Effect of Nanoparticle Sizes and Number Densities on the Evaporation

The total domain for tomography spans a semicircular area of 3.05 mm in radius [0.05 .... The largest 47-nm Al2O3 nanofluid shows the largest temperat...
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Langmuir 2007, 23, 2953-2960

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Effect of Nanoparticle Sizes and Number Densities on the Evaporation and Dryout Characteristics for Strongly Pinned Nanofluid Droplets Chan Hee Chon,† Sokwon Paik,‡ Joseph B. Tipton Jr., and Kenneth D. Kihm* Department of Mechanical, Aerospace, and Biomedical Engineering, UniVersity of Tennessee, KnoxVille, Tennessee 37996 ReceiVed June 9, 2006 A microfabricated linear heater array operating in a constant voltage mode has been used to study the effect of nanoparticle size on the evaporation and dryout characteristics of strongly pinned nanofluid droplets. Four different nanofluids have been tested, containing 2-nm Au, 30-nm CuO, 11-nm Al2O3, and 47-nm Al2O3 nanoparticles, each of 5-µL droplets with 0.5 vol % in water. Nanofluid droplets show strong pinning along the droplet perimeter and, upon evaporation, leave a ring-shaped nanoparticle stain. Particle size is seen to have a clear and strong effect on the dryout stain pattern, while heater temperature seems to have little effect. With the assumption of axi-symmetry, tomographic deconvolution of measured data from the linear heater array allows for examination of the spatially and temporally resolved temperature and heat flux characteristics of the evaporating nanofluid droplets.

1. Introduction When a particle-laden liquid droplet, such as a colloidal fluid or nanofluid, evaporates, uneven progress of dryout tends to deposit the residual particles in a ring-shaped pattern along the original wet surface boundary.1 These ringlike dryout patterns can be seen in many practical examples, ranging from soap water droplet stains to the recent DNA mapping techniques, where particle-DNA-laden microscale flows stretch and deposit the DNA molecules onto a substrate, also termed as “fluid fixation”.2 The history of published interest in the ring stain formation goes back to Denkov et al.,3 who analytically described the mechanism of particle formation as the capillary effect existing between particles. About a decade later, the same team of researchers comprehensively studied these capillary forces, as well as ring formation of deposited particles.4 A series of publications by Deegan et al.1,5,6 presented physical explanations of colloidal fluid evaporation and ring formation and growth in naturally occurring events, such as a coffee ring stain. Uno et al.7 and Tay and Edirisinghe8 studied the particle deposition on both hydrophilic and hydrophobic surfaces. Conway et al.9 studied the size and concentration effects of polystyrene beads on ring formation, while Maenosono et al.10 studied the ring growth of semiconductor nanoparticles in liquids. Nguyen and Stebe * To whom correspondence should be addressed. Tel. (865) 974-5292. E-mail: [email protected]. Homepage: http://minsfet.utk.edu/. † Current address: Department of Mechanical Engineering, Vanderbilt University, Nashville, TN. ‡ Current address: Oak Ridge National Laboratory, Oak Ridge, TN. (1) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827-829. (2) Dugas, V.; Broutin, J.; Souteyrand, E. Langmuir 2005, 21, 9130-9136. Wang, W.; Lin, J.; Schwartz, D. C. Biophys. J. 1998, 5 513-520. (3) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183-3190. (4) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383-401. (5) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. ReV. E 2000, 62, 756-764. (6) Deegan, R. D. Phys. ReV. E 2000, 61, 475-485. (7) Uno, K.; Hayashi, K.; Hayashi, T.; Ito, K.; Kitano, H. Colloid Polym. Sci. 1998, 276, 810-815. (8) Tay, B. Y.; Edirisinghe, M. J. Proc. R. Sco. Lond. A 2002, 458, 20392051. (9) Conway, J.; Korns, H.; Fisch, M. R. Langmuir 1997, 13, 426-431. (10) Maenosono, S.; Dushkin, C. D.; Saita, S.; Yamaguchi, Y. Langmuir 1999, 15, 957-965.

