Published: November 03, 2011

r 2011 American Chemical Society 14923 dx.doi.org/10.1021/la203320g | Langmuir 2011, 27, 14923–14929

ARTICLE

pubs.acs.org/Langmuir

Impact of Pinning of the Triple Contact Line onElectrowetting PerformanceRohini Gupta, Danica M. Sheth, Teno K. Boone, Arianne B. Sevilla, and Jo€elle Fr�echette*

Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States

bS Supporting Information

’ INTRODUCTION

Spreading (or actuation) of a conductive liquid drop on adielectric in response to an electric field (Figure 1a) is referred toas electrowetting on dielectric (EWOD),1�3 and is employed fordigital microfluidics,4,5 variable-focus lenses,6,7 and electronicdisplays.8,9 Recent needs for the design of low-power and efficientelectrowetting systems have triggered studies to understand themechanism and limitations of electrowetting. According toelectromechanics, the fringe fields distributed over a distancecomparable to the dielectric thickness away from the solid�liquidinterface (indicated by the red dashed box in Figure 1b) lead to anonuniform drop curvature. The electric field beyond the reddashed box (Figure 1b) is negligible, and the macroscopic dropshape away from the solid�liquid interface, therefore, correspondsto a spherical cap with an apparent (macroscopic) contact angle(θapp) whose variation with the applied potential is described bythe electrowetting equation (eq 1). The electrowetting equationoriginates from the quasi-static macroscopic balance of forces(excluding the pinning forces) that act on the liquid drop incontact with a charged dielectric parallel to the plane of thesurface. The electrostatic force at the triple contact line is zero,and the potential and charge density across the electric doublelayer is never sufficient to alter the solid�liquid interfacial energy.The balance of forces that act on the triple contact line, therefore,indicates that there is no change in the local (microscopic) contactangle (θlocal) with applied potential.10�14

cos θapp ¼ cos θlocal þ cV 2

2γlvð1Þ

In eq 1, γlv is the liquid�vapor interfacial energy, c is the capa-citance per unit area of the dielectric stack, and V is the applied

potential. The local angle (θlocal) represents the apparent anglemeasured at zero applied potential (or the initial angle), and isequal to Young’s angle (θY) for substrates with negligible contactangle hysteresis. Because of limitations in the optical resolutionof the imaging technique used to estimate contact angles, only theapparent angle and not the local angle are accessible in macroscopicelectrowetting experiments. The nonuniform drop curvature has,however, been observed experimentally by extending the distanceover which the fringe fields are distributed via the use of thickerdielectrics, combined with imaging at higher magnification.12

The performance of electrowetting systems is determined bythe fluid�surface interactions. Contact angle hysteresis is a mani-festation of the pinning of the triple contact line due to surfacechemical and physical heterogeneities. The presence of signifi-cant contact angle hysteresis (or appreciable difference betweenadvancing and receding angles) leads to a threshold potentialbelow which liquid actuation is inhibited,4,15�22 and adverselyaffects the reversibility of electrowetting response (or differencebetween apparent angles for increasing and decreasing poten-tials).18,23,24 Pinning also manifests itself as a stick�slip motionof the triple contact line for sessile drops,25 and adversely affectsthe switching speed.22Moreover, models for drop dynamics needto account for pinning to make reasonable predictions regardingthe correct shape and time scale of drop motion, splitting, andmerging.26�29 Electrowetting systems operating at the oil�waterinterface6,15,20,23,30�35 on low contact angle hysteresis sub-strates18,24,36�39 are, therefore, preferred to ensure reliability

Received: August 23, 2011Revised: October 1, 2011

ABSTRACT: Pinning of the triple contact line adversely affects electrowetting on dielectric.Electrowetting response of substrates with contact angle hysteresis ranging from 1� to 30� hasbeen characterized, and the results are interpreted within the framework of electromechanicscorrected for pinning. The relationship between contact angle hysteresis, threshold potentialfor liquid actuation, and electrowetting hysteresis is quantified. Our results demonstrate that amodified electrowetting equation, based on balance of forces (including the pinning forces)acting on the triple contact line and on the drop, describes the electrowetting response ofsubstrates with significant contact angle hysteresis. Finally, the surface properties of PDMSSylgard 184 were found to be influenced by the electric field.

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and reversibility. To expand the range of fluids and substrates forreliable electrowetting systems, it is important to gain a betterunderstanding of how pinning influences the electrowettingperformance.

