Journal of Colloid and Interface Science 363 (2011) 640–645

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Journal of Colloid and Interface Science

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Effect of submicron particles on electrowetting on dielectrics (EWOD)of sessile droplets

Debapriya Chakraborty a, Gogineni Sai Sudha b, Suman Chakraborty a, Sunando DasGupta b,⇑a Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721 302, Indiab Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721 302, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 February 2011Accepted 22 July 2011Available online 31 July 2011

Keywords:EWODSubmicron particlesContact anglesContact angle saturationDroplet

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.07.077

⇑ Corresponding author.E-mail address: sunando.dasgupta@gmail.com (S.

The present study elucidates the effects of included submicron-sized particles on the wetting behavior ofsessile droplets under the influence of applied electric field in an electro-wetting-on-dielectric (EWOD)configuration. A thermodynamic description using an energy minimization approach is used to analyzethe experimental results related to the effects of the included particles on the EWOD phenomenon, con-sidering the effects of line tension as well. The effects of particle size and concentration on interfacialareas are included in the model to analyze the wetting characteristics. Experiments are also conductedwith submicron-sizes latex beads, in an effort to elucidate the related phenomena. It is further postulatedthat these beads act as suspended dielectrics in the droplet, thereby mimicking a system of two capaci-tors in series. An effective electrical permittivity of the composite medium is used to study the experi-mental results related to contact angle changes at different concentrations and diameters of submicronparticles in the droplet.

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1. Introduction

Rapid developments in the field of biomedical, bio-technologi-cal and micro-electronics applications have resulted in extensiveuse of droplet based systems in miniaturized fluidic applications,commonly known as Digital Microfluidics [1,2]. Such micro-scalesystems are often characterized by the presence of solid–liquid–vapor triple junctions [3] and the motion of the contact line formedby two immiscible fluids and the solid boundary. The shape of a li-quid droplet on a surface is determined by a delicate balance be-tween the solid–liquid, liquid–vapor and solid–vapor interfacialtensions present in the three phase contact line, morphology ofthe underlying solid surface, and the volume and composition ofthe liquid at equilibrium [4]. The contact angle, defined as the localtangent at the junction of three phase contact line, is the com-monly observable parameter governing the drop shape which isan implicit indicator of the wettability and nature of the substratewith respect to the liquid. Local modulation of contact angle pro-vides a flexible means of flow actuation catering to the needs ofseveral microfluidic systems, where the favored scaling [5] of thesurface forces in comparison to the other body forces over reducedlength scales plays a pivotal role. Different methods like electrical,chemical [6], thermal [7], electrochemical [8], and photochemical[9] have been used to alter the contact angle of the three phase

ll rights reserved.

DasGupta).

contact line. The electrical method probably is the most promisingone for lab-on-a-chip based microfluidic applications, offering thepromise of high energy efficient and inherently integrable compactelectromechanical systems.

On application of an electrical potential, the interfacial tensiongets modified which leads to an asymmetric deformation of themeniscus at the two ends and thus a motion of droplet can be actu-ated. The principle of altering the wetting properties of fluid bymodifying the interfacial tension of the contact line with the appli-cation of electric potential is commonly known as electrowetting[10]. However, electrochemical reaction between the electrodeand the aqueous medium, leading to current flow from the sub-strate to the solution, restricts the application of higher voltages.In order to avoid such undesirable reactions, the liquid and theelectrodes are separated by a thin dielectric layer and such a con-figuration is commonly known as electrowetting on dielectric orEWOD [11–21]. The dielectric serves both to block the electrontransfer and also to provide a hydrophobic surface that enableslarge changes in contact angle. The change in contact angle is afunction of the thickness and permittivity of the dielectric.