researched ring formation by surfactant-enhanced MaragnoniBenard instability,11 and Gokhale et al. studied contact line dynamics of surfactant-laden microdrops.12 While these studies focus on how the ring stain is formed and which parameters can alter ring growth, further study is still needed to quantitatively explain the unique evaporation/dryout characteristics associated with ring stains of particle-laiden liquid droplets. For the case of single-phase liquid evaporation/dryout phenomena, an extensive publication list is available for both experimental and numerical investigations. Just naming a few, Birdi et al.13 examined the mass transfer rate for evaporating water droplets and presented observation of pinned contact lines on a glass surface. Shanahan and Bourges14,15 measured the timevarying heights, contact angles, and contact-line radii of evaporating water droplets and presented an explanation for “stickslip” evaporation. Hisatake et al.16 experimentally studied the evaporation rate as a function of temperature, humidity, air velocity, and vessel dimensions. Anderson and Davis17 performed numerical predictions for the effects of capillarity, thermocapillarity, vapor recoil, viscous spreading, contact angle hysteresis, and mass loss during liquid droplet evaporation. Fisher18 and Hu and Larson19 studied the internal flow fields inside evaporating droplets using lubrication theory and computational methods, respectively. With regard to liquid droplet evaporation heat transfer, Michiyoshi and Makino20 examined the heater surface temperature profiles underneath an evaporating droplet contacting different (11) Nguyen, V. X.; Stebe, K. J. Langmuir 2003, 19, 8271-8279. Nguyen, V. X.; Stebe, K. J. Phys. ReV. Lett. 2002, 88, 164501. (12) Gokhale, S. J.; Plawsky, J. L.; Wayner, Jr. P. C. Langmuir 2005, 21, 8188-8197. (13) Birdi, K. S.; Vu, D. T.; Winter, A. J. Phys. Chem.1989, 93, 3702-3703. Birdi, K. S.; Vu, D. T. J. Adhes. Sci. Technol. 1993, 7, 485-493. (14) Shanahan, M. E. R.; Bourges, C. Int. J. Adhes. Sci. Technol. 1994, 14, 201-205. Bourges, C.; Shanahan, M. E. R. Langmuir 1995, 11, 2820-2829. (15) Shanahan, M. E. R. Langmuir 1995, 11, 1041-1043. (16) Hisatake, K.; Tanaka, S.; Aizawa, Y. J. Appl. Phys. 1993, 73, 73957401. (17) Anderson, D. M.; Davis, S. H. Phys. Fluids 1995, 7, 248-265. (18) Fischer, B. J. Langmuir 2002, 18, 60-67. (19) Hu, H.; Larson, R. G. J. Phys. Chem. B 2002, 106, 1334-1344. Hu, H.; Larson, R. G. Langmuir 2005, 21, 3963-3971. Hu, H.; Larson, R. G. Langmuir 2005, 21, 3972-3980. (20) Michiyoshi, I.; Makino, K. Int. J. Heat Mass Transfer 1978, 21, 605613. Makino, K.; Michiyoshi, I. Int. J. Heat Mass Transfer 1984, 34, 781-791.

10.1021/la061661y CCC: $37.00 © 2007 American Chemical Society Published on Web 12/09/2006

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Figure 1. Schematic illustration of the constant-voltage experimental system for droplet evaporation: (a) experimental setup and a detailed microheater array with 32 gold line heater elements, which are 100 µm wide, 0.5 µm thick, and 1.5 cm long, individually, and are spaced 100 µm apart; and (b) a voltage divider circuit diagram.

heated surfaces. Klassen et al.21 presented measurements of the temperature distribution of an evaporating droplet with infrared thermography, while Chandra et al.22 evaluated the effect of initial contact angle on evaporation and calculated surface temperature and heat flux during droplet evaporation. Xiong and Yuen23 experimentally studied the plate bulk temperature and overall heat flux during droplet evaporation. In 1999, Rule and Kim24 fabricated a complex microheater array to achieve spatially resolved heat flux measurements for the case of pool boiling of FC-72. More recently, Paik et al.25,26 fabricated a 32-linear microheater array to study the microscale heat and mass transport for slowly evaporating sessile water droplets. A nanofluid is a mixture of metallic nanoparticles (Au, CuO, Al2O3, etc.) with a base fluid (water, ethylene glycol, etc.) that is known to have substantially enhanced thermal conductivity with a relatively small concentration of nanoparticles.27 (21) Klassen, M.; di Marzo, M.; Sirkis. ASME J. Heatt Transfer 1990, 141, 117-121. di Marzo, M.; Tartarini, P.; Liao, Y.; Evans, D.; Baum, H. J. Int. Heat Mass Transfer 1993, 36, 4133-4139. (22) Chandra, S.; di Marzo, M.; Qiao, Y. M.; Tartarini, P. J. Fire Safety 1996, 27, 141-158. (23) Xiong, T. Y.; Yuen, M. C. Int. J. Heat Mass Transfer 1991, 34, 18811894. (24) Rule, T. D.; Kim, J. Asme J. Heat Transfer 1999, 121, 386-393. (25) Paik, S. Ph.D. Dissertation, Texas A&M University, College Station, TX, 2005. (26) Paik, S. W.; Kihm, K. D.; Lee, S. P.; Pratt, D. M. ASME J. Heat Transfer 2006, in press. (27) Lee, S.; Choi, S. U. S.; Li, S.; Eastman, J. A. ASME J. Heat Transfer 1999, 121, 280-289.