Experiments where the triple contact line is forced to advanceor recede via the use of either tilted plane or drop expansion andcontraction have revealed that the magnitude of contact anglehysteresis in electrowetting systems is independent of the appliedpotential,40�42 and can be reduced significantly via the use ofalternating applied potential.41,43 This independence has beenexplained via electromechanics by incorporating the pinningforces in the macroscopic balance of forces that act on thedrop.41 These experiments, however, did not address the pre-sence (or magnitude) of the threshold potential for actuation orthe degree of irreversibility of the electrowetting response. Modelsthat take into account pinning have been proposed to predict thethreshold potential for actuation of translating drops.18,20 How-ever, these models are not applicable for sessile drops. Moreover,neither of these models has been directly interpreted to predictthe electrowetting response beyond the threshold potential or toquantify the impact of pinning on the reversibility of electro-wetting. Therefore, systematic experiments and their analysisaimed at quantifying the correlation between pinning and electro-wetting performance are needed.

In this Article, we establish the quantitative relationship betweenthe contact angle hysteresis, the threshold potential for liquidactuation, and the electrowetting hysteresis for sessile drops. Wehave characterized the electrowetting response of substrates (115samples) with contact angle hysteresis ranging from 1� to 30�using the modified electrowetting equation, which is based onthe balance of forces (including the pinning forces) that act onthe triple contact line and on the drop. Our results reveal thatthe electrowetting hysteresis and the contact angle hysteresis areequal in magnitude. We also observe that the electrowetting re-sponse deviates from the modified electrowetting equation intwo transition regions: a threshold region prior to the onset ofdrop spreading and a transition region as the drop begins torecede with a decrease in applied potential. We show that thesetransition regions correspond to a change in the local angle froman initial angle to the advancing angle necessary for the triple con-tact line to advance, and from the advancing angle to the recedingangle necessary for the triple contact line to recede, respectively.The magnitude of the threshold potential for actuation observedexperimentally during this study is consistent with that estimatedusing the modified electrowetting equation. Finally, we observethat for PDMS Sylgard 184, the contact angle hysteresis is largerthan the electrowetting hysteresis.

’MATERIALS AND METHODS

Hydrophobic Dielectrics. Polymer films (thickness: 6�12 μm)were used to systematically vary contact angle hysteresis (see Table 1).For spin-coatable polymers (polydimethylsiloxane and Cytop), a cleanp-doped silicon (100) wafer (WRS materials) coated with 20 nm ofchromium (99.9% purity, Kurt J. Lesker) followed by 200 nm of silver(99.999% purity, Alfa Aesar) acted as both support and electrode.Commercially available 10 μm thick polytetrafluoroethylene (PTFE) films(Goodfellow Corporation, εr = 2.1) were used as is, and the electricalcontact was made via 200 nm of silver thermally evaporated directly onthe rear of the films.

Two varieties of polydimethylsiloxane (PDMS) were used: PDMSSylgard 184 (two-part elastomer containing base and curing agent inratio 10:1 by weight, Dow Corning, εr = 2.65) and soft elastomer PDMS7-9600 (two-part elastomer containing base and curing agent in ratio 1:1by weight, Dow Corning mixed with copolymer HMS-082, Gelest Inc.).The polymermixtures were degassed for 15min prior to spin-coating (slowspreading step of 500 rpm for 30 s and fast smoothing step of 5000 rpmfor 60 s), followed by curing for 4 h at 80 �C in air for Sylgard 184 and 1 hat 140 �C in air for 7-9600. It has been reported that the presence of silicafiller particles in PDMS (e.g., Sylgard 184) leads to contact anglehysteresis ranging from 20� to 40� as compared to <5� for filler-freePDMS (e.g., 7-9600).44�46 Cytop (CTL-809M, Asahi Co., εr = 2.1) wasdeposited on both PDMS Sylgard 184 and PDMS 7-9600 via spin-coating (slow spreading step of 500 rpm for 10 s and fast smoothing stepof 4000 rpm for 20 s), followed by annealing for 15 min at 180 �C undervacuum. Multiple coatings of Cytop were used to vary the contact anglehysteresis between 2� and 10�. Finally, the polymer films were impreg-nated with silicone oil (Fisherbrand) by soaking them in oil overnight. Aclean kimwipe was used to absorb the excess oil, while leaving behind amicroscopic film of oil sufficient enough to reduce the contact angle hys-teresis. Care was taken not to rub off against the surface of the polymerfilm so as to avoid damage. The films upon impregnation with silicone oilvia this protocol exhibited negligible contact angle hysteresis (e1�).

