oscillating nanofluid droplet for micro-cooling
TRANSCRIPT
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Sensors and Actuators B 239 (2017) 562–570
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
jo u r nal homep age: www.elsev ier .com/ locate /snb
scillating nanofluid droplet for micro-cooling
onojit Chakraborty a, Rahul Anand a, Pujari Srinivasa Rao a, Shubhatam Sen b,unando DasGupta a,∗
Department of Chemical Engineering, Indian Institute of Technology Kharagpur, 721302, IndiaAdvanced Technology Development Centre, Indian Institute of Technology Kharagpur, 721302, India
r t i c l e i n f o
rticle history:eceived 7 April 2016eceived in revised form 16 June 2016ccepted 25 June 2016vailable online 26 June 2016
eywords:
a b s t r a c t
AC electrowetting is used to induce oscillation in a nanofluid droplet with significant changes in the shapedependent parameters at an optimized frequency. The presence of the synthesized silver nanoparticlesnot only enhances the wetting characteristics of the resulting nanofluid droplets, but leads to the augmen-tation of their heat extraction capability from a hot spot. The low frequency AC electrowetting induceddroplet shape deformation generates surface waves and associated internal flow inside the droplet. Thisaugments the convective heat transfer process resulting in additional evaporative cooling. The imposed
C electrowettingroplet oscillationanofluids
internal flow and mixing also contribute to the reuse of the residual (after evaporation) nanoparticlesto recreate the nanofluid droplet on the substrate. The generated surface waves are characterized usingimage analysis of the oscillating droplets in terms of their amplitude, frequency and damping. A modelbased on Stoke’s drift phenomenon is used to analyze the results indicating augmented heat transfer inthe low frequency regime.
© 2016 Elsevier B.V. All rights reserved.
. Introduction
Decreasing sizes and increasing package densities in Microelec-romechanical systems (MEMS) and integrated circuit (IC) industryre making thermal issues considerably important. MEMS extendhe fabrication techniques developed for the integrated circuit (IC)ndustry and provide multiple options of augmenting additional
echanical elements, leading to the construction of integratedlectromechanical systems [1]. Non-uniform thermal distributionnd the generation of spots on a circuit cause physical stressnd performance degradation of the fabricated devices [2,3]. Fastnd efficient cooling of hot spots has remained a major challengehile designing the latest cooling strategies. In this respect, Digitalicrofluidics (DMF) [4,5] technology may offer a better alternative
or chip cooling as it can efficiently handle discrete droplets of smallolume, and provide enhanced process rate due to the high surfaceo volume ratio [6]. A recent study has shown an enhancement in
he cooling efficiency by pulsating DC voltage induced oscillatingroplets [7].∗ Corresponding author.E-mail addresses: [email protected], [email protected]
S. DasGupta).
ttp://dx.doi.org/10.1016/j.snb.2016.06.145925-4005/© 2016 Elsevier B.V. All rights reserved.
DMF technology, built on the physical phenomenon of Elec-trowetting on Dielectric (EWOD) [8], provides the advantage ofcontrolling and manipulating individual droplets of tiny volumeby placing them on arrays of electrodes[9]. Various applications ofDMF [10] platforms ranges from DNA handling [11], protein andpeptide purification [12], cell manipulation [13] to lab-on-a-chipdevices [14], micro-chip cooling [15] etc. In a DMF based platform,a droplet of conductive liquid is placed between an activated anda non-activated electrode. The activated electrode exerts an elec-tromechanical force on the contact line (in addition to the capillaryforce) located above it. The resultant of these forces, parallel to thesurface and directed towards the electrically actuated region, per-turbs the equilibrium of the droplet and it starts to move towardsthe actuated electrode under the effect of the exerted electrome-chanical force [4]. Droplets can be split, merged, and transported[16,17] by electrically controlling the contact angle of the dropletsusing EWOD on a DMF platform [9].
The phenomenon of alteration of the wetting properties of amodified surface on the application of electric field is termed aselectrowetting. A significant change in the contact angle of the con-ductive liquid droplet, placed on a dielectric surface, can be broughtabout with the help of an external electric field and is commonly
known as Electrowetting on Dielectric (EWOD) [8]. Chemical [18],thermal [19,20] and electrochemical [21] methods are also used tochange the contact angle of liquid droplets. The problem of elec-
and Ac
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M. Chakraborty et al. / Sensors
rolysis has been eliminated using a thin insulating layer, actings a capacitor, to separate the conductive liquid from the metalliclectrode [8]. The electric charges that are accumulated near thehree phase contact line (TCL) induce additional stress, commonlynown as Maxwell stress. The stress generated near the TCL pullshe droplet in the direction of the applied electric field [22,23].
