Evaporation of Sessile Droplets Affected by Graphite Nanoparticles

Nov 5, 2014 - School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798. J. Phys. Chem. B , 2014, 118 (47), p...
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Evaporation of Sessile Droplets Affected by Graphite Nanoparticles and Binary Base Fluids Xin Zhong and Fei Duan* School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798 ABSTRACT: The effects of ethanol component and nanoparticle concentration on evaporation dynamics of graphite− water nanofluid droplets have been studied experimentally. The results show that the formed deposition patterns vary greatly with an increase in ethanol concentration from 0 to 50 vol %. Nanoparticles have been observed to be carried to the droplet surface and form a large piece of aggregate. The volume evaporation rate on average increases as the ethanol concentration increases from 0 to 50 vol % in the binary mixture nanofluid droplets. The evaporation rate at the initial stage is more rapid than that at the late stage to dry, revealing a deviation from a linear fitting line, standing for a constant evaporation rate. The deviation is more intense with a higher ethanol concentration. The ethanol-induced smaller liquid−vapor surface tension leads to higher wettability of the nanofluid droplets. The graphite nanoparticles in ethanol−water droplets reinforce the pinning effect in the drying process, and the droplets with more ethanol demonstrate the depinning behavior only at the late stage. The addition of graphite nanoparticles in water enhances a droplet baseline spreading at the beginning of evaporation, a pinning effect during evaporation, and the evaporation rate. However, with a relatively high nanoparticle concentration, the enhancement is attenuated.

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INTRODUCTION Nanofluids are kinds of fluids that normally contain insoluble nanosized particles. The evaporation of a nanofluid droplet on a solid substrate has attracted extensive interest as the selfassembly and deposition pattern forming after drying have plenty of important applications, such as coating, inkjet printing,1 DNA molecular stretching,2 and micropatterning of electronic devices. Controlling deposition patterns is useful in particular fields. A coffee-ring, for instance, is desirable in the print of repetitive fine lines,3,4 while a uniform profile is required in coating. The pattern formation is dependent on the evaporation dynamics of the droplets. The prerequisite for forming a coffee-ring is a pinned contact line,3 while a uniform pattern can be attained by driving particles to the liquid−vapor interface to achieve “skin formation”.5 The transition between the coffee-ring and uniform deposition can be managed by changing the particle shapes6,7 and adding the surfactant.8,9 In addition, the alteration of pinning and depinning of the contact line can produce concentric rings, known as the stick−slip behavior.10 Thus, fully understanding the evaporation dynamics of nanofluid droplets makes it possible to meet the challenge of producing desirable patterns. The evaporation process of a nanofluid droplet is a complex process. The suspended nanoparticles cannot be simply treated as passively driven insoluble solutes, but they exert great impact on the wetting dynamics and evaporation of the droplet, accordingly varying the profile of the deposition. The nanoparticles can extend the pinning duration of a sessile droplet and lead to more long-lasting radial outward flow.11,12 Nanoparticles have also been found to enlarge the contact © XXXX American Chemical Society

angle. It is attributed to the deposition of nanoparticles, particularly the self-assembly near the three-phase line,13 which modifies liquid−solid surface tension,14 and the interaction with the solid substrate.15,16 Besides, nanoparticles have influence on the evaporation rate of a droplet. Iron oxide (Fe2O3), zirconium dioxide (ZrO2), or nickel/iron (Ni/Fe) nanoparticle addition into the droplets was observed to reduce the evaporation rate, while the presence of clay nanoparticles increased evaporation.17 On one hand, the evaporation could be enhanced due to a prolonged pinning stage caused by nanoparticles; on the other hand, the aggregation of nanoparticles at the three-phase line, and the increased fluid viscosity could attenuate evaporation. So far, the mechanism of the nanoparticle effect on evaporation is not fully understood, and further systemic exploration is necessary. Combined with the nanoparticles, the manipulation of base fluids has been made with the employment of mixtures comprising liquids with different thermal and physical properties. Hitherto, some works have been contributed to probe the complex flow regimes led by the different volatility in the droplets of pure binary fluids without nanoparticles.18−22 Sefiane et al. investigated the evaporation behavior of a pure ethanol−water sessile droplet on a polytetrafluoroethylene (PTFE) substrate20 and found three evaporation regions in terms of contact angle, base diameter, and volume. They indicated that the ethanol, which is more volatile, evaporated during the first phase, while the water Received: August 8, 2014 Revised: November 5, 2014