Figure 1. Schematic of (a) the setup employed for macroscopic electrowetting measurements, and (b) the nonuniform drop curvature resulting fromthe forces (excluding pinning forces) that act on the triple contact line and on the drop.

Table 1. Advancing Angle (θadv), Receding Angle (θrec), andContact Angle Hysteresis for Water on the HydrophobicDielectrics Used

hydrophobic dielectrics

contact angle

hysteresis

advancing

angle

receding

angle

PDMS Sylgard 184 11�31� 108�116� 84�102�PTFE 14�23� 111�116� 93�98�Cytop on PDMS 2�10� 110�115� 100�108�PDMS 7-9600 3� 107� 104�films soaked in silicone oil e1� 105�111� 104�110�

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Contact Angle Hysteresis Measurements. A goniometer (FirstTenAngstroms FTA125) was used to analyze the drop profile and estimatethe contact angle of water on polymer films with a confidence of (1�.Contact angle hysteresis was measured using the captive needle approach(expansion and contraction of liquid drop).47 A syringe pump (Aladdin,World Precision Instruments) was used to pump and withdraw deionizedwater at a rate of 9 μL/min via a pulled-glass capillary needle (tip innerdiameter: 5 μm) inserted in the drop. A nonspherical drop shape analysiswas used to estimate the contact angle of water drops (approximately10μL) so as to avoid any distortions thatmay arise due to gravity or the glasstip inserted in the drop. Contact angle hysteresis was estimated from thedifference between the advancing and receding angles (see SupportingInformation, Figure S1 for a typical measurement).Electrowetting Measurements. The apparent angle of deion-

ized water drops (approximately 10 μL) were measured with a confidenceof(1� as a function of the potential applied between the silver contactingthe rear of the polymer film (ground) and a platinum wire inserted insidethe drop (Figure 1a). The applied potential was altered in increments of10 V every 5 s, and was controlled using a power supply (Burleigh HighVoltage DC Op Amp PZ-70). The current was monitored using anammeter (Keithley 177 microvolt DMM). Negligible current (1�2 nA)was observed throughout the experiments. The electrowetting responsewas characterized by measuring apparent angles as a function of appliedpotential as the drop spreads and recedes with an increase and decreasein applied potential, respectively (Figure 2). Throughout this study, themaximum applied potential was chosen to avoid contact angle saturation.

It has been shown using molecular dynamics simulations that aprecursor film can propagate ahead of and faster than the triple contactline for a drop spreading on metal when subjected to an electric field.48

However, no significant precursor film has been observed for electro-wetting on dielectric using spectroscopic ellipsometry.49 Spreading ofthe drop during electrowetting on dielectric is not accompanied by anychange in the local angle, which may prevent the formation andpropagation of the precursor film. We have, therefore, not taken intoconsideration the precursor film while understanding the impact ofpinning on electrowetting performance.

’RESULTS AND DISCUSSION

Pinning of the triple contact line has profound effects on theelectrowetting performance, as evident from the comparison of

the electrowetting response of substrates with negligible (Figure 2a)and significant (Figure 2b) contact angle hysteresis. A reversibleelectrowetting response is observed for the substrate with neg-ligible contact angle hysteresis (Figure 2a): the apparent angle atany given applied potential is the same whether the drop isspreading or receding (i.e., if the applied potential is increased ordecreased). The three key features of electrowetting responsecaused by pinning that are observed in Figure 2b are (a) athreshold potential (Vth) for the onset of drop spreading (“O” inregion I), (b) hysteresis between spreading and receding curves(shaded region), and (c) a transition region as the drop begins torecede with a decrease in applied potential (“O” in region III).Although the data shown in Figure 2a,b are for individual samples,these features were consistently observed for 15 samples withnegligible (e1�) and 100 samples with significant (2�30�) con-tact angle hysteresis (unless otherwise mentioned).