Figure 2. (a) Schematic of the axi-symmetric, concentric, tomographic deconvolution zones; and (b) illustration of the geometrical correlation between the concentric zones and the heater element zones. Table 1. Thermo-Physical Properties of Water and the Tested Nanoparticlesa particle size (nm) density (kg/m3) thermal conductivity (W/m‚K) a

water

Au

CuO

Al2O3

0.3231

2 19 300 317

30 6500 20

11 and 47 3600 25

997.0 0.613

At T ) 300 K.

Nanofluids have broad potential as a next-generation coolant in various energy-saving applications where effective cooling, smallscale heat dissipation, and high-density power system management are required. Additional applications include nanopatterning and electrical circuitry fabrication by nanofluid evaporation.28 These applications require a commanding knowledge of the fluidic and heat-transfer mechanisms peculiar to nanofluid droplet evaporation, dryout, and nanoparticle deposition. In an effort to elucidate nanofluid droplet evaporation characteristics, this paper presents experimental results of dryout (28) Szczech J. B.; Megaridis, C. M.; Gamota, D. R.; Zhang, J. IEEE Trans. Electron. Packaging Manufact. 2002, 25, 26-33. Yarin A. L.; Szczech, J. B.; Megaridis, C. M.; Zhang, J.; Gamota, D. R. J. Colloid Interface Sci. 2006, 294, 343-354.

Strongly Pinned Nanofluid Droplets

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Figure 3. Evolution of 11-nm Al2O3 nanofluid droplet evaporation/dryout with sequential photographs and schematic sketches. Just after placement on the microheater substrate, the droplet is pinned at the edge (a). During the liquid-dominant evaporation (b), the strong pinning of nanoparticles acts to congregate them to the rim. The droplet thickness and contact angle decrease while the wet diameter remains constant. With further evaporation of liquid, the contact angle exceeds the critical angle, and the thin core liquid region begins to break away from the rim, i.e., depinning (c). The depinned core liquid then shrinks toward the center as the evaporation and dryout further progresses (d). Finally, the resulting ring-shaped nanoparticle stain is formed along the rim, and the evaporation is completed (e). protection. This surface was then plasma-treated to convert the naturally hydrophobic surface into a hydrophilic one.33 For further reference, detailed heater design and fabrication processes are presented in the dissertation work of one of the co-authors.23 Over the tested temperature range, 40-80 °C, the temperatureresistance relation of gold is approximated to be linear, as29 R ) R0 (1 + R(T - T0))

Figure 4. Evolvement of droplet wet diameters (D/D0) as functions of evaporation time for water and four different nanofluids placed on the hydrophilic microheater surface at 80 °C initial temperature. The strong pinning of nanofluid droplets maintains their wet diameters at the initial wet diameter (D0) until completion of dryout. In contrast, the water droplet diameter remains unchanged during the pool evaporation of water, which occupies ∼90% of the total evaporation time, and drastically shrinks thereafter as the peripheral thin film rapidly recedes toward the center. The vertical arrows indicate dryout times for different nanofluids.

and heat transfer characteristics for evaporating nanofluid droplets using a microfabricated linear heater/detector array consisting of 32 gold heater lines, 100 µm wide and 0.5 µm thick. Section 2 further describes the microheater array, and Section 3 describes tomographical deconvolution of line-average data into axisymmetric radial data. Section 4 presents the results for the evaporation and dryout patterns and discusses the temperature and heat flux measurement data for different nanofluids tested. Section 5 presents conclusive remarks.