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before the evaporation experiments. Clean silicon wafers (Bonda Technology Pte Ltd., hydrophilic) were used as evaporation substrates. Evaporation experiments were performed using a goniometer and a bright-field microscope to study the evaporation dynamics and the deposition patterns of the suspensions, respectively. For evaporation dynamics, the goniometer employed to record the images of the evaporating droplets was a “Wet Angle” Theta Optical Tensiometer, as shown in Figure 1. A controlling dosing system was set to generate sessile

evaporated at the third stage. In the second stage, the contact angle increased. Cheng et al. studied the evaporation of the ethanol−water mixing droplets on the coated gold surface under ambient conditions,21 and Liu et al. studied the evaporation of the sessile binary liquid droplets on an oxygen-plasma-treated silicon plate without heating or cooling in a controlled chamber.22 They both verified the three evaporation regions in the binary liquid droplets before drying. Sefiane et al. and Cheng et al. indicated that the droplet baseline shrank at the very early stage.20,21 However, the influence of nanoparticles on both droplet wettability and evaporation still remains unsolved. In addition, the forming process of a pattern affected by the complex flows in the droplets containing nanoparticles has not been studied extensively. In this study, we quantitatively investigate the effect of ethanol concentration in water for nanofluid droplets and the graphite nanoparticle concentration for water-based droplets on the wettability, evaporation rate, and deposition patterns. We use a goniometer to study the evaporation dynamics and microscopy to monitor the nanoparticle motion in the drying nanofluid droplet. We provide a primarily direct glimpse of the nanoparticle-modified evaporation dynamics in binary droplets, the formation of the nanoparticle aggregation at the droplet surface and the final deposition patterns affected by ethanol concentration, and the initial spreading of the sessile waterbased droplets due to graphite nanoparticle addition.

Figure 1. Schematic of the goniometer. The inset is the photo of the test rig.

droplets with a required volume of 2.09 ± 0.11 μL. The evaporation process was recorded with a high-resolution digital camera at a frequency of 1 frame per 5 s. Entire evaporation times ranged from 20 to 30 min. Ball calibration was conducted before each experiment to minimize experimental error. After the evaporation experiments, all of the images were postprocessed by executing the graphic analysis function in the program, through which the contact angle, baseline length, and droplet volume were obtained and recorded. For part of the deposition patterns, a microscopy system (LV100D-U from Nikon Inc.) with an objective lens at 2× magnification was employed to observe the transport of nanoparticles during evaporation and the formed patterns after evaporation. The droplets were evaporating under the open conditions with the pressure at 1 atm, the temperate at 24 ± 1 °C, and the relative humidity at about 50 ± 5%. To ensure the repeatability of our experiments, at least three tests were conducted for each set of experimental conditions in Table 1. The measurement accuracy of the contact angle or the volume was ±0.1° or ±5%. The uncertainties of the postprocessing were evaluated within ±5%.

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EXPERIMENTAL METHODS To investigate the effects of ethanol solvent and graphite nanoparticles on the droplet evaporation, a water-based suspension of 1.5 g/L nanoparticles was selected as a reference. The ethanol concentration in the binary fluids and the nanoparticle concentration are the two variables in the two prepared batches of samples, respectively: (1) mixtures of 1.5 g/L graphite nanoparticles and water-based binary fluids with 0, 10, 25, 40, and 50 vol % ethanol; and (2) water-based suspensions with graphite nanoparticle concentrations at 0, 0.5, 1.0, 1.5, and 2.0 g/L. The details are listed in Table 1. The Table 1. Experimental Conditions of Sessile Nanofluids for Droplet Evaporation ethanol concentration, vol %

nanoparticle concentration, g/L

exp. no.