To compare the measured electrowetting response with theelectrowetting equation (eq 1), the local angle and the capaci-tance of the dielectric stack were used as the fitted parameters.Throughout this study, the fitted dielectric thickness (or capacitance)was in agreement with the thickness measured independently(see Supporting Information, Figure S2). The electrowetting re-sponse of substrates with negligible contact angle hysteresis isreversible, and well-described by the electrowetting equation(solid line in Figure 2a), with the local angle corresponding toYoung’s angle (θY) for both spreading and receding curves, which isin agreement with the published work showing the validity ofeq 1.1 Young’s angle used in the electrowetting equation is unique;therefore, eq 1 does not predict the separate spreading and recedingelectrowetting curves observed for substrates with significantcontact angle hysteresis in Figure 2b. Consequently, a modifiedelectrowetting equation is needed to predict the entire electro-wetting response (spreading and receding curves, as well as thetransition regions).

We use a modified electrowetting equation based on electro-mechanics obtained by incorporating the pinning forces (fp =γlv(cos θY � cos θadv or rec)) in the quasi-static balance of forcesthat act on the triple contact line and on the drop at each appliedpotential. The pinning forces are assumed to be independent ofthe applied potential for electrostatic force that acts away from

Figure 2. Electrowetting response of substrates with contact angle hysteresis of (a) 1� (Cytop on PDMS Sylgard 184 impregnated with oil) and (b) 8�(Cytop on PDMS Sylgard 184). The solid lines represent the (a) electrowetting equation and (b)modified electrowetting equation. The “O” in regions Iand III represent the threshold and transition regions, respectively, and the shaded region represents the hysteresis between the spreading and recedingcurves.

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the triple contact line.41 A similar approach has been used topredict the dependence of advancing (or receding) angle on theapplied potential in electrowetting systems, as measured byforcing the triple contact line to advance (or recede) via theuse of drop expansion (or contraction). This dependence isconsistent with the electrowetting equation such that the localangle is equal to the advancing or receding angle, implying thatcontact angle hysteresis is independent of the applied potential inelectrowetting systems.41 This interpretation of the force bal-ance, however, does not address the presence (or magnitude) ofthe threshold potential for actuation or the degree of irreversi-bility of the electrowetting response as the applied potential isincreased or decreased. We hypothesize that during electro-wetting on dielectric, the local angle, instead of being constantand equal to Young’s angle, must remain equal to the advancing(or receding) angle for increasing (or decreasing) potential. Thishypothesis accounts for the pinning of the triple contact line dueto surface chemical and physical heterogeneities, which forces thelocal angle to vary between the advancing and receding angles,50�52

and leads to a modified electrowetting equation that describesthe dependence of the apparent angle on applied potential forboth increasing and decreasing potentials (see the SupportingInformation for derivation):

cos θapp ¼ cos θadv or rec þ cV 2

2γlvð2Þ

This modified electrowetting equation predicts that the apparentangle at the end of an electrowetting cycle (once the potential isremoved) should be equal to the receding angle measuredindependently, and that the electrowetting hysteresis and thecontact angle hysteresis should be equal in magnitude.

We test the validity of this modified electrowetting equationbased on experimentally measured electrowetting response ofsubstrates with contact angle hysteresis ranging from 2� to 30�combined with independent contact angle hysteresis measure-ments. To compare the spreading curve with the modifiedelectrowetting equation (eq 2), the local angle and the capaci-tance of the dielectric stack were used as the fitted parameters.The apparent angles for potentials less than the thresholdpotential (Vth) required for the onset of drop spreading (“O”in region I in Figure 2b) were excluded from the fits to eq 2.Beyond the Vth, the apparent angles measured for increasingpotential agree with the modified electrowetting equation, suchthat the fitted local angle (or fitted advancing angle) correspondsto the independently measured advancing angle (θadv). Thereceding curve is compared with the modified electrowettingequation (eq 2) using the value for capacitance obtained from thespreading curve and the apparent angle measured at the end ofthe electrowetting cycle as the local angle. We refer to the apparentangle measured at the end of the electrowetting cycle as theelectrowetting receding angle. Past the transition region (“O” inregion III in Figure 2b), the apparent anglesmeasured for decreasingpotential agree with the modified electrowetting equation with thelocal angle equal to the electrowetting receding angle. We alsoobserve that the electrowetting receding angle is equal to theindependently measured receding angle (θrec). These observationsare found to be independent of the maximum applied potential (seethe electrowetting measurements for maximum applied potentialof 100 and 150 V shown in Figure 3), the number ofelectrowetting cycles (Figure 4), and the initial angle prior toelectrowetting (Figure 5). The magnitude of threshold potential

for actuation, however, depends on the initial angle (apparentangle at 0 V) for each run in accordance with eq 3.