(1)

where the resistance-temperature coefficient is R ) 0.003715 K-1 for gold and R0 is the resistance of gold at the reference temperature T0 ) 25 °C. Equation 1 implies that the same gold heater can serve as a temperature sensor via measurement of the resistance. The resistance value uncertainty occurring from the linearization approximation of eq 1 is estimated as 0.0062 Ω, and the corresponding temperature uncertainty is estimated as 0.12%.25 The dry heater temperature remains steady and uniform to within less than (1 °C for both spatial and temporal fluctuations. This is attributed to the large differential in thermal conductivities between the soda lime glass substrate (0.937 W/m‚Κ at 300 K) and the gold heater electrodes (317 W/m‚Κ). It should be noted that the thermal conductivity is 0.026 W/m‚Κ for air and 0.613 W/m‚Κ for water at the same temperature. The experimental setup consists of two subsystems: (1) the heater power control and recording unit for constant voltage operation and (2) the imaging unit with a Canon Macrolens FD 50-mm CCD camera (640 × 480 pixels at 1 fps) to record the droplet evaporation and dryout progress. The constant voltage circuit consists of a parallel arrangement of the 32 heater lines, each line connected in series with a fixed 51 Ω resistor. Because each fixed resistor, Rdiv, acts as a voltage divider, the current may be calculated as iH ) Vdiv/Rdiv with knowledge of the supply voltage and the voltage drop, Vdiv, across the fixed resistor. When a droplet contacts the heater array, the resistance of each line heater, RH, varies accordingly and can be calculated as RH ) VH/iH by measurement of the voltage across each heater line, VH. Subsequently, the line heater temperature is calculated using eq 1, and the heat flux from each heater element is calculated as

2. Microheater Array A microheater array was designed and fabricated using standard MEMS lithography techniques within a class-1000 clean room facility. As shown in Figure 1, the microheater array consists of 32 gold line heaters that are individually 100 µm wide, 0.5 µm thick, and 1.5 cm long, and spaced 100 µm apart. In total, the 32 linear heaters provide a heated area of 0.945 cm2 (0.63 cm wide × 1.5 cm long). In the final step of construction, the microheater array was coated with a thin layer of SU-8 to provide electrical insulation and

PH ) i2HRH )

V2div R2div

RH(T)

(2)

Before each evaporation test, the microheater array surface was thoroughly cleaned with 99.9% isopropyl alcohol and a specified (29) Young, H. D.; Freedman, R. A. UniVersity Physics, 9th ed.; Addison Wesley: New York, 1996; pp 805.

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Figure 5. Nanofluid dryout patterns for the case of 80 °C initial heater temperature, primarily depending on the nanoparticle sizes for 5-µL droplets individually containing the same volume concentration of 0.5 vol % of (a) 2-nm Au particles, (b) 11-nm Al2O3 particles, (c) 30-nm CuO particles, and (d) 47-nm Al2O3 particles. Smaller nanoparticles are deposited thicker and more uniformly in the center region, and the edge-stains are loosely defined. Larger nanoparticles show more distinctive ring-shaped stain at the droplet edge, while very little stain is resulted in the center region.

Figure 6. Schematic illustration of nanofluid droplet evaporation and dryout process depending on particle sizes: (a) smaller nanoparticles (such as 2-nm Au); and (b) larger nanoparticles (such as 47-nm Al2O3). The more densely populated smaller nanoparticles and their higher nanofluid viscosity tend to quench the thermally driven nanoparticle motion, and this results in a thicker and more uniform dryout pattern in the center region with a loosely defined wider ring in the rim area. On the other hand, larger nanoparticles with lower viscosity tend to readily move to the rim because of the more active distillation there, and the resulting dryout leaves a highly distinctive ring-shaped stain. supply voltage was provided to ensure a steady heater surface temperature condition. Once a micro pipet was used to gently place a 5-µL water or nanofluid droplet onto the heater surface, the history of the heater voltage drop was recorded during the entire evaporation/ dryout progress, and the development of the evaporating droplet shape was simultaneously recorded.