0

0.0

N1 N2 N3 N4 N5 N6 N7 N8 N9

√

10

25

40

50

0.5

1.0

2.0

√ √ √ √ √

√ √ √ √ √ √ √ √

1.5

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RESULTS AND DISCUSSION Effect of the Volume Concentration of Ethanol in the Binary Fluids. The role taken by ethanol in nanofluid droplet evaporation is examined by varying its concentration within the range from 0 to 50 vol %. The parameters examined here are droplet volume, baseline length, and contact angle. The visual changes of droplet drying sequences of experiments N1−N5 for the side view are in Figure 2a−e, while the resulting deposition patterns are shown in Figure 2f. Similar to the evaporation behavior of the pure liquid droplets on the no heating or cooling substrates reported by Sefiane et al.,20 Cheng et al.,21 and Liu et al.,22 the droplets with graphite nanoparticles evaporate faster with an increase in the ethanol concentration, represented by the droplet volume, which decreases as the ethanol concentration increases at 400 s (Figure 2e). The initial droplet baseline is more extended with a higher ethanol

√ √ √ √

nanofluids were prepared with graphite powders at 2−3 nm from Skyspring Nanomaterial Inc., ethanol (C2H5OH, Reagent ACS grade), and deionized water with the resistivity at 18.0 MΩ−cm. The surface tension of pure water was occasionally measured as it was exposed in the laboratory environment for 1 h, and no variation was observed. The samples were mixed first and then kept in an ultrasonication bath (Fisher Scientific model 500) for 5 h to obtain well-distributed mixtures just B

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Figure 2. Drying sequences and patterns of 1.5 g/L nanofluid droplets with 0, 10, 25, 40, and 50 vol % ethanol concentrations at (a) 0, (b) 50, (c) 100, (d) 200, and (e) 400 s. The dotted−dashed lines show the edges of the droplets at 0 s. The dried patterns are shown in (f).

Figure 3. Drying sequence of a (a) 1.5 g/L nanofluid droplet with 10 vol % ethanol and (b) 1.5 g/L nanofluid droplet with 40 vol % ethanol. The evaporation time is normalized with the entire evaporation lifetime for each experiment. For the 1.5 g/L nanofluid droplet with 10 vol % ethanol (N2), the sporadic nanoparticle fragments were formed at 17% of the lifetime. The chaotic flows ended at around 25% of the lifetime, and the inward flow to the centerline of the droplet, or Marangoni flow, was found; the Marangoni flow finished roughly at 70% of the lifetime. For the 1.5 g/L nanofluid droplet with 40 vol % ethanol (N4), from the moment of 5 to 20 and 25% of the lifetime, the aggregate at the droplet interface grew rapidly due to vortices; the aggregate fully covered the interface at about 55% of the lifetime, and the Marangoni flow started to act; it finished at around 83% of the lifetime.

relatively uniform distribution. With only 10 vol % ethanol concentration, however, most nanoparticle residuals stay at the central area and detach from the outer ring. As the ethanol concentration continues to increase to 25 vol %, the deposit approaches uniformity again. However, the detachment between the interior deposit and the outer ring emerges and intensifies with the ethanol concentration increasing to 40 and 50 vol %. Even though we cannot measure the flowing in the nanofluid droplet with particle image velocimetry now, we can refer to the findings by Christy et al.18 in explaining the flow regimes in a pure water−ethanol droplet. They classified three regimes in evaporation: (1) the ethanol-dominated stage full of chaotic flows and vortices; (2) the transient stage of Marangoni

concentration. With time going, the evaporating droplets with relatively low ethanol concentrations remain pinned, while those at 40 and 50 vol % of ethanol exhibit a depinning behavior at the late stage. Different from the observation of the fast shrinkage of the baseline at the first stage for the pure ethanol−water droplets,20,21 the added graphite nanoparticles in the binary mixtures are seen to prolong the pinning duration for the nanofluid droplets. To quantify the evaporation behavior shown qualitatively in Figure 2a−e, we plot the contact angle, baseline length, and volume as a function of time, which are discussed further. The dried patterns in Figure 2f are sensitive to the variation of ethanol. The pattern from the water-based suspension shows C

