2009 Trans Nelson Sliding Resistance EWOD Droplet
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Transcript of 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
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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
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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.
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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
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[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
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[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
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2017