3. Tomographic Deconvolution of Line-Averaged Data Strictly speaking, a single measurement of the line heater temperature (eq 1) and the heat flux (eq 2), as determined from the heater voltage drop measurements described above, is valid only if the heater is imposed to have a uniform temperature at any given instant of time. In reality, however, substantial temperature gradients exist, as the different heat transfer characteristics between the dry and wet sections prevail on a single heater surface, and the temperature gradient distribution varies in time as the droplet evaporation progresses. In order to determine the distributed temperature profiles of the droplet considering the temperature gradients, tomographic conversion30 was conducted to deconvolute the line-averaged temperature into radially distributed temperature profiles, assuming axi-symmetric evaporation and heat transfer. The largest axi-symmetric tomographic conversion possible considers 16 concentric deconvolution zones corresponding to the 16 heater lines covering one-half of a droplet’s wet surface and (30) Kak, A. C.; Slaney, M. Principles of Computerized Tomographic Imaging; IEEE Press: New York, 1987; pp 275-296. (31) Franks, F.; Water: A matrix of life, 2nd ed.; Royal Society of Chemistry: Cambridge, UK, 2000.

sufficient surrounding dry region (Figure 2). The semi-circular zone 1 covers one-half of the central region of heater line A, and the concentric zone 2 extends to cover the central region of line B, as well as the two sub regions of line A. In a similar manner, the concentric zone 16 includes partial regions of all heater lines, from A to P. The axi-symmetry assumption allows for the use of only 16 heater lines for the deconvolution calculations, while all 32 heater lines are powered. The total domain for tomography spans a semicircular area of 3.05 mm in radius [0.05 mm for the zone 1 + (16 - 1) × (0.1 mm for the heater line width + 0.1 mm spacing)], which is more than ample, as this study considers 5-µL droplets with ∼1.45 mm radii. The 16 unknown temperatures for the 16 concentric zones, Ti, must be determined from the line-averaged measured temperatures from the 16 line heater elements, T h k. The k-th line heater, as illustrated in Figure 2b, consists of concentric zones of i ) k, k + 1, ..., N (N ) 16), and its resistance is given as N

R hk )

∑ i)k

N

Rk,i )

∑ i)k

F

Lk,i

Lk,i

N

)

wd

F0L

∑ F [1 + R(T - T )] wd ) wd + 0

i

0

F0R

N

i)k

∑ (T - T )L wd i

0

k,i

(3)

i)k

where F is the resistivity of gold with the subscript 0 indicating a reference temperature of 25 °C, w and d are the line heater width N and thickness, respectively, and ∑i)k Lk,i ) L. Assuming a rectangular area for each subzone of the k-th heater line, the equivalent heater length of the sub-zone is given as, Lk,i ) A′k,i/w. Therefore, eq 3 now reduces to R hk )

F0L wd

+

F0R

N

∑ (T - T )A′ wdw i

0

k,i

(4)

i)k

while the measured line-averaged resistance is expressed as R h k ) F0[1 + R(T h k - T0)]

L wd

(5)

where T h k represents the line-averaged temperature of the k-th line heater. Combining eqs 4 and 5, and using a dimensionless area Ak,i ) A′k,i / L · w, gives: N

T hk )

∑ TA

i k,i

(6)

i)k

where the left-hand side presents the measured line-averaged temperature data. The deconvolved temperature data for each concentric zone, Ti, is thus calculated one by one starting from the outmost heater where the heater temperature is uniform and equal to the dry heater temperature.

Strongly Pinned Nanofluid Droplets

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Figure 7. Average temperature (left column) and average heat flux (right column) of different nanofluids and water droplets for initial microheater surface temperatures of (a) 80 and (b) 60 °C. The temporally developed average temperature and heat flux data can be conceptually divided into three periods: (I) Liquid Dominant Evaporation, showing the constant temperature from the initial sharp drop of the data to the point of discontinuity, (II) Dryout Progress, from the discontinuity to the recovery of initial temperature and heat flux, and (III) Formation of Nanoparticle Stain, after completion of dryout. The water droplet, on the other hand, does not show as distinct a discontinuity as for the case of nanofluid droplets. The tomographic deconvolution is progressively performed starting from the outmost concentric zone toward the center. The associated uncertainties will also be progressive. More detailed estimation calculations26 show that the uncertainties range from (0.2 °C for the outmost concentric zone (k ) 16) to (0.8 °C for the center zone (k ) 1). With the aforementioned heater resistance uncertainty accounted for as (0.12%, the overall measurement uncertainty is estimated as (1.00 °C, including both system bias and the accumulated uncertainties resulting from tomographic deconvolution.