Fluid elements with particle inclusion, mostly nano-scale parti-cles in suspended forms (also known as nanofluids), have been stud-ied in the literature, with a motivation of achieving high transportcoefficients as well as imposing tuneable controllability on the resul-tant interfacial phenomena. Interestingly, the well-established con-cepts of spreading and adhesion of simple liquids do not apply tofluids containing particles of nanometer dimensions [22]. Asolid-like ordering of suspended spheres occurs in the confined

D. Chakraborty et al. / Journal of Colloid and Interface Science 363 (2011) 640–645 641

three-phase contact region at the edge of the spreading fluid, whichtends to become more disordered and fluid-like towards the bulkphase. The pressure arising from such colloidal ordering in the con-fined region also enhances the spreading behavior of nanofluids. Onthe contrary, the behavior for non-wetting fluids with suspendedparticles has not been extensively studied till date. There are a fewinvestigations about the interfacial effects during evaporation ofthe droplet of nanofluids. The presence of nanoparticles leads to areduction in the evaporation rate compared to a simple fluid. Thedeposition of nanoparticles into the triple contact line wedge duringevaporation causes a greater pinning of the nanofluid droplets [23].The contact angle of a sessile droplet has been found to be decreasingwith decrease in the particles size [24]. Apart from the size, concen-tration also plays an important on the contact angle. Nanoparticles ofsmaller size leads to larger changes in contact angle at the same massconcentration [25]. EWOD has been studied for nanofluid comprisedof an aqueous suspension of bismuth telluride nanoparticles cappedwith thioglycolic acid (TGA) [26]. Nanofluid droplets have beenobserved to exhibit enhanced stability with the absence of contactangle saturation. Surface tension lowering and behavior of theparticles has been studied for colloidal particle suspension nearthe interface [27,28]. However, no study has yet been reported inthe literature on the explanation of the fundamental interfacialphenomena, including the contact line thermodynamics, associatedwith the incorporation of submicron-sized particles in a sessiledroplet in EWOD configuration.

In the present work, the variation of contact angle of dropletscontaining submicron-sized particles under the influence of ap-plied electric field in EWOD configuration is investigated. Moti-vation behind this study stems from the transport of biologicalmacromolecules like suspended cells, coated submicron beadsand DNA fragments. The effects of droplet based cell manipula-tion [29] have been studied for cell vitality and cytotoxicity as-say by EWOD which has been proven to be more sensitivethan the macroscale methods. The sizes of these cells are typi-cally in the micron or submicron range, altering the characteris-tics of conventional EWOD. In another context, bead basedimmunosorbent assay systems have been developed with humansecretory immunoglobulin A (s-IgA) adsorbed on the bead sur-face for reaction with colloidal gold conjugated anti-s-IgA anti-body [30]. Such bead based assays using droplet, in contrast tothe continuous flow, have proven to be highly specific and effi-cient from economic perspectives. Keeping such significantimplications in view, here we explore the changes in contact an-gle for a droplet containing submicron particles under the effectof electric fields. However, unlike previous studies in this area,the effects of varying concentrations and diameters of the sus-pended particles are addressed herein. Conventionally, the sus-pended nanoparticles contribute only to the increase of theeffective electrical permittivity and thermal conductivity. How-ever, in the present context, we have studied the effects of ap-plied potential on the three phase contact line of a droplet inpresence of submicron particles. A thermodynamic descriptionusing an energy minimization approach is developed to analyzethe experimental results. The effects of line tension and modifiedinterfacial areas due to the submicron particles are included inthe model. The theoretical trends are consistent with the exper-imental results and the significance of the model parameters isestablished. It is postulated that the submicron spherical latexbeads act as suspended dielectric in the droplet thereby mimick-ing a system of two capacitors in series. An effective electricalpermittivity of the composite medium is used to examine thevoltage drop across the insulating layer and the submicron par-ticles and utilized to analyze contact angle changes and its sat-uration at varying concentrations and sizes of the suspendedparticles.

