrowetting effect is mostly evident at tair interface, where 20 V causes the contafrom about 110° to less than 90°, corre

increased surface tension force compone

plate motion. Furthermore, it is clear pronounced comparison, Figure 6(b), in wh20 V (solid) and 40 V (dashed) are ovEWOD force greatly inhibits sliding, indicalarge in-plane deformations occurring atsurfaces of the droplet. An interesting effethe voltage from 40 V to 60 V appears incomparison, Fig 6(c), at the advancing conwe see reduced in-plane meniscus stretresistance to sliding at the highest voltage.

CONCLUSIONSWe developed a method for tracking m

and measuring dynamic contact angles of under electrowetting. The results showforce varies over ranges of Ca and voltagEWOD actuation in droplet microfluidic sy10

-3, static contact angle hysteresis is domi

by a weak dependence of contact angle onand apparently independent of voltage. Be

the speed- and voltage-dependent mechanis

V (dashed); (b) 20 V (solid) & 40 V (dashed); (c) 40 V

isms includingance to motion.

h sliding speedas sensitive tonamic contactfull meniscus

y conclude thaten a stationary

closer look atclude that this

e EWOD force

laid in order to

at the highestofiles illustrate

components of Fig. 6(a), the

e rear droplet-t angle to drop ponding to an

nt opposite to

for the mostich menisci for rlaid, that theted by the veryfront and rear

ct of increasingthe third shapeact line, wherehing, i.e. less

niscus profiles

roplets slidinghow resistancee applicable tostems. At Ca <

nant, as showndroplet speed,

yond Ca • 10-3,

ms dominate.

ACKNOWLEDGEMENTSThis work was supported

program, and a NSF IGERT thrCreation Training Program (done in a laboratory of theMimetic Space Exploration (CM

REFERENCES[1] S. K. Cho, H. Moon, a

transporting, cutting, andelectrowetting-based actuaticircuits," Journal of Microevol. 12(1), pp. 70-80, 2003.

[2] H. Ren, R. B. Fair, M. G.“Dynamics of electro-weSensors and Actuators B, vo

[3] R. Baviere, J. Boutet, anddroplet transport induced byMicrofluid Nanofluid, vol. 4

[4] P.-G. deGennes, F. Broch

"Capillarity and Wetting PPearls, Waves," Springer, N

[5] T. D. Blake, in Wettability,Marcel Dekker, New York.[6] T. D. Blake, A. Clarke, an

Investigation of ElectrostWetting,” Langmuir, vol. 16

[7] S. Schuetter, T. Shedd, K.“Measurements of the dyconditions relevant to imMicrolith., Microfab., M 023002, 2006.

[8] F. Li and F. Mugele, “Hoslippery: Contact angle hwith alternating voltage,” A92, pp. 244108, 2008.

CONTACT* W. Nelson: wyattnelson@ucla

solid) & 60 V (dashed).

y the DARPA HERMITough the UCLA MaterialsCTP). Experiments wereNASA Institute for CellISE) at UCLA.

d C.-J. Kim, "Creating,

erging liquid droplets byon for digital microfluidiclectromechanical Systems,

ollack, E. J. Shaughnessy,tting droplet transport,”l. 87, pp. 201-206, 2002.Y. Fouillet, “Dynamics of electrowetting actuation,”

, pp. 287-294, 2008.rd-Wyart, and D. Quere,

enomena Drops, Bubbles,ew York, 1st. ed., 2004.

J. C. Berg, Editor. 1993,. 251 - 309.d E. H. Stattersfield, “An

atic Assist in Dynamic(6), pp. 2928-2935, 2000.

Doxtator, and G. Nellis,namic contact angle for mersion lithography,” J.

crosyst, vol. 5(2), pp.

to make sticky surfacessteresis in electrowetting plied Physics Letters, vol.

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2009 trans nelson sliding resistance ewod droplet

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