4. Results and Discussion Four different nanofluid samples containing 0.5 vol % nanoparticles were tested: (1) 2-nm Au nanoparticles (nanoComposix, Inc.), (2) 11-nm Al2O3 nanoparticles (Nanostructured and Amorphous Materials, Inc.), (3) 30-nm CuO nanoparticles (Nanophase, Inc.), and (4) 47-nm Al2O3 nanoparticles (Nanophase, Inc.). Table 1 shows thermo-physical properties of the three tested nanoparticles at 300 K along with water properties for comparison. 4.1. Evolution and Dryout of Evaporating Nanofluid Droplets. Figure 3 details the evolution of 11-nm Al2O3 nanofluid droplet evaporation as one representative example. Immediately after placement on the microheater substrate, the droplet is pinned along the wet perimeter due to the capillary forces between nanoparticles that are close to the surface4 and the irregularity of the surface, including its roughness and contact potential5,6 (Figure 3a). During the liquid-dominant evaporation (Figure 3b), the strong pinning of nanoparticles acts to congregate them to the rim. The droplet height and contact angle decrease, while the wet diameter remains constant. With further evaporation of liquid, the contact angle exceeds the critical angle15,19 and the thin core liquid region begins to break away from the rim (Figure 3c). It should be noted that the rim region, where most nanoparticles are distilled with little water, dries out first as a result of the expedited evaporation by the extended thin film monolayer of

particles at the droplet edge.32 The extended thin film of the droplet plays a role similar to that of a meniscus where it is reported that the evaporative mass flux of the thin film region is much higher compared to the bulk fluid region.33 The depinned core liquid then shrinks toward the center as the evaporation further progresses (Figure 3d). The resulting ring-shaped nanoparticle stain is formed along the rim, and the evaporation is completed (Figure 3e). Figure 4 shows evolvement of droplet wet diameters (D/Do) as functions of evaporation time for water and four tested nanofluids heated by the microheater array at an initial surface temperature of T0 ) 80 °C. (It should be noted that the heater array is coated with SU-8 for electrical insulation and protection.34) The strong pinning of all nanofluid droplets sustains their wet diameters to remain the same as the initial wet diameter (D0) until their complete dryout.8 The water droplet wet diameter remains unchanged during the pool evaporation of water, which occupies more than 90% of the total evaporation time, and the wet diameter drastically shrinks during the dryout as the peripheral thin film rapidly recedes toward the center. Figure 5 shows different dryout patterns for the case of an 80 °C initial heater temperature primarily depending on nanoparticle sizes at the same volume concentration of 0.5 vol %: (a) 2-nm Au particles, (b) 11-nm Al2O3 particles, (c) 30-nm CuO particles, and (d) 47-nm Al2O3 particles. The greatly increased number density of the finer gold particles enhances the resulting viscosity35 and interparticular capillary actions,36 which in turn make the nanoparticle distillation less pronounced and tend to spread the nanoparticles out as the evaporation progresses to dryout (Figure (32) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 10571060. (33) Wee, S. K.; Kihm, K. D.; Hallinan, K. P. Int. J. Heat Mass Transfer 2005, 48, 265-278. (34) Vijayendran, R. A.; Motsegood, K. M.; Beebe, D. J.; Leckband, D. E. Langmuir 2003, 19, 1824-1828. (35) Pak, B. C.; Cho, Y. I. Exp. Heat Transfer 1998, 11, 151-170. (36) Son, S. Y.; Kihm, K. D. Atomization Sprays 1998, 8, 503-519.

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Figure 8. Deconvoluted temperature drop profiles for (a) 2-nm Au, (b) 30-nm CuO, and (c) 47-nm Al2O3 nanofluids. The left column presents the temperature drop (from initial T0 ) 80 °C) with time at different radial locations, and the right column presents the corresponding radial profiles with time.