2. Materials and methods

Glass slides were cleaned in Piranha solution (1:1 H2O2:H2SO4)for 5 min, rinsed in DI water and dried using nitrogen gas jet. Thesecleaned slides were used to coat gold to serve as one of the elec-trodes for the EWOD configuration. Chrome was deposited overthe glass substrate using a vacuum coating system (Model 12A4-D, HindHiVac, India) which uses evaporation induced coating onsubstrate to a thickness of 150 nm in order to improve the adhe-sion of gold. 22 carat gold was deposited in a similar manner overchrome coated glass slide with a thickness of 0.1 lm. Sylgard-184(a two-part PDMS elastomer; Dow Chemicals, USA) consisting of anoligomer and cross linking agent was used to prepare the dielectriclayer by mixing them in 10:1 ratio by weight. The mixed partswere desiccated to eliminate the presence of trapped bubbleswhile mixing. A small amount of sylgard mixture was poured onthe glass substrate for deposition of a thin layer over the glassusing a spin coater which was spun at 600 rpm for 10 s for initialramming, and then gradually increased to 3000 rpm for 60 s. Thesylgard coated substrate was cured in an oven at 70 �C for 2 h.The thickness of the deposited film was measured to be 28.3 lmusing profilometer (DekTak, Veeco, USA). The Sylgard coated sur-face was characterized using Atomic Force Microscope to charac-terize the quality of the coated film (see Supplementary materialfig. S1). Submicron polystyrene beads (Sigma–Aldrich, India) ofdiameters 53 nm, 500 nm and 1 lm were dispersed in DI waterat different volume concentrations ranging from 0 to 0.005 andthe suspensions were homogenized by ultrasonic agitation for10 min just before the experiment. The particles are neutral beads(usually hydrophobic surface) obtained in aqueous solutions fromthe manufacturer. The experiments were performed immediatelyafter ultrasonication to avoid the agglomeration of the particles.

The substrate was placed on a goniometer (Rame Hart, Ger-many) platform for contact angle measurements as shown inFig. 1. With the help of a controlled dispenser, a small droplet ofthe test fluid was placed over the substrate. A platinum electrodewas placed at the tip of the droplet and the gold layer acted asthe other electrode. Using a DC Sourcemeter, (Model2410 Keithley,USA), voltage ranging from 0–200 V was applied between thedroplet and the gold electrode. The change in contact angle wasmeasured, both while increasing as well as decreasing the appliedvoltage. All the measurements were repeated at least three timesand only the average values of the observations are reported here.The standard deviations of the measurements of contact angles arewithin 1.5%. The contact angle hysteresis observed (i.e., the differ-ence between contact angle measured at the same voltage and par-ticle concentration with increasing and decreasing voltages) isfound to be quite small – in the range of the experimental errors.Interestingly, it has been observed that the presence of suspendedparticles significantly reduces contact angle hysteresis as com-pared to traditional EWOD experiments without suspended parti-cles. This has recently been reported by other researchers as well[31,32]. However, a physical mechanism leading to the completeunderstanding of this effect is yet to evolve.

3. Results and discussion

EWOD experiments are performed initially with pure (no sus-pended particles) DI water as a control. The trends match well withthe traditional Young–Lippmann equation:

cos h ¼ cos h0 þ vV2 ð1Þ

where h and h0 are the contact angles with and without external ef-fects respectively, V is the applied external potential, v ¼ e0er

2rlv d is theslope of the resulting linear characteristic line of cos h vs. V2 line,

Glass + chrome

Gold

Sylgard

Camera Droplet

Light Source

Pt Wire

Voltage Sourcemeter

Image Analysis

Fig. 1. Schematic of the experimental setup.

642 D. Chakraborty et al. / Journal of Colloid and Interface Science 363 (2011) 640–645

with e0 being the permittivity of vacuum, er the relative permittivityof the dielectric, rlv the liquid–vapor surface tension and d as thethickness of the dielectric medium. It is observed from experimen-tal data that for water at room temperature of 25.6 �C, the variationof cos h � cos h0 with V2 is linear with a slope of 5.2 � 10�6 V�2,yielding the thickness of the dielectric to be 30 lm (the dielectricconstant of PDMS is taken as 2.5).