For potentials where the apparent angles for both spreadingand receding curves agree with eq 2 (region II in Figure 2b), weconsistently observed that the electrowetting hysteresis (or thedifference between the inferred local angles when the appliedpotential is increased or decreased) is equal in magnitude to thecontact angle hysteresis (see Supporting Information, Figure S4for electrowetting response in linear form). To estimate theelectrowetting hysteresis in region II (Figure 2b), we calculatethe difference between the fitted advancing angle and the elec-trowetting receding angle. Figure 6 shows that the electrowettinghysteresis and contact angle hysteresis are equal in magnitude,and this equality is independent of the magnitude of contactangle hysteresis and the nature of substrate. Each of the datapoints and the corresponding vertical error bars in Figures 6 and

Figure 3. Electrowetting response of a substrate with contact anglehysteresis of 6� (Cytop on PDMS 7-9600) for a drop at the advancingangle prior to electrowetting for different maximum applied potentials.Note the absence of a threshold potential. The solid lines represent themodified electrowetting equation. For 0�50 V, the apparent angle atmaximum applied potential and as such the electrowetting recedingangle are larger than the independently measured receding angle.

Figure 4. Electrowetting response of a substrate with contact anglehysteresis of 8� (Cytop on PDMS Sylgard 184) for multiple cyclesperformed with the same drop. The solid lines represent the modifiedelectrowetting equation.

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7 represent the average and standard deviation, respectively, ofmultiple measurements performed on a given sample. The hori-zontal error bars represent the accuracy of estimating contactangle. We will next discuss the threshold and transition regions,where the electrowetting response deviates from the modifiedelectrowetting equation (“O” in regions I and III in Figure 2b).

The inhibition of drop spreading (“O” in region I in Figure 2b)is expected to occur because the triple contact line remainspinned as the local angle increases from its initial value to theadvancing angle. When the triple contact line remains pinned, anincrease in electrostatic force would tend to decrease the apparentangle, while increasing the local angle to the advancing angle.Combined with the constant volume constraint, pinning couldresult in a negligible (or no) change in the apparent angle with anincrease in applied potential for potentials belowVth. BeyondVth,the triple contact line is free to move with the local angle beingequal to the advancing angle for increasing potentials, and theapparent angle follows the modified electrowetting equation.

Therefore, the threshold potential can be estimated from themodified electrowetting equation for the apparent angle beingequal to the initial angle (θinitial).

Vth ¼ 2γlvcðcos θinitial � cos θadvÞ

� �1=2ð3Þ

The validity of eq 3 is confirmed by the agreement between theestimated and experimentally observed Vth throughout thisstudy. We also verify the relationship between Vth and the initialangle by using liquid drops at different initial angles (i.e., theadvancing angle, the receding angle, and any angle between thetwo) prior to electrowetting (Figure 5). Electrowetting responseof a liquid drop at the advancing angle prior to electrowettingshould not exhibit a threshold potential, even in the presence ofsignificant contact angle hysteresis. We indeed observe an absenceof threshold potential for a drop at the advancing angle prior toelectrowetting (Figure 3), which confirms the origin of the thresh-old potential.

Similarly, the transition region as the drop begins to recedewith a decrease in applied potential (“O” in region III in Figure 2b)is expected to occur because the triple contact line remains pinned asthe local angle changes from the advancing to the receding angle.When the triple contact line remains pinned, a decrease in elec-trostatic force would tend to increase the apparent angle, whiledecreasing the local angle to the receding angle. Combined withthe constant volume constraint, pinning could result in a negli-gible (or no) change in the apparent angle with a decrease inapplied potential. We do, however, observe an increase in theapparent angle that deviates from the modified electrowettingequation, implying that it may be possible for the apparent angleto increase as the local angle decreases from the advancing to thereceding angle, while satisfying the constant volume and pinnedtriple contact line constraints. This is possibly due to the nonuni-form curvature of the drop or the presence of the electrode wirein the drop. Past this transition region, the triple contact line isfree to recede such that the local angle remains equal to thereceding angle for decreasing potentials, and the apparent anglefollows themodified electrowetting equation. The electrowettingreceding angle should be equal to the receding angle measured

Figure 5. Electrowetting response of a substrate with contact anglehysteresis of 6� (Cytop on PDMS 7-9600) for different initial anglesprior to electrowetting. The solid lines represent the modified electro-wetting equation.