5a). The relatively higher specific gravity of gold (SG ) 19.3) also contributes to hinder the distillation of nanoparticles to the rim. A thicker and more uniform dryout pattern results in the core with a loosely defined, wider ring in the rim, as schematically illustrated in Figure 6a. At the other extreme among the tested, the dryout pattern of the largest (47-nm) Al2O3 nanoparticles (Figure 5d and Figure 6b for schematic illustration), which are about 12 000 times less populated than the gold particles, shows a highly distinctive ring-shaped stain because of the relatively lower viscosity of the nanofluid containing large nanoparticles. The low SG of 3.6 also makes the Al2O3 nanoparticles readily move to the rim during the distillation. The in-between cases of 11-nm Al2O3 and 30-nm CuO nanofluids (Figures 5b and c) with a 6.5 SG display thin dryout layers in the core and loosely defined wide ring stains in the rim, reflecting a cross between the two extreme cases. 4.2. Temperature and Heat Flux Characteristics of Evaporating Nanofluid Droplets Figure 7 plots the average temperature (left column) and average heat flux (right column) of different nanofluids and water droplets for initial microheater surface temperatures of (a) 80 and (b) 60 °C. Tomographically deconvoluted thermal properties, either temperature or heat flux, are multiplied by the fraction of each concentric ring area to the

initial droplet wet area, and the summation of all these weighted properties provides the average properties. The final stage of water droplet evaporation shows a more gradual recovery in temperature, as well as heat flux, than in nanofluids. The final stage of nanofluid droplet evaporation reveals substantially more rapid temperature and heat flux recovery. It is believed that this is attributed to the strongly pinned nanoparticles, which keep the droplet diameter constant during evaporation and hold water molecules between nanoparticles until sudden release after the depinning process upon liquid depletion. The temporally developed average temperature and heat flux data of nanofluids can be conceptually divided into three periods: (I) Liquid Dominant Evaporation, showing the constant temperature from the initial sharp drop of the data to the point of discontinuity (Figure 3a and b), (II) Dryout Progress, from the discontinuity to the recovery of initial temperature and heat flux (Figure 3c and d), and (III) Formation of Nanoparticle Stain, after completion of dryout (Figure 3e). The data discontinuity occurs due to the competing evaporation/dryout process in that the high thermal conductivity of nanoparticles dramatically expedites the evaporation of the surrounding liquid. The water droplet, on the other hand, does not show a discontinuity as

Strongly Pinned Nanofluid Droplets

Figure 9. Temperature spans between the droplet center and edge averaged over the period from 10% to 90% of the evaporation time. The largest 47-nm Al2O3 nanofluid shows the largest temperature difference, reflecting more distinct separation of deposited particles to the edge area from the center region. For the case of the 2-nm Au nanofluid, both nanoparticle concentration and temperature distribution deviate less between the center and edge regions. The corresponding dryout pattern shows a loosely defined rim with relatively thickly coated nanoparticle stain in the center. Both the 11-nm Al2O3 nanofluid and the 30-nm CuO nanofluid are approximately the same and are consistent with their similar deposition patterns.

distinct as that for the case of nanofluid droplets because of the phase change nature of a pure liquid. The water droplet displays a smooth transition from the constant-temperature pool evaporation to the gradually increasing dryout evaporation. For the case of the smallest Au nanoparticles (2-nm average diameter), the delayed transition from period I to period II is persistently observed, while virtually no distinction is seen for the rest of the nanofluids containing nanoparticles ranging from 11 to 47 nm in diameter. It is conjectured that the high population of Au nanoparticles and the increased nanofluid viscosity are expected to slow down the convective heat/mass transport inside the droplet, resulting in a slower evaporation of the surrounding liquid. Figure 8 shows deconvoluted temperature profiles for (a) 2-nm Au, (b) 30-nm CuO, and (c) 47-nm Al2O3 nanofluids for the case of an initial microheater surface temperature of T0 ) 80 °C. The left column shows the development of the temperature variations from the initial 80 °C with time starting from droplet contact. The family of curves are the corresponding profiles at different radial locations, ranging from r/r0 ) 0-2 where r0 is the droplet wet radius at t ) 0. For the case of Au nanofluid, the relatively high viscosity tends to slow down the densely populated fine particles and constrains them to a more even distribution during the evaporation (Figure 6a). This is consistent with the relatively small variations in temperature distribution inside the wet area from r/r0 ) 0-1, as shown in Figure 8a. The spread of temperature distributions inside the wet area increases with increasing nanoparticle size (Figure 8b and c). The more distinctively defined rim area is packed with nanoparticles (see Figure 6b), and the local temperature drop at r/r0 ) 1 tends to be smaller due to the enhanced heat transfer from the heater to those highly conducting nanoparticles. Most evaporation occurs in the center region containing water, and the local temperature at r/r0 ) 0 decreases due to the latent heat release. As a result, the radial temperature distribution on the contact surface spreads more with increasing nanoparticle size.