Contact angles (h0) are measured for the droplets containing dif-ferent sized submicron particles in varying concentrations in ab-sence of electric potential. It may be observed from Fig. 2 thatboth particle size (d) and volume fraction (/) affect the equilibriumcontact angle of the sessile droplet. The contact angle increaseswith particle volume fraction, even though the surface tensioncoefficient remains unaltered as measured by pendant dropletanalysis (see Supplementary material fig. S2). At very low particleconcentration, all the values tend to 101.1� ± 1�. This value closelymatches with the contact angle of a droplet of pure DI water rest-ing over a cured PDMS substrate. As the volume fraction is in-creased, the contact angle increases but reaches an asymptoticvalue. For lower particle diameter (53 nm), the volume fractionof the particles cannot be increased further as the particles tendto coagulate quickly. Hence, the concentrations of the particlesare restricted up to a maximum volume fraction of 0.005 v/v, for

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

x 10-3φφ

Experimental Theoretical

53 nm (Expt)500 nm (Expt)1000 nm (Expt)

53 nm 500 nm1000 nm

-0.36

-0.34

-0.32

-0.3

-0.28

-0.26

-0.24

-0.22

-0.2

-0.18

cos

θ

Fig. 2. Effect of concentration of the submicron particle on the static contact anglefor different particle diameters. Experimental results are compared with thetheoretical fit.

all particle sizes. It needs to be further mentioned that particlesmay not remain homogeneously dispersed as the electric field isincreased beyond a threshold limit that depends on experimentalconditions. However, in the experiments reported herein, the par-ticle concentrations are dilute (<0.005 v/v) and the applied electricpotentials low enough (<200 V) to ensure no coagulation betweenthe particles. Experimental observations under bright-field micro-scope also confirm that the particles remain well dispersed in thedroplets and therefore the assumption of a homogeneously dis-persed model, is appropriate for the experimental conditions usedin this study. For higher concentrations and electric fields the par-ticle suspension may not remain homogeneous, however, address-ing such cases is beyond the scope of the present analysis.

A thermodynamic description using an energy minimizationapproach is developed to analyze the experimental results underequilibrium conditions. The developed model assumes homoge-neously dispersed particles in the droplets which is consistent withthe experimental conditions as explained in the previous para-graph. The energy (E) of a constant volume, ("), sessile droplet overa solid surface in equilibrium with the surrounding gaseous phase(air), is given by:

E ¼Xi–j

Aijrij þ gL� k8 ð2Þ

where Aij is the interfacial area that demarcates the phases i and j,with the corresponding surface energy being designated as rij, grepresents the line tension [33–35] and L is the length of the so-lid–liquid two phase contact line. The subscripts ‘l’, ‘s’ and ‘v’ referto the liquid, solid and vapor phases, respectively. k is a Lagrangemultiplier to enforce a constant volume constraint (physically, k isequal to the pressure drop across the liquid–vapor interface, ther-modynamically consistent with the definition of free energy ofthe system). In presence of suspended submicron particles, themodified interfacial areas A0sl and A0lv can be expressed as a functionof the volume ratio of the particles to the total volume (denoted by/). The solid–liquid interface is assumed to be covered uniformly byparticles and the modified interfacial areas can therefore be repre-sented as A0sl ¼ aAsl and A0lv ¼ bAlv where a and b are parametersdependant on / and will be discussed in greater details later. Theinterfacial areas may be written as: Asl ¼ pR2 sin2 h andAlv ¼ 2pR2ð1� cos hÞ. The volume (") and the length (L) of the threephase contact line of the spherical sessile drop of radius R and con-tact angle h are given by [4]: 8 ¼ pR3 2

3� 34 cos hþ cos 3h

12

� �and L = 2pR

sin h. The energy as given by Eq. (2) may hence be written as:

D. Chakraborty et al. / Journal of Colloid and Interface Science 363 (2011) 640–645 643

E ¼ ðrsl � rsvÞpR2ðsin2 hÞaþ rlv2pR2ð1� cos hÞbþ g2pR sin h

� kpR3 23� 3

4cos hþ cos 3h

12

� �ð3Þ

The energy should be minimized in terms of the independentvariables, namely R and h, which is obtained by partial differentia-tion of E with respect to R and h and setting them to zero, i.e.,