Figure 6. The electrowetting hysteresis and the contact angle hysteresisare equal in magnitude, independent of the magnitude of contact anglehysteresis and the nature of substrate (PTFE, 9; Cytop on PDMS, b;PDMS 7-9600, 0; and films soaked in silicone oil, O). The solid linerepresents a line of slope 1.

Figure 7. Unlike for PTFE, Cytop on PDMS, PDMS 7-9600, and filmssoaked in silicone oil (b), the electrowetting receding angle is greaterthan the measured receding angle for PDMS Sylgard 184 (O). The solidline represents a line of slope 1.

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independently, which is consistent with our experiments (seeFigures 2�5 and “b” in Figure 7).

We observe that when the apparent angle at the maximumapplied potential is greater than the receding angle, the electro-wetting receding angle does not agree with the receding anglemeasured independently (see the electrowetting measurementfor maximum applied potential of 50 V in Figure 3). This dis-agreement is expected. For the drop to recede, the apparent anglewould have to decrease to the receding angle, while the localangle decreases from the advancing to receding angle. This iscontrary to the fact that a decrease in electrostatic force increasesthe apparent angle, while decreasing the local angle to the re-ceding angle. We, therefore, observe negligible (or no) change inthe apparent angle upon decrease in the applied potential, as seenfor the receding curve between 50 and 0 V in Figure 3. Thisspecial case was excluded from the estimation of the electrowett-ing hysteresis shown in Figure 6 and the electrowetting recedingangle shown in Figure 7.

For PDMS Sylgard 184, however, the electrowetting recedingangle is consistently larger than the receding angle measuredindependently (“O” in Figure 7). A higher electrowetting reced-ing angle implies that for decreasing potential the triple contactline recedes at a local angle higher than the independentlymeasured receding angle. To test if the applied potential alteredthe receding angle, the triple contact line was forced to recede at agiven applied potential by withdrawing liquid out of the drop,followed by the removal of potential. The local angle at the end ofthis measurement was indeed observed to be equal to the electro-wetting receding angle (see Supporting Information, Figure S5a).Furthermore, we observe that for multiple electrowetting cycles,the apparent angle as a function of increasing potential beyondthe threshold fits the modified electrowetting equation with thelocal angle less than the advancing angle for all but the first cycle(see Supporting Information, Figure S5b). These observed anoma-lies are not caused by changes in the bulk dielectric properties ofPDMS Sylgard 184 with applied potential. Indeed, these anoma-lies disappear for PDMS Sylgard 184 coated with Cytop or im-pregnated with silicone oil, which exhibit electrowetting responseconsistent with the theoretical expectations. Moreover, no perma-nent changes in interfacial properties due to the electric field havebeen observed: the contact angle hysteresis values measuredbefore and after an electrowetting cycle are equal. This impliesthat in the case of PDMS Sylgard 184, the heterogeneity andinterfacial properties of the solid�liquid interface may be rever-sibly affected by the applied potential. PDMS Sylgard 184 isknown to contain silica filler particles in the near surface region thatnot only improve its strength but also impart it high contact anglehysteresis.44�46 We suspect that there is a potential-inducedreorganization of silica filler particles near the solid�liquidinterface, which may lead to a lower effective contact anglehysteresis. There is no direct evidence available to support ourhypothesis, but it is consistent with the fact that PDMS 7-9600(soft elastomer without the silica filler particles) exhibits lowcontact angle hysteresis, and electrowetting response in agree-ment with the theoretical expectations.

’CONCLUSIONS

We characterized the electrowetting response of substrateswith hysteresis ranging from 1� to 30� to quantify the relation-ship between the contact angle hysteresis, the threshold potentialfor liquid actuation, and the electrowetting hysteresis. We establish