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flow led by the nonuniform distribution of ethanol on the liquid−vapor interface; and (3) the water-dominated radial flow. These combined flow regimes can result in our observed patterns. The deposition patterns in Figure 2f are distinctively varied by a small change in ethanol, particularly from 0 to 10 vol %, from 10 to 25 vol %, and from 40 to 50 vol %. It implies that the component of ethanol exerts high influence in the relative weight of the three regimes discussed by Christy et al.,18 and the presence of nanoparticles may modify the flow regimes. To confirm it, we employed the microscopy to record the evaporating graphite−ethanol−water droplets. On the top view, we can observe the flow pattern on the basis of the motion and assembly of the nanoparticles at the droplet surface. At the initial time, strong vortices were observed to carry nanoparticles to the liquid−vapor interface and make them form aggregates. The flow in this stage was intense and rapid. Then, the flow slowed down gradually, and the formed aggregates along the liquid−vapor interface started to move to the droplet centerline. This is seen to be generated by the Marangoni flow, originating from the surface tension variation that is induced by the nonuniform local evaporation flux with the maximum value at the three-phase line.23,24 Ultimately, the aggregates stopped moving inward but descended toward the substrate. However, we found that both the duration for each flow regime and the intensity of the fluid flow depend significantly on the ethanol concentration. The top views of the drying sequences of the nanofluid droplets with 10 and 40 vol % ethanol concentrations are demonstrated in Figure 3. Because the evaporation lifetimes for both of the droplets are different, we normalized the evaporation time with the whole drying time for each nanofluid droplet. As shown in Figure 3a for 10 vol % ethanol (experiment N2), the regime of vortices due to chaotic flows ended at around 25% of the lifetime, during which the intense flow moved the nanoparticles and the sporadic aggregate to the droplet surface. Afterward, the fragments were transported to the centerline with the flow at the liquid−vapor interface until 70% of the lifetime. With a higher ethanol at 40 vol % (N4), the finishing moment of the vortices and chaotic flow was roughly at 55%, and the flow toward the centerline was almost gone at 83% of the lifetime. The detaching gap of the nanoparticle deposition in Figure 2f was a result of depinning at the last stage. The distinction between the patterns from these two droplets is attributed to the concentration of ethanol. The induced vortices in experiment N2 were not as strong as those in experiment N4; thus, the speed for the formation of the aggregates was low. Moreover, the much stronger chaotic flows lasting for a longer time in the droplet of 40 vol % ethanol led to more rapid formation of the aggregates. One noticeable difference in the observation of the surface flow is that the nanoparticles prolonged the flow toward the centerline, or the Marangoni flow, in the water−ethanol-based nanofluid droplet in comparison with the pure liquid water− ethanol droplet. In the study of Christy et al.,18 the Marangoni flow in the transient stage occupied only 8% of the entire time, while in our observation, the slow inward movement of the aggregate took over about 30% of the lifetime. Although the mechanism underlying is still unclear at present, there are two possible reasons for the longer-lasting Marangoni flow in the nanofluid droplet: (1) A number of nanoparticles were seen to accumulate at the droplet rim in the experiments; therefore, the pinning effect was enhanced and could maintain the nonuniform local evaporation at droplet surface; and (2) the coverage

of the graphite aggregates at the liquid−vapor interface could modify the liquid−vapor surface tension, resulting in a modified gradient of surface tension and a subsequent fluid circulation even when less ethanol was left. However, in order to clarify the mechanism and to quantify the influence of ethanol concentration on the flow regimes and the drying patterns of the nanofluid droplets, further detailed visualization investigation would help. Figure 4a shows the trend of the volume of the sessile droplets containing an ethanol component. The starting