@E@R¼ 2pR sin2 hðrsl � rsvÞaþ rlv4pRð1� cos hÞbþ g2p sin h

� k 3pR2 23� 3

4cos hþ cos 3h

12

� �� �¼ 0 ð4aÞ

@E@h¼ 2pR2 sin h cos hðrsl � rsvÞaþ rlv2pR2 sin hb

þ g2pR cos h� k pR3 34

sin h� sin 3h4

� �� �¼ 0 ð4bÞ

Upon eliminating k from Eqs. (4a) and (4b), we obtain modifiedYoung’s equation taking into consideration the effect of line ten-sion and modified interfacial areas because of the presence of sub-micron particles and using the relation cos ha ¼ rsl�rsv

rlv, to obtain:

cos h0 ¼a cos ha

b� g

brrlvð5Þ

where r is the radius of the solid–liquid contact area of the dropleton the surface. Absence of particles implicates that the values ofboth a and b will turn out to be unity, resulting in a modified formof the Young’s equation accounting for line tension. For large valuesof r the effect of line tension gets reduced but becomes progres-sively more important as droplets of smaller sizes are considered.

Before assessing the theoretical predictions on EWOD charac-teristics in presence of suspended submicron particles with corre-sponding experimental observations, it may become imperative tocharacterize the parameters a and b from a theoretical perspective,thereby establishing a fundamental basis for equilibrium descrip-tion of the contact angle under no-voltage condition for the parti-cle-laden droplets. It has already been assumed that the particledistribution in the droplet is fairly homogeneous. The particlesnear the solid–liquid interface augment the interfacial area Asl bya factor denoted by a. Each particle increases the surface area bypd2 (corresponding to the surface area of a single sphere) as theparticle touches the solid–liquid surface at a single point as shownin Fig. 3. The volume of the solid particles at the solid liquid inter-face may be denoted by the product of the number of the particles(n) times the volume of each particle (1

6 pd3). Again, the volumecoverage of the interface by the solid particles can also be ex-pressed by (assuming homogeneous distribution) as: (Asld)/,where / is the particle volume fraction in the interfacial region.Equating the volume of the solid particles from both of these con-siderations, it follows that, ðAsldÞ/ ¼ n 1

6 pd3, which yields n ¼ 6Asl

pd2 /.The incremental solid–liquid interfacial area, due to the pres-

ence of these particles, becomes DAsl = npd2, which impliesDAsl = 6Asl/. Thus, the modified solid–liquid interfacial area is

Fig. 3. Schematic used to describe the theoretical model to predict the effect ofsubmicron particles. The dotted lines show the modified solid–liquid and liquidvapor interfaces.

given by: A0sl ¼ Aslð1þ 6/Þ, so that a = 1 + 6/. Furthermore, thewetting nature of the submicron particles augments the interfacialarea of the liquid vapor interface as well. It is assumed that half ofeach submicron particles at the interface is exposed to the vaporside while the other half remains submerged in the liquid, asshown in Fig. 3. Thus each particle contributes an additional areaequal to pd2/2 (increase in interfacial area due to presence of thespherical particle) minus 1

4 pd2 (area of the liquid–vapor interfaceblocked by the particle). This results in a net increase of liquid–va-por interfacial area by 1

4 pd2 contributed by one particle. The totalincrease in interfacial area of the liquid–vapor interface is theproduct of the contribution of the surface area of a single particletimes the number of particles at the liquid–vapor interface (n0).Similar to the earlier case, the number of particles at the liquid–va-por interface is given by n0 ¼ 3Alv

pd2 /.However, the volume coverage of the liquid–vapor interface is

given by Alvd2

� �/ because particles are considered half immersed

in the liquid as shown in Fig. 3. Equating the volume of the solidparticles from both of these considerations, it follows thatAlv

d2

� �/ ¼ n0 1

6 pd3, which yields n0 ¼ 3Alvpd2 /. The incremental liquid–

vapor interfacial area, due to the presence of these particles,becomes DAlv ¼ n0 pd2

4 , which implies DAlv ¼ 34 Alv/. Hence, the

modified interfacial area of the liquid–vapor interface is given as:A0lv ¼ Alv 1þ 3

4 /� �

, which yields b ¼ 1þ 34 /.