that the electrowetting hysteresis and the contact angle hysteresisare equal in magnitude, and use a modified electrowetting equationbased on the balance of forces (including pinning forces) that acton the triple contact line and on the drop to explain this equality.There exist two transition regions where the electrowettingresponse deviates from the modified electrowetting equation.The first is the threshold region prior to the onset of dropspreading, which is consistent with a change in the local anglefrom the initial angle to the advancing angle to allow for the triplecontact line to advance. The apparent angle as a function ofapplied potential beyond the threshold potential fits themodifiedelectrowetting equation with the local angle being equal to theadvancing angle. The second is the transition region as the dropbegins to recede with a decrease in applied potential, which weattribute to the change in the local angle from the advancingangle to the receding angle to allow for the triple contact line torecede. Past this transition region, the apparent angle as afunction of applied potential fits the modified electrowettingequation with the local angle being equal to the electrowettingreceding angle, which agrees with the measured receding angle.Electrowetting response is independent of the initial angle priorto electrowetting, the number of electrowetting cycles, themaximum applied potential, the nature of substrate, and themagnitude of contact hysteresis. The magnitude of thresholdpotential for actuation, however, depends on the initial angle. ForPDMS Sylgard 184, the contact angle hysteresis is larger than theelectrowetting hysteresis, which we suggest is due to the poten-tial-induced reorganization of silica filler particles near thesolid�liquid interface, and is an exception to the responsepredicted by the modified electrowetting equation.

’ASSOCIATED CONTENT

bS Supporting Information. Contact angle hysteresis mea-surement, comparison of fitted and measured dielectric thicknesses,derivation of the modified electrowetting equation, electrowett-ing response in linear form, and the results from electrowettingmeasurements for PDMS Sylgard 184 for multiple cycles, andwhere the triple contact line is forced to recede at a given appliedpotential, followed by the removal of potential. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Telephone: (410) 516-0113. Fax: (410) 516-5510. E-mail:jfrechette@jhu.edu.

’ACKNOWLEDGMENT

This material is based upon work supported by the NationalScience Foundation under Grant CMMI-0709187 and 3M Corp.We thank Mark Robbins and Patricia McGuiggan for helpfuldiscussions, and acknowledge the contribution of Miriam Vish-niac toward optimizing the contact angle hysteresis measurementprotocol.

’REFERENCES

(1) Mugele, F.; Baret, J. C. J. Phys.: Condens. Matter 2005, 17, R705–R774.

(2) Shamai, R.; Andelman, D.; Berge, B.; Hayes, R. Soft Matter 2008,4, 38–45.

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(3) Gras, S. L.; Mahmud, T.; Rosengarten, G.; Mitchell, A.; Kalantar-Zadeh, K. ChemPhysChem 2007, 8, 2036–2050.(4) Pollack, M. G.; Fair, R. B.; Shenderov, A. D. Appl. Phys. Lett.

2000, 77, 1725–1726.(5) Squires, T. M.; Quake, S. R. Rev. Mod. Phys. 2005, 77, 977–1026.(6) Berge, B.; Peseux, J. Eur. Phys. J. E 2000, 3, 159–163.(7) Kuiper, S.; Hendriks, B. H. W. Appl. Phys. Lett. 2004, 85,

1128–1130.(8) Hayes, R. A.; Feenstra, B. J. Nature 2003, 425, 383–385.(9) Zhou, K.; Heikenfeld, J.; Dean, K. A.; Howard, E. M.; Johnson,

M. R. J. Micromech. Microeng. 2009, 19, 065029-1–12.(10) Buehrle, J.; Herminghaus, S.; Mugele, F. Phys. Rev. Lett. 2003,

91, 86101-1–4.(11) Jones, T. B. J. Micromech. Microeng. 2005, 15, 1184–1187.(12) Mugele, F.; Buehrle, J. J. Phys.: Condens. Matter 2007, 19,

375112-1–20.(13) Mugele, F. Soft Matter 2009, 5, 3377–3384.(14) Gupta, R.; Olivier, G. K.; Frechette, J. Langmuir 2010,

26, 11946–11950.(15) Pollack, M. G.; Shenderov, A. D.; Fair, R. B. Lab Chip 2002,

2, 96–101.(16) Cho, S. K.; Moon, H. J.; Kim, C. J. J. Microelectromech. Syst.

2003, 12, 70–80.(17) Cooney, C. G.; Chen, C. Y.; Emerling, M. R.; Nadim, A.;

Sterling, J. D. Microfluid. Nanofluid. 2006, 2, 435–446.(18) Berthier, J.; Dubois, P.; Clementz, P.; Claustre, P.; Peponnet, C.;

Fouillet, Y. Sens. Actuators, A 2007, 134, 471–479.(19) Fair, R. B. Microfluid. Nanofluid. 2007, 3, 245–281.(20) Song, J. H.; Evans, R.; Lin, Y. Y.; Hsu, B. N.; Fair, R. B.