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Also, the magnitude of the maximum temperature drop at r/r0 ) 0 increases with increasing nanoparticle size. The right column shows the same data of the left column displayed as radial temperature profiles evolving with time ranging from t/τ0 ) 0.02-2 where τ0 is the dryout time for each case. It is clearly seen that the magnitude of the maximum temperature drop increases with increasing nanoparticle size. Figure 9 shows the temperature differentials between the droplet center and edge, averaged over the period from 10% to 90% of the evaporation time, with their variations shown with extended bars, for two different initial heater temperatures of 60 and 80 °C. The largest 47-nm Al2O3 nanofluid shows the largest temperature difference, reflecting a more distinct separation of deposited particles in the edge area (Figure 5d). During evaporation, the high thermal conductivity of pinned nanoparticles enhances the edge temperature over the center water region temperature. The temperature differential progressively decreases with decreasing nanoparticle size. The temperature differentials for both the 11-nm Al2O3 nanofluid and the 30-nm CuO nanofluid are approximately the same due to their similar deposition patterns, namely, loosely defined wider rim areas and thinly coated nanoparticle stains in the center region (Figure 5b and c). For the case of the 47-nm nanofluid, the nanoparticles distributed in the central area, even at low population, tend to enhance the heat transfer in comparison with the case of pure water, and this is attributed to the smaller temperature differentials. The densely populated 2-nm Au nanofluid with higher viscosity results in more evenly distributed nanoparticles in the wet area (Figure 5a) and further lowers the temperature differential. This is attributed to the enhanced thermal conductivity by the nanoparticles in the center region.

5. Conclusions Thermal characteristics of evaporating nanofluid droplets are experimentally studied using a microheater array of 32 line elements that are 100-µm wide, 0.5-µm thick, and 1.5-cm long under a constant-voltage mode. Four different nanofluids have been tested, containing 2-nm Au, 30-nm CuO, 11-nm Al2O3, and 47-nm Al2O3 nanoparticles, each of 5-µL droplets with 0.5 vol % in water. Strongly pinned nanofluid droplets are considered for a sequential evaporation process of (1) pinning, (2) liquid dominant evaporation, (3) depinning, (4) dryout progress, and (5) formation of nanoparticle stain (Figure 3). Upon completion of the evaporation process, ring-shaped nanoparticle stains are left, the pattern of which strongly depends upon the nanoparticle sizes (Figure 5). Smaller nanoparticles result in relatively wider edge accumulation and more uniform central deposition, whereas larger nanoparticles make narrower and more distinctive stains at the edge with less central deposition (Figures 5 and 6). Tomographic deconvolution of measured data obtained from the linear heater elements reveals spatially and temporally resolved temperature/heat flux profiles on the wet droplet surface (Figures 7 and 8). Nanofluid evaporation consists of three periods. First, Liquid Dominant Evaporation (I) occurs with steady thermal properties that are nearly identical to those of pure water with little effect of suspended nanoparticles on the overall heat and mass transfer. Next, the Dryout Progress (II) characterizes the later part of evaporation, when the nanoparticle effect dominates and water level is receded. This period shows a discontinuous surge of temperature and heat flux due to the high thermal conductivity of nanoparticles, which in turn rapidly recovers to the dry heater condition while the recovery process for pure water droplet is gradual and continual. Finally, Formation of

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Nanoparticle Stain (III) period occurs, which strongly depends on nanoparticle size. The temperature span between the droplet center and edge (Figure 8) increases with increasing particle size. This is consistent with the different stain patterns of different nanoparticle sizes. The more distinct ring-shaped edge accumulation of larger nanoparticles results in a larger temperature span because of the large difference of thermal conductivity between nanoparticles and water. Accordingly, for the case of smaller nanoparticles,

Chon et al.

less distinction between the edge ring and the central deposits can result in a relatively small temperature span. Acknowledgment. This work was partly supported by the U.S. Department of Energy, Office of Basic Energy Science under Contract No. DE-FG02-05ER46182, and partly by the NASA Project Grant No. NNC05GA18G to the University of Tennessee. LA061661Y