The electrical wetting tension (Maxwell stress) [24] is a weakunction of the polarity of the applied voltage [22,25,26]. The usef alternating (AC) voltage may provide a better alternative toirect (DC) voltage due to its inherent advantages such as reduc-ion of contact angle hysteresis [27,28], increase of contact angleaturation voltage [29,30], and reduction in the molecular adsorp-ion at the liquid-substrate interface [31]. The time-averaged meanomponent of the electrical stress (or Maxwell stress) acting athe TCL in AC electrowetting contributes to changes in the con-act angle. The other component of the Maxwell stress is theime-periodic oscillatory component that induces oscillation to theroplet [32,33]. This induced oscillatory motion can cause phe-omenon such as generation of steady streaming due to the effectf viscosity [34], synthetic jetting [35] etc.
The shape of the droplet, exposed to AC electrowetting, dependsn the frequency of the applied AC voltage. High frequency AColtage induces flow within the droplet, which is due to thelectro-thermal effect wherein electrical charges are generated byonductivity and permittivity gradients [24]. On the other hand,fficient mixing inside the droplets has been achieved at low fre-uency because of the hydrodynamic flows and shape oscillation36,37]. Surface waves [37] are found in case of low frequencyriven oscillating droplets. The surface waves travelling from theCL to the apex trigger an asymmetry in the liquid-air interface andhus resulting in a net movement of the fluid element (similar toStokes drift velocity’ [38]) inside the droplet. This internal flow ofuid parcels is responsible for the enhancement of mixing in casef low frequency AC electrowetting [37].
Recent observations show an increase in the spreading ofanofluid droplets in EWOD configuration [39]. These fluids areolloidal suspension of nano-sized particles (generally 1–100 nmean diameter) that are dispersed in the fluid due to the Brow-
ian motion, surmounting the settling forces such as gravity [40].anoparticles in liquids can alter the effective permittivity of the
apacitive system requiring smaller voltage for a change in contactngle [41]. The enhancements of the thermo-physical propertiesthermal diffusivity, thermal conductivity, viscosity, convectiveeat transfer coefficient) leading to better heat transfer perfor-ance are one of the main advantages of a nanofluid [42]. For
xample, nanofluids can be applied to cool automobile engines andigh power microwave tubes. Nanofluid as a coolant is used for
mproving the efficiency of MEMS as well [43]. Based on these find-ngs, it can be summarized that suspended nanoparticles in a baseiquid can be an effective way to enhance both the wetting propertynd the heat removal capability.
The present work utilizes the favorable thermo-physical andnhanced wetting properties of a synthesized nanofluid to augmenthe cooling efficiency of micro-scale devices. The heat extractionfficiency of a specially prepared nanofluid droplet, oscillatingnder the influence of a suitably selected AC electric field, is investi-ated. The input frequency of the AC voltage is optimized to ensureaximum droplet shape deformation and is used in subsequent
xperiments. The AC voltage induced surface waves give rise tonternal flow inside the droplet resulting in augmented convection,aster evaporation and improved cooling of a hot spot. The gener-ted hydrodynamic flow inside the droplet is subsequently used
or the mixing of the residual nanoparticles (after the evaporation)ith the base liquid to be reused again. A theoretical model basedn Stokes drift phenomenon in conjunction with optical character-
tuators B 239 (2017) 562–570 563
ization of the surface wave is used to analyze the cooling potentialof the oscillating droplet.
2. Materials and methods
2.1. Preparation of the silver nanoparticle
The nanofluid, containing dispersed silver nanoparticles, wassynthesized using a standard protocol [44,45]. In the first step,4 ml of 52 mM AgNO3 was added to 10 ml of 6.92 mg/ml chitosansolution (dissolved in 1% acetic acid) while stirring at room tem-perature for 15 min. The mixture was then kept in a water bathat 95 ◦C for 12 h to obtain the desired silver nanoparticles. The sil-ver nanoparticles were purified by centrifugation at 7000 rpm for30 min and redispersed in milli-Q water (twice). The average size ofthe particles was ∼30 nm, as obtained by high resolution transmis-sion electron microscopy (JEM 2100), operating at an acceleratingvoltage of 200 kV. The particle size distribution was measuredusing dynamic light scattering (DLS) (Malvern Zetasizer, NanoZS,Germany) and the average hydrodynamic diameter was found tobe 33.1 nm. The thermal conductivity of nanofluid was 0.69 W/m◦C,measured using KD2 Pro-Decagon device.