Figure 4. (a) Droplet volume change with time of 1.5 g/L nanofluid droplets with 0, 10, 25, 40, and 50 vol % ethanol concentrations; below the dashed line, the evaporation stage is dominated by the less volatile component. (b) The averaged evaporation rate of 1.5 g/L nanofluid droplets as a function of ethanol concentration and the comparison with the pure water−ethanol sessile droplets,20−22 in which the substrates were not heated and cooled. The inset is the comparison between the average evaporation rate at the late stage (0.7 μL before dry-out) and the overall average rate of the graphite−ethanol−water droplets.

volume for each evaporating droplet is about 2.09 μL. As ethanol is more volatile than deionized water, the droplet with a higher concentration of ethanol is expected to evaporate faster, consistent with the steeper trend of the volume change at a higher ethanol concentration (Figure 4a). The droplet volume of the suspension without ethanol (experiment N1) declines almost linearly, suggesting an approximately constant evaporation rate. With the addition of ethanol, the reducing rate of the volume at the initial stage is more rapid than that at the late stage, representing a deviation of the evaporation rate from a constant value. The deviation is more intensified with a higher ethanol concentration, resulting from the fact that a relatively larger amount of ethanol that evaporates initially induces a greater difference between the initial stage with fast evaporation D

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and the late stage of the water-dominated evaporation with a relatively low rate. The data are summarized and plotted in Figure 4b to show the average volume evaporation rates of the graphite−ethanol− water droplets as a function of ethanol concentration in the binary mixtures. The evaporation rate of the nanofluid droplets increases approximately linearly with the loading of ethanol from 0 to 40 vol %, but at 50 vol % of ethanol, the evaporation rate exhibits a remarkable increase. The average evaporation rate for the water-based nanofluid droplet (N1) is roughly 1.53 × 10−3 μL/s, almost half of that of the droplet with 40 vol % ethanol (N4) and about one-fifth of that for the droplet with 50 vol % ethanol (N5). The results are compared with the evaporation rates of the pure ethanol−water droplets without any particles20−22 in Figure 4b. The ethanol−water droplets were evaporating on a PTFE surface at 1 atm in the investigation of Sefiane et al.20 As the ethanol concentration is at 0, 25, and 50%, the evaporation rates of the pure binary droplets20 are all below those of the binary fluids containing 1.5 g/L graphite nanoparticles in this study. Further comparisons are made with the evaporation experiments of the pure ethanol−water droplets on the monolayer-coated gold surface at the temperature of 23 °C with the relative humidity of 40− 50%21 and on the no heating or cooling silicon plate with the relative humidity of 47−64%.22 The evaporation rates with the nanoparticles are higher than those of the pure binary liquid droplets. It could be a result of the enhanced thermal conductivity of the nanofluids. Another reason is related to the enhanced pinning effect led by nanoparticles. Moreover, we investigate the average evaporation rate at the late stage of the binary nanofluid droplets. The data of the last 0.7 μL (under the dashed line in Figure 4a) are selected, plotted, and illustrated in the inset of Figure 4b. The average evaporation rate at the late stage is much lower than the related overall rate, and it still increases with the ethanol loading and stays higher than that of the pure water droplet with 1.5 g/L graphite nanoparticles (N1). One possible factor is that the left solvents still have ethanol; the other, more important, factor is the enhanced pinning effect of the three-phase line led by the nanoparticles. The baseline length is found to be extended with the higher loading of ethanol; therefore, the nanofluid droplet with a higher ethanol concentration has a larger contact area with the substrate, leading to a more rapid evaporation.14 Besides, the nanoparticle deposit at the contact line helps pinning of the contact line. Even though the ethanol loss in the binary mixtures becomes higher with the proceeding of evaporation, the deposited particles retard the contact line shrinkage; thus, the droplet can maintain a relatively large contact area. We can see that the last-stage baseline of the nanofluid droplet with 50 vol % ethanol (N5) is still way longer than that of the pure water-based nanofluid droplet (N1). On the basis of the above discussion, we measure the contact angle and the baseline to reveal the role taken by ethanol in evaporation. Figure 5a is for the raw data of the contact angle, and Figure 5b is for the normalized data with a reference to the initial contact angle. Figure 6a,b illustrates the raw data of the baseline length as a function of time and the relative values with a reference to the initial baseline, respectively. As the droplets maintain pinned for most of the lifetime, the evaporationinduced reduction of volume is primarily reflected in the decrease of the contact angle. Similar to the trend of volume, the contact angle shows a faster reducing rate with a higher ethanol component in the nanofluid droplets in Figure 5. The