The experimental data are fitted to Eq. (5) with a calibration ofthe theoretical particle volume fraction with the concentration ofthe experimentally employed particle-laden sample. This calibra-tion factor, for the present set of experiments, has turned out tobe 0.004 (i.e., particle volume fraction = 0.004 � sample concentra-tion). Further, in an effort to assess the implications of the particlevolume fraction corrected interfacial areas from experimental con-siderations, it is assumed that a = 1 + t1/, b = 1 + t2/, where t1 andt2 are treated as unknowns, to be determined from experimentalconditions. The parameters ha and t3 ¼ g

rrlvare obtained from the

present experimental conditions as 92.9� and 0.14, respectively.Based on these input parameters and model considerations, a com-parison between experimental data and theoretical predictions isdepicted in Fig. 2. The trends in the data are correctly explainedby the model with deviations at a few points. The deviations aredue to the experimental uncertainties associated with the preci-sion of the measurement of contact angles. From these compari-sons, the value of t2 is found to be equal to 3

4, which is inremarkable agreement with the proposed theory. However, the va-lue of t1 is found to be a weak function of the diameter of the par-ticles (t1 equal to 5.85, 5.35 and 5.0 corresponding to particlediameters of 53 nm, 500 nm and 1000 nm respectively); but is stillclose to the constant theoretical value of 6.0. In the limit when /tends to zero (i.e., a droplet without any particle), the value ofthe contact angle h becomes cos�1ðcos ha � t3Þ and is found to beequal to 101.03�. This is in excellent agreement with the experi-mentally measured value of 101.1�. On the other extreme, when/ tends to infinity, the value of cos h tends to a constant asymptoticvalue of t1

t2.

With an establishment of the underlying interfacial phenomenaunder no-voltage conditions, investigations are extended to assessthe pertinent behavior in a EWOD environment. For illustration,the variations in the contact angles of droplets containing homoge-neously dispersed submicron particles of diameters 53 nm, 500 nmand 1 lm are investigated with varying particle volume fractions:/ = 0, 0.0005, 0.002 and 0.005 under the influence of external elec-trical potential.

The variations of contact angle (h) with V for different particleconcentration at fixed diameter (d = 500 nm) are shown in Fig. 4and variation of cos h � cos h0 as a function of V2 is shown inFig. 5 for a particular concentration (/ = 0.005) but varying diame-ters of the particles. It may be observed that increase in particle

Fig. 4. Variation of contact angle with voltage for varying particle concentration (/= 0, 0.0005, 0.002, 0.005) for a fixed particle diameter of 500 nm.

644 D. Chakraborty et al. / Journal of Colloid and Interface Science 363 (2011) 640–645

concentration leads to increase in the contact angle values which isconsistent with the theoretical predictions as well as with theexperimental results presented in Fig. 2 for no applied potentialcases. The increase in contact angles is a direct consequence ofincrements in the liquid–vapor and solid–liquid interfacial areas,as explained before. However, the introduction of particles modi-fies the electrical permittivity of the composite droplet in such away as to oppose the previous phenomena, based on the consider-ation that an increment in the effective permittivity results in anincrement in the effective interfacial energy, and a consequentreduction in the effective contact angle. Increase in particle volumefraction, thus, leads to increase in the electrical permittivity of thecomposite droplet resulting in decrease in contact angle (Eq. (1)). Itis to be noted that the variations in contact angle by the presenceof particles is more pronounced at lower values of applied poten-tials. The contact angle increases from 101.1� to 108.9� at V = 0when the concentration is progressively increased to a value of/ = 0.005. However, the effect of submicron particle concentrationson contact angle progressively decreases with increase in appliedvoltage as can be seen from Fig. 4, the contact angle values becomevirtually identical at V = 210 V for a particle concentration rangingfrom 0 to 0.005. This implies that at low voltages, the particle con-centration contributes more to the modification of the interfacialareas (and the consequent increases in contact angles), whereasat higher voltages, the effect of increased electrical permittivityof the composite droplet dominates. The difference in diameter