2.2. Experimental method
The experimental setup consisted of an Indium Tin Oxide (ITO,In2O3/SnO2) coated glass, further coated with Polydimethylsilox-ane (PDMS) and Teflon (AF-1600 from Dow Chemicals, USA). PDMSwas used as the dielectric layer (dielectric constant of 2.8) andTeflon acted as a protective shield of the dielectric layer. The Teflonlayer also imparted enhanced hydrophobicity to the substrate andreduced contact angle hysteresis. The PDMS coating was preparedby mixing the base (SYLGRAD 184) with the cross-linker in 10:1 wtratio. The mixture was placed in a vacuum desiccator to get rid ofthe air-bubbles formed during the mixing process. The PDMS andTeflon were spin-coated on the conductive side of the ITO-coatedglass in a Spin coater (SüssMicroTec), using a two-step protocol [7].In the first step the PDMS was spin-coated at 500 rpm for 30 s, fol-lowed by at 3000 rpm for 70 s and left at 95 ◦C overnight. In thenext step, a thin layer of Teflon was spin coated on the PDMS at3000 rpm for 30 s. The substrate was heated at 110 ◦C for 10 minfollowed by 175 ◦C for 20 min for final curing. The thickness of thePDMS film and Teflon layer were measured by a profilometer andwere found to be 13 �m and 23.3 nm respectively.
Droplets (10 �l of 0.01 M NaCl solution in DI water and Nanofluidcontaining 4.4 �M synthesized Ag nanoparticles) were placed onthe top of the prepared surface. A small portion of the modifiedITO coated glass substrate was left uncoated for connecting theground electrode. The droplet and the substrate were electricallyconnected in an EWOD setup as shown in Fig. 1. An AC power source(Model 33250A source meter Agilent) was used to initiate AC fieldinduced oscillations to the droplet.
The AC power source (Model 33250A, 80 MHz waveform gen-erator Agilent) was programmed for a sine wave input voltage andwas amplified 50 fold using an amplifier (High Voltage Wide BandAmplifier 9200). A schematic of the output sine wave signal fromthe AC power source is presented in Fig. 1. The nature of the oscil-lating droplet depended on the amplitude and frequency of theapplied AC voltage. The peak to peak value of the input AC voltagewas kept fixed at 200 V and the frequency was varied from 5 to500 Hz. The frequency of the oscillation of a droplet is defined as
the inverse of the time taken by the droplet to undergo reversiblechange in shape when a pulse cycle is completed. The tempera-ture and relative humidity were kept at 25 ± 0.5 ◦C and 35 ± 2%respectively.
564 M. Chakraborty et al. / Sensors and Actuators B 239 (2017) 562–570
atic of the set up.
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The output to the amplifier was connected to a platinum wireFig. 1) which was dipped into the droplet. A high speed cameramodel V641, Phantom) was used to capture the dynamic changesn the droplet shape during oscillation (See Electronic supplemen-ary information for slow motion video of the oscillating droplets).he high speed camera was connected to the image processingomputer to analyze the captured images. Special light sourceLeica CLS150 LED) was placed diagonally opposite to the cameraFig. 1). The droplet oscillations were captured using the Phantomamera control (PCC, version 2.2) software at 2000 fps and a framey frame analysis was used to evaluate the radius of the dropletootprint, contact angle, the height of the drop and the surface
aves using ImageJ (version 1.47), DropImage Advanced (version.5.02) and ImagePro-Plus (version 6.0) software. An infrared (IR)amera (FLIR SC500) was placed right on the top of the oscillat-ng droplet to measure the temperature distribution. A strip heater
ith dimensions comparable to that of the drop footprint area, wasttached to the bottom of the ITO coated glass (Fig. 1) to mimic theresence of a hot spot.