Figure 5. (a) Contact angle change with time in droplets with 0, 10, 25, 40, and 50 vol % ethanol concentrations in 1.5 g/L nanofluids. (b) The relative contact angle change with time. The initial contact angle of 1.5 g/L nanofluid droplets as a function of ethanol is in the inset.

Figure 6. (a) Baseline length change as a function of time of droplets with 0, 10, 25, 40, and 50 vol % ethanol−water binary nanofluids. (b) The relative baseline length change with time.

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Figure 7. Drying sequences of droplets with 0, 0.5, 1, 1.5, and 2 g/L nanoparticle concentrations at (a) 0, (b) 20, (c) 100, (d) 500, and (e) 1100s. The dotted−dashed lines show the edges of the droplets at 0 s. The dried patterns are shown in (f).

contact angle displays a deviation as well in the nanofluid droplets comprising ethanol, in contrast to the linear reduction of the contact angle of the nanofluid droplet without ethanol addition. The reducing contact angle throughout the evaporation is significantly different from the ones of pure ethanol−water mixtures, which increase in the stage of fast evaporation of ethanol,20−22 resulting from the enhanced pinning effect of the three-phase contact line due to the nanoparticle deposition. Due to the lower surface tension of ethanol, the droplet with a higher ethanol concentration exhibits a smaller contact angle and stronger wettability. As shown in the inset of Figure 5, for the water-based nanofluid droplet, the initial contact angle is 50.8°. With the ethanol increasing to 10, 25, 40, and 50 vol %, the corresponding contact angle is 47.6, 41.2, 36.9, or 23.9°, respectively. A higher ethanol concentration results in a relatively lower apparent surface tension in the water−ethanol binary fluids. The force balance at the contact line can be expressed in principle by the Young equation,25 γSV = γSL + γLV cos θ, in which γSV, γSL, γLV, and θ are the solid−vapor surface tension, solid−liquid surface tension, liquid−vapor surface tension, and contact angle. As the droplets are added with more ethanol, the γLV is found to reduce significantly;26 therefore, the contact angle θ should decrease to maintain the balance while there is not much change in γSV and γSL. Thus, a higher ethanol concentration leads to a smaller initial contact angle for the nanofluid droplets in the inset of Figure 5 and a longer baseline in Figure 6. Corresponding to the reduced contact angle in experiments N1−N5, the droplet baseline length at the beginning of evaporation is evidently elongated with more addition of ethanol. The pure water-based nanofluid droplet maintains pinned on the substrate for almost the whole drying time in experiment N1. As the ethanol concentration increases from 10 (N2) to 50% (N5), the duration of the pinned baseline length