Microfluid. Nanofluid. 2009, 7, 75–89.(21) Chang, J. H.; Choi, D. Y.; Han, S.; Pak, J. J. Microfluid.

Nanofluid. 2010, 8, 269–273.(22) Yang, S.; Zhou, K.; Kreit, E.; Heikenfeld, J. Appl. Phys. Lett.

2010, 97, 143501-1–3.(23) Janocha, B.; Bauser, H.; Oehr, C.; Brunner, H.; G€opel, W.

Langmuir 2000, 16, 3349–3354.(24) Wan, Z. L.; Zeng, H.; Feinerman, A. J. Fluids Eng. 2007,

129, 388–394.(25) Sen, P.; Kim, C. J. Langmuir 2009, 25, 4302–4305.(26) Walker, S. W.; Shapiro, B. J. Microelectromech. Syst. 2006,

15, 986–1000.(27) Arzpeyma, A.; Bhaseen, S.; Dolatabadi, A.; Wood-Adams, P.

Colloids Surf., A 2008, 323, 28–35.(28) Walker, S. W.; Shapiro, B.; Nochetto, R. H. Phys. Fluids 2009,

21, 102103-1–16.(29) Schertzer, M. J.; Gubarenko, S. I.; Ben-Mrad, R.; Sullivan, P. E.

Langmuir 2010, 26, 19230–19238.(30) Quilliet, C.; Berge, B. Curr. Opin. Colloid Interface Sci. 2001,

6, 34–39.(31) Berry, S.; Kedzierski, J.; Abedian, B. J. Colloid Interface Sci. 2006,

303, 517–524.(32) Kedzierski, J.; Berry, S. Langmuir 2006, 22, 5690–5696.(33) Berry, S.; Kedzierski, J.; Abedian, B. Langmuir 2007, 23, 12429–

12435.(34) Maillard, M.; Legrand, J.; Berge, B. Langmuir 2009, 25, 6162–6167.(35) Paneru, M.; Priest, C.; Sedev, R.; Ralston, J. J. Am. Chem. Soc.

2010, 132, 8301–8308.(36) Welters, W. J. J.; Fokkink, L. G. J. Langmuir 1998, 14,

1535–1538.(37) Verheijen, H. J. J.; Prins, M. W. J. Langmuir 1999, 15,

6616–6620.(38) Verheijen, H. J. J.; Prins, M. W. J. Rev. Sci. Instrum. 1999, 70,

3668–3673.(39) Seyrat, E.; Hayes, R. A. J. Appl. Phys. 2001, 90, 1383–1386.(40) Vallet, M.; Berge, B.; Vovelle, L. Polymer 1996, 37, 2465–2470.(41) Li, F.; Mugele, F. Appl. Phys. Lett. 2008, 92, 244108-1–3.(42) Nelson, W. C.; Sen, P.; Kim, C. J. Langmuir 2011, 27,

10319–10326.

(43) ’t Mannetje, D. J. C. M.; Murade, C. U.; van den Ende, D.;Mugele, F. Appl. Phys. Lett. 2011, 98, 014102-1–3.

(44) Owen,M. J.; Smith, P. J. J. Adhes. Sci. Technol. 1994, 8, 1063–1075.(45) Kim, J.; Chaudhury, M.; Owen, M. IEEE Trans. Dielectr. Electr.

Insul. 1999, 6, 695–702.(46) Efimenko, K.; Wallace, W. E.; Genzer, J. J. Colloid Interface Sci.

2002, 254, 306–315.(47) de Gennes, P. G.; Brochard-Wyart, F.; Qu�er�e, D. Capillarity

and Wetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer Verlag:New York, 2004; Chapter 3.

(48) Yuan, Q. Z.; Zhao, Y. P. Phys. Rev. Lett. 2010, 104, 246101-1–4.(49) Decamps, C.; DeConinck, J.Langmuir 2000, 16, 10150–10153.(50) Joanny, J. F.; de Gennes, P. G. J. Chem. Phys. 1984, 81, 552–562.(51) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827–863.(52) Bonn, D.; Eggers, J.; Indekeu, J.; Meunier, J.; Rolley, E. Rev.

Mod. Phys. 2009, 81, 739–805.