. Results and discussion
.1. Application of alternating and direct voltage
The efficacy of the use of alternating voltage over a pulse DC [7]nput for droplet oscillation has been established by two separatexperiments. Pulsating DC voltage [7] with a specific delay time,, and with a peak voltage of 100 V is applied (DC power source,410 Source Meter Keithley) to individual droplets (10 �l) of water0.01 M NaCl solution in DI water) and nanofluid separately usinghe EWOD configuration. The videos of the droplets oscillation areecorded at 2000 fps using the high speed camera. The changesn the contact angles (from the equilibrium contact angle) of thescillating droplets are measured from the extracted images of the
ecorded video using ImageJ software. The maximum change inhe contact angle during the electrowetting process, for a specificelay time of the pulsating DC voltage, for water (�(CA)max,w)andanofluid (�CAmax,nf) droplets are noted. The difference betweenFig. 2. Variation in the relative change in contact angle with cycle time/delay timefor an input sinusoidal AC voltage and Pulsating DC voltage.
the maximum change in the contact angles of nanofluid and water(�CAmax,nf − �CAmax,w) at a specific delay time of the input pul-sating DC voltage is denoted by �CAR. The delay time of the inputpulsating DC voltage is varied from 10 ms to 200 ms and the cor-responding �CAR values are calculated. Sinusoidal AC voltages ofcycle time twice that of the delay time (i.e., cycle time equal to 2t),are applied using the AC power source and the amplifier.The peakvoltage of the input pulsating DC is fixed at 100 V whereas the peakto peak voltage of the sinusoidal AC voltage is kept at 200 V and thevalues of �CAR for different cycle times are calculated. A compari-son of the �CAR for the input sinusoidal AC voltage (�CAR|AC) withthe input pulsating DC voltage (�CAR|DC) is presented in Fig. 2.
The results from the experimental observations indicate that
the relative change in the contact angle is more prominent in ACcompared to the DC input voltage, signifying increased change inthe droplet morphology for AC electrowetting. Furthermore, theadditional advantages of AC electrowetting, namely low contact
M. Chakraborty et al. / Sensors and Actuators B 239 (2017) 562–570 565
he osc
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3
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Fig. 3. Dynamic changes in the shape related parameters of t
ngle hysteresis [28], high saturation voltage [29,30] etc. have madeC the ideal choice for all experiments reported herein.
.2. Effect of input frequency on shape oscillation
The sequential occurrence of wetting and de-wetting promotes dynamic change in the contact angle which strongly dependsn the frequency of the input AC voltage. The changes in the con-act angle (from its equilibrium value) are found to be significantlyigher for nanofluid droplets compared to the water droplets andre in tune with recent findings [39]. The frequency of the input AColtage is kept within the low frequency range (from 5 to 500 Hz).he induced internal flow inside the droplet at a low frequencyrises due to the shape dependent oscillation of the droplet. Atigher frequencies, the flows inside the droplet are reported to beenerated due to electrothermal effect inside the droplet [33] andan lead to Joule heating [24]. The characteristic frequency thateparates the two regimes are given by the reciprocal of the timeo charge the capacitor (i.e. the insulating layer) and can be calcu-ated as �RC = �dL/�cd, where �d and d are the dielectric constant24.78 × 10−12 F/m) and thickness of the insulator (13 �m) respec-ively, �c is the liquid conductivity (1120 �S/m for 0.01 M NaClolution in DI water and 1800 �S/m for the nanofluid used herein;easured using Multi-Parameter PCSTestrTM 35) and L is the dis-
ance of the needle to the electrode (0.48 mm) [46]. The maximumperating frequency (500 Hz) is kept well below the characteristicrequencies (∼ 1224 KHz for 0.01 M NaCl in DI water and ∼1967 KHzor the nanofluid solution) for the system used herein to elim-nate any chances of voltage drop inside the droplet which canead to electrothermal effect. The Bond number (0.17–0.40) andhe capillary number (1.38 × 10−5–7.3 × 10−5) of the droplets usederein are calculated and found to be less than unity, signifying theominance of surface forces over gravity and viscous forces. Thehange in the shape of the oscillating droplet is characterized by theariations in the shape dependent parameters such as amplitudeheight from the substrate), contact angle and droplet footprintadius. These parameters are evaluated (for the water and nanofluidroplets) from the extracted images of the recorded video at differ-
nt input frequencies. The input frequency for maximum droplethape deformation is obtained by quantitative analysis of thesehape dependent parameters and is used for subsequent experi-ents.illating droplet at 10 Hz (a) contact angle (b) contact radius.