Fig. 5. Plot of cos h� cos h0 with V2 at / = 0.005 for varying bead diameters of53 nm, 500 nm and 1000 nm. The solid lines are guide to the reader’s eyes only.

of the particles also influences the effective electrical permittivityof the droplet system and thereby significantly affects the contactangles of the submicron particle containing droplets. However, it isinteresting to note that the linearity of the cos h � cos h0 vs. V2 isstill retained. It may be noted that the behavior of the cos h � cos h0

vs. V2 curves at lower values of applied potential are more complexas the effect of gravity and other environmental factors start influ-encing the shape of the composite droplet in a non-linear waywhich is beyond the scope of the present study.

What are the contrasting demarcating electrostatic features ofEWOD in a particle-laden environment as compared to the stan-dard EWOD conditions without particles? In an effort to answerthis question, it may first be noted that in standard EWOD config-uration, the droplet (without any particles present in it) is consid-ered to be conducting and the charge develops only across thedielectric layer. The submicron spherical latex particles having adielectric constant equal to 2.5 act as suspended dielectric in theliquid droplet. Hence, unlike the conventional case, the decreasein potential is both across the droplet as well as the static dielectriclayer. With the enhancement of the dielectric constant of thewhole capacitive system, it may be noted from Eq. (1) that a smal-ler voltage may be required to produce the same effective changein contact angle. It may also be observed that the saturation of con-tact angle is not present for the experimental conditions studiedherein. The particle laden droplets act like moving/recirculatingdielectric unlike the static dielectric medium (insulating layer).The capacitance of the composite droplet and the capacitance ofthe static insulating layer act in series. The voltage drops acrossboth these dielectrics, resulting in comparatively lower voltagedrop than conventional EWOD under identical operating condi-tions. Hence, the phenomenon of charge trapping occurring be-yond a threshold high voltage is delayed to an even highervoltage. Thus, saturation in contact angle in presence of submicronparticles is likely to take place at a much higher voltage and suchhigher voltages are usually not encountered in most of the applica-tions. The limiting voltage for saturation cannot be evaluated in thecurrent experiments due to dielectric break down at such elevatedvoltages.

4. Conclusions

The variations in the contact angles of water droplets contain-ing homogeneously dispersed submicron particles of diameters53 nm, 500 nm and 1 lm are investigated under the influence ofexternal electric potential in EWOD configuration. The contact an-gle increases with particle concentration but decreases with in-crease in diameters, at identical applied voltages. Athermodynamic description using an energy minimization ap-proach has been developed. The changes in the solid–liquid andthe liquid–vapor interfacial areas due to the presence of the parti-cles are considered and the associated parameters, evaluated usingthe experimental data, successfully describe the physics of theprocess. The measured values of the contact angles for differentconcentrations and sizes of the suspended particles followYoung–Lippmann equation for the experimental applied potentialsstudied herein. The experimental results indicate that the contactangle increases with increase in particle concentration, thoughthe particle concentration dependence tends to get offset at rela-tively higher voltages, as a consequence of counterbalancing influ-ences of increment in contact angle due to elevated effectiveinterfacial areas and decrements in contact angle due to enhancedeffective permittivities. Importantly, the presence of the submicronparticles alters the effective permittivity of the capacitive systemin such a way that a smaller voltage is necessary for identicalcontact angle changes as compared to a sessile pure water drop.

D. Chakraborty et al. / Journal of Colloid and Interface Science 363 (2011) 640–645 645

Acknowledgments

This work is partially supported by a grant from Intel Technol-ogy India Pvt. Ltd., Bangalore, India. Any opinions, findings andconclusions expressed in this paper are those of the authors anddo not necessarily reflect the views of Intel. We acknowledge thehelp from Mr. Pradip Kr. Dey for deposition of gold over glass sub-strate using the facilities of the Advanced Technology DevelopmentCentre, IIT Kharagpur. The authors express their appreciation toProf. Rabibrata Mukherjee for his help with the AFM.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jcis.2011.07.077.

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