3.2.1. Change in the contact angle and droplet footprint radiusThe contact angle (CA) of the droplet decreases on the applica-
tion of electric field and results in an enhancement of the wettedarea. The droplet spreading reaches a maximum when the inputsinusoidal voltage reaches its peak value and returns nearly to itsprevious shape on completion of the cycle. The observed dropletoscillation frequency is twice that of the input frequency indicat-ing no time lag. The dynamic changes in the droplet CA and thefootprint radius (CR), from their equilibrium values are evaluatedfrom the extracted images using ImageJ (version 1.47) software andare presented in Fig. 3.
Significant enhacements in the droplet wetted area and reduc-tion in the CA for the nanofluid droplet (compared to the waterdroplet) are observed due to the substantial higher spreading of theoscillating nanofluid droplets [39]. The hysteresis of the oscillatingdroplet is also noticeably less due to the use of AC voltage [28]. Thefrequency of the input AC voltage is varied (from 5 to 200 Hz) andthe variation in the CA and CR are measured for 10 s. As has beenmentioned before, the peak to peak value of the input AC voltagewas kept fixed at 200 V. The trends in the variation of the maximumchange in droplet CA (�CAmax) and CR (�CRmax) with frequencyare presented in Fig. 4.
The experimental results reveal that the maximum changes inCA and CR for the oscillating nanofluid droplets are higher as com-pared to the pure water droplets with a maximum at 10 Hz forboth the liquids. The presence of nanoparticles alters the interfa-cial energy near the TCL [39] and also increases the microscopiccurvature close to the TCL [47]. The ‘lubricating effect’ induced bythe presence of nanoparticles augments the structural disjoiningpressure [48] near the TCL and helps in increasing the contact linespreading [49]. The combined effect results in a decrease in thecontact angle and amplification of the contact radius. The maxi-mum changes in CA for the nanofluid and water droplets are 55.3◦
and 36.9◦ respectively at 10 Hz, decreasing considerably as thefrequency increases to 500 Hz. (Fig. 4(a)). Similarly, the values ofCRmax for a nanofluid and a water droplet are 1.11 mm and 0.87 mmrespectively at 10 Hz, while it decreases to ∼0.33 mm and 0.30 mmrespectively as the frequency increases to 500 Hz. (Fig. 4(b)).
3.2.2. Amplitude of the oscillating droplet
At low frequencies, the internal flow inside the droplet is gov-erned by the oscillation and steady streaming[32,33]. Under thiscondition, the variation in the amplitude of the droplet oscillationplays an important role. The amplitude of the oscillating droplet
566 M. Chakraborty et al. / Sensors and Actuators B 239 (2017) 562–570
Fig. 4. Variation in the maximum change in shape dependent parameters of the droplet with frequency (a) contact angle (b) contact radius.
at 10
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Fig. 5. Amplitude of oscillating droplet (a) dynamic variation
s defined as the difference of the instantaneous droplet heightrom the height of the droplet in absence of electric field. Theynamic height of a droplet is measured using the DROPimagedvanced software from the extracted images of the oscillatingroplet. The dynamic changes in droplet amplitude (at 10 Hz) foroth the nanofluid and water are presented in Fig. 5(a) showing sig-ificant amplification in amplitude for the nanofluid. The dynamicmplitude values, at different frequencies (from 5 to 500 Hz) areeasured for 10 s (∼100 cycle at 5 Hz to ∼10,000 cycles at 500 Hz)
rom the extracted images for the water and nanofluid droplets. Thealues of the maximum amplitudes at different frequencies are pre-ented in Fig. 5(b). It is clear that the amplitude of the oscillatingroplet is maximum at 10 Hz for both types of the droplets and isignificantly higher for the nanofluid. The maximum amplitude ofscillation decreases significantly (from 620 �m to 70 �m for theanofluid droplet and from 280 �m to 50 �m for the water droplet)ith increase in frequency from 10 Hz to 500 Hz.
It is clear that a significant amplification of the shape defor-ation induced hydrodynamic internal flow [24] is possible for
nanofluid droplet at low frequency. The thermal conductivityf the nanofluid used herein is 0.69 W/mK, (measured using KD2
Hz (b) variation of the maximum amplitude with frequency.
Pro-Decagon device at 25 ◦C) which is higher compared to that ofthe water (0.58 W/mK at 25 ◦C). These two features of the oscillat-ing nanofluid droplet make it an attractive choice as a coolant formicro-scale devices.
3.3. Potential of oscillating nanofluid droplet for coolingenhancement
A strip heater (4.5 mm × 2 mm), attached to the uncoated sideof the modified ITO coated glass substrate, is used to create a localhot spot (Fig. 1). The dimension of the heater is comparable to thatof the droplet wetted area. The strip heater is connected to a powersupply (TESTRONIX 93C) and delivers a gross power of 1.46 W tothe base. The temperature of the hot spot is monitored by an IRcamera (FLIR SC500), placed above the hot spot (Fig. 1) on whichdroplets of both the nanofluid and water (3 �l–10 �l) are placed
separately. The evaporation induced by the elevated temperatureof the substrate is monitored along with the effect of oscillationsinduced by AC voltage of variable frequencies (5–500 Hz). The totalevaporation times of the static and the oscillating droplets of iden-
M. Chakraborty et al. / Sensors and Ac
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ig. 6. Percentage enhancement in heat extraction rate of the oscillating nanofluidroplet as compared to an identical static droplet under the same conditions.
ical volumes subjected to the same heat load are noted, which areirectly related to their heat extraction efficiencies.
The relative change in the heat extraction rate can be calculateds [7]
=[�1Q1�1
�2Q2�2− 1
](1)
here, �1, Q1, �1 and �2, Q2, �2 are the density, average rates ofvaporation and latent heats of vaporization for the oscillating andtatic fluid respectively.
The average evaporation rates are calculated from the totalvaporation time of the droplet, The dynamic temperature distri-ution measurement (using ALTAIR: Version 5.80.016 connected to
R camera) technique [7] is used for the accurate measurements ofhe time required for complete evaporation. A significant reductionn the total evaporation time for the oscillating nanofluid droplet isbserved. The enhancements in the heat removal rate using thescillating nanofluid droplets of varying volumes (3–10 �l) and
nput frequencies (5–500 Hz) are evaluated and are presented in
ig. 6. It is possible that a change in the nanoparticle concentra-ion will affect the deformation parameters (i.e. CAmax, CRmax andmplitude). The nanoparticle concentrations for the heat extrac-ion experiments and the nanoparticle concentration used forFig. 7. Dynamic variation of D2 for (a) Water (b) Nanofluid. To enhanc
tuators B 239 (2017) 562–570 567
the droplet morphology validation experiments are kept identical(4 �M) in the present study.
It is clear from Fig. 6 that the increase in heat transfer rate ismaximum at 10 Hz and decreases with increase in the volume of thedroplet. An enhancement of 26.4% in evaporation rate is observedfor a 3 �l droplet at 10 Hz whereas the increase is only 1.3% for10 �l droplets at 500 Hz. The low-frequency oscillation induces aninternal hydrodynamic flow [24,32] inside the droplet resulting inenhanced convective heat transfer and higher rates of evaporation.The results are consistent with the previous observation that thedeformation of the droplet is maximum at 10 Hz.
To examine the combined effect of nanoparticles and oscillation,the oscillating nanofluid droplets are compared to the static water(0.01 M NaCl solution in DI water) droplets of the same volume.The relative change in the latent heat of vaporization (�1/�2), iscalculated using the well known D2-law [50,51], which is valid forboth reactive and non-reactive evaporating liquid droplets includ-ing water and fuels [52,53]. The law proposes a linear variation ofthe evaporating droplet diameter with time, as [51]
D2 = D20 − Kt (2)
where, D0 is the initial diameter of the liquid droplet and K is theevaporation rate constant (L2/T). The rate constant, K is a functionof the thermo-physical properties of the evaporating liquid and isfound to be approximately inversely proportional to the latent heatof vaporization for water and nanofluids [51].
The droplet diameters are measured using the Pendant dropmethod, (goniometer 250 G1 Ramé-hart Germany) with the help ofthe image processing software (DROPimage Advanced, 2.5.02) [51].The dynamic variation of D2 for water and nanofluid are presentedin Fig. 7(a) and (b).
An increase in the slope for the nanofluid droplets(K1 = 5.1 × 10−3 mm2/s) compared to the water droplets(K2 = 4.3 × 10−3 mm2/s) signifies a decrease in the latent heatof vaporization (as � ∞ 1/K) of the nanofluids as compared towater droplets under identical conditions[51]. Latent heat ofvaporization decreases by a factor ≈1.186 (�2/�1 = K1/K2) for thenanofluid. The density of the nanofluid used herein (�1) is found tobe 1.01 times higher than that of the water (i.e. �1/�2 = 1.01). Sub-stituting these values, the increases in the rates of heat extraction
for different volumes of droplets are calculated using Eq. (1) andpresented in Fig. 8.As can be seen from Fig. 8, substantial increases in the heatextraction rate take place as compared to the static water droplet.
e readability experimental data at every tenth point are shown.
568 M. Chakraborty et al. / Sensors and Ac
Fig. 8. Effects of oscillation and the use of a nanofluid on the percentage enhance-m
TrwatgsfiaM
taiitofmchs
number of images at every input frequencies, which are further
ent in heat extraction rate as compared to a static droplet of pure water.
he increase in heat removal rate is 73.4% when the evaporationate of a 3 �l oscillating nanofluid droplet is compared with a staticater droplet of the same volume and 36.1% in comparison to
static nanofluid droplet. It has been reported in the literaturehat low frequency AC electric field induced shape deformationenerates surface waves [37] (discussed in detail in the next sub-ection). These surface wave induced internal hydrodynamic floweld [32,37] increases with the reduction in the droplet size [7],ugmenting heat transfer with applicative potential in DMF basedEMS devices.
The reusability of the nanoparticles present in the droplet isested by adding a fresh droplet of water to the spot from wheren oscillating nanofluid droplet has completely evaporated, leav-ng the nanoparticles on the substrate. The newly formed droplets then subjected to the same oscillating AC voltage at 10 Hz and theime for complete evaporation is noted. The low frequency inducedscillation helps in mixing [32,36] the dried nanoparticles with thereshly added water creating a nanofluid droplet. The measure-
ents indicate that this process of recreating a nanofluid droplet
an be continued without any noticeable change in the enhancedeat extraction efficiency of an oscillating nanofluid droplet and istriking from potential applications point of view.Fig. 9. Schematic of th
tuators B 239 (2017) 562–570
3.4. Characterization of the surface waves
The modelling of the low frequency AC electrowetting is com-plex and yet to be completely analysed. At low frequencies, themixing inside the droplet and thus the enhancement in heat extrac-tion, is due to the surface perturbations [7,32,33,37,54]. Theseperturbations start from the three phase contact line (TCL) wherethe maxwell stresses are concentrated, and they move towards theapex of the droplet (needle electrode) [37]. This movement of thesurface results in a net displacement of the fluid elements presentbelow the surface up to a significant depth into the droplet. Thevelocity of these fluid elements, as a function of depth, can beobtained by assuming these waves to be similar to those gener-ated on the surface of an ocean due to gravity [55]. The velocity offluid parcels is given by the ‘Stokes drift velocity’ [38]. The averagevelocity of fluid parcels for the case of ‘deep waves’ (wavelength ofwaves comparable to the depth of the fluid) is given as [37,55]
Us= 2kfA2e−kz (3)
where k is the wave number, f is the frequency of the wave, A is theamplitude of wave and z is the depth from the surface (Fig. 9). Thisexpression is derived by taking the difference between the averageLagrangian flow velocity of a fluid parcel and the average Eulerianvelocity at a fixed position and is valid for inviscid, incompressibleand constant mass density fluids.
The modified expression for the surface velocity (at z = 0) can bewritten as
Us = 4�f 2A2
Vp(4)
where, Vp is the phase velocity of the surface wave, which is yetto be determined. This surface wave velocity is obtained in thepresent case from the images of the oscillating droplet, using themeasured droplet surface deformation patterns (Fig. 9). The wave-length of the surface wave is determined from the extracted imagesusing image processing technique (in MATLAB). The time period ofsurface waves, involved in the analysis, varies between 0.1 s (corre-sponding to an input frequency of 5 Hz) to 0.0125 s (correspondingto an input frequency of 40 Hz). The images of the waves are cap-tured using high speed camera at 2000 fps. This ensured sufficient
used in reconstructing the waves. At each of the frequencies, thewavelength (�) is calculated for all such images by tracking theinterface and then locating the crests. The average of these wave-
e surface wave.
M. Chakraborty et al. / Sensors and Ac
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Fig. 10. Variation in the average surface velocity with frequency.
engths obtained is taken as the final value of wavelength (�) athat particular frequency. The velocity can then be obtained usinghe following expression
p = �
t(5)
ere, � is the wavelength and t is the time period of the surfaceerturbation. The surface perturbations, generated at the TCL, areaused by Maxwell stresses, and are experimentally measured. Theurface wave generation frequency is twice that of the applied inputrequency. The amplitude of the waves (A) continually gets decayeds the waves travel towards the apex (Fig. 9) due to the inertial andiscous effects. The amplitude at the TCL is the change in the contactadius from its initial value (A1 in Fig. 9) to A2 at the apex as depictedn Fig. 9. Both the values (A1 and A2) reported herein are averagesf multiples image sequences (at different time intervals) from thexperimental images using ImageJ (version 1.47) software. A spatialverage of the amplitude is calculated by assuming an exponentialecay of amplitude with distance from the TCL. The wave equations a function of space is expressed as
(x) = Ce−�x cos(kx) (6)
here C and � are constants and � is defined as the decay parameterFig. 9). Time averaged values of the amplitude (average of A1 and2 at different time intervals) are used to construct the wave usingq. (6). The variation in � with frequency is presented at the inset ofig. 10. It can be seen from the figure that decay parameter assumedts least value at 10 Hz. The amplitude of the wave, A (to be usedn Eq. (4) for the evaluation of Us) is then calculated by obtaining
spatial average of the amplitude (Ce−�x). By putting these valuesn Eq. (4) the surface velocity (Us) as a function of frequency of thepplied AC field is obtained and is shown in Fig. 10.
The significant increases in the surface velocity for the case ofroplets with silver nanoparticles result in the increase in internalirculation and the enhancement in heat extraction. Additionally,he maximum in the values of Us at 10 Hz implies that the extractionf heat should be maximum at this frequency itself, corroboratinghe experimental observations. At frequencies more than 40 Hz, i.e.,
n the case of 100 Hz and 500 Hz, the change in contact radius is less,esulting in low amplitude surface wave generation and reducednternal circulation and heat transfer.[
tuators B 239 (2017) 562–570 569
4. Conclusions
Significant enhancements in the deformation of a nanofluiddroplet over the base fluid (contact angle, footprint radius, ampli-tude and frequency of the surface waves) are observed on theapplication of AC voltage. The shape deformation is a strong func-tion of the frequency of the applied AC voltage and is foundto be maximum at 10 Hz for a peak to peak voltage of 200 V.The enhancements in the maximum change in the contact angle,droplet footprint radius and amplitude of a nanofluid droplet ofvolume (10 �l) with the introduction of nanoparticles are ∼49%,∼27.5% and ∼121% respectively at 10 Hz. The AC field induced oscil-lation results in a significant increase (∼73%) in the heat extractionrate (for a 3 �l droplet, at 10 Hz) from a hot-spot as compared to astatic droplet of the base fluid alone. The results are consistent withthe fact that a change in the droplet morphology induces surfacewaves which enhance the internal hydrodynamic flow leading toan increase in the convective heat transfer and the overall coolingefficiency. A theoretical model based on stokes’ drift phenomenonis successfully used to explain the experimental results. The studyestablishes the potential of an oscillating nanofluid droplet forenhanced heat removal in micro-cooling applications.
Acknowledgements
The authors gratefully acknowledge the financial sup-ports provided by the Indian Space Research Organization,[IIT/KCSTC/STC/Chair/Appr/NEW/P/14-15/09, Dated 22-08-2014]and by the Indian Institute of Technology Kharagpur, India[Sanction Letter no.: IIT/SRIC/ATDC/CEM/2013-14/118, dated19.12.2013].
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.snb.2016.06.145.
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Biographies
Monojit Chakraborty obtained his PhD degree in theDepartment of Chemical Engineering at the Indian Insti-tute of Technology Kharagpur, India. His research interestincludes droplet manipulation, microflow enhancement,thin film dynamics, microchip cooling and moleculardynamics of micro and nano scale systems.
Rahul Anand received his Master of Technology degree inChemical Engineering from Indian Institute of TechnologyKharagpur, India in 2015. His research interest is in thefields of droplet oscillation and micro-cooling.
Pujari Srinivasa Rao is pursuing his Bachelor of Engineer-ing degree in the Department of Chemical Engineering atthe Indian Institute of Technology Kharagpur, India. Hisresearch interest is in the fields of microfludics and simu-lations of microscale systems.
Shubhatam Sen is pursuing his PhD degree in theAdvanced Technology Development Centre at the IndianInstitute of Technology Kharagpur, India. His researchinterests includes the synthesis of nanoparticles and studyof the effect of different external factors e.g., nanoparticlesand electric field on protein fibrillation phenomenon.