gradually decreases. The sessile droplets exhibit depinning behavior at only the late stage of evaporation at 40 and 50 vol % of ethanol. As aforementioned, the ethanol addition leads to fluid circulation in the droplets; thus, the inward flow along the liquid−vapor interface can move the nanoparticles toward the centerline of the droplet, while the outward flow, due to the higher evaporation loss at the contact line,8,23 transfers the nanoparticles to the three-phase line. More particle aggregation reinforces the pinning effects. However, a higher inward flow from the contact line at a higher ethanol concentration can reduce the pinning effect. The two mechanisms compete and result in the pinning behavior at most lifetimes in the series of experiments and the depinning behavior at the last period of experiments N4 and N5. The pinning three-phase line during evaporation in the nanofluid droplets is quite different from the experiments of pure ethanol−water droplets.20,21 The droplets without particles were observed to shrink rapidly from the beginning of evaporation, and then the retraction continued but with a slower rate. The receding trend was seen more intense with a higher ethanol component. With the nanoparticle addition in the binary fluids, the droplets containing ethanol maintain pinned on the solid surface for most of the lifetime, as shown in Figure 6. Even for the one with 50 vol % ethanol (N5), the pinning behavior can be seen in the first 50 s. Then, the droplet is depinned. However, its baseline length at the late stage is still much longer than that of the other droplets (N1−N4). Recalling Figure 4 for the average evaporation rate, we can conclude that the pronounced evaporation in ethanol−water nanofluid droplets in the last stage is mainly attributed to the extended baseline by the ethanol and the long pinning stage by the nanofluids. The large baseline length and the high ethanol component make the nanofluid droplet with 50 vol % ethanol (N5) evaporate much more rapidly than the others in the series of experiments. F

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In addition, we have noticed a repeatedly continuous spreading phenomenon in the water-based nanofluid droplet without ethanol (experiment N1) at the very beginning of the evaporation stage, as shown in Figure 6b. It promotes us to study the effect of nanoparticle concentration on evaporation dynamics of nanofluid droplets. Effect of Graphite Nanoparticle Concentration. In the evaporation of sessile nanofluid droplets, both increased and decreased evaporation rates were found to result from the addition of nanoparticles.17 Although the nanoparticles have been confirmed to enhance liquid film spreading by generating the structural disjoining pressure,27,28 the enlarged contact angle of the nanofluid droplet compared to that of a pure base fluid droplet seems to contradict it. Thus, we investigate waterbased nanofluids with the addition of 0.0, 0.5, 1.0, 1.5, and 2.0 g/L graphite nanoparticles to examine the influence of nanoparticles on sessile water droplet evaporation. Figure 7 shows the drying sequences and the deposition patterns of water droplets with the various concentrations of graphite nanoparticles. At a first glance of Figure 7a−e, it is easy to find out that the pure water droplet is flatter as compared to the other nanofluid droplets. The water droplet retracts from its initial position, different from the long-lasting pinning of the droplets containing nanoparticles. The nanoparticle deposits are shown in Figure 7f. Different from the coffee-ring observation,23 the nanoparticle deposition covers almost the area circumscribed by the initially formed ring mainly due to the nanoparticle sticking to the agglomerate in drying.8,29 With the increasing concentration of graphite nanoparticles, both the interior residual and the coffee-ring are intensified, denoted by the darker deposition color. Moreover, there are formed aggregates as the nanoparticle concentration increases, especially in the droplet with 2.0 g/L nanoparticle concentration. Interestingly, the droplet with the graphite nanoparticles exhibits a fine but obvious behavior: the contact line spreads at the very early stage of evaporation. The initial spreading is not observed in the water droplet. It is suggested that the nanoparticles are the inducement of the spreading. Wasan et al.27,28 observed nanoparticle-induced slow spreading of the nanofluid in a wedge film confined by a glass slide and an oil droplet that was immersed in the nanofluids. The spreading of sessile droplets led by the nanoparticles, however, has not been observed extensively. Here, we provide both direct observation and quantitative analysis for the initial spreading behavior. To investigate the impact of nanoparticles on evaporation, we plot Figures 8 and 9 to show the trend lines of the baseline length and contact angle for experiments N1 and N6−N9. Demonstrated in Figure 8a, the baseline of the deionized water droplet initially is much longer than those of the nanofluid droplets, suggesting that the initial wetting of the pure water droplet (N6) is stronger. Due to the deposition of nanoparticles, the superficial structure and the roughness of the wafer are modified.14 The nonuniform deposition of the nanoparticles particularly near the three-phase line produces inhomogeneity of the surface energy of the substrate, thus attracting the three-phase line to attach to a new site13,14 and varying the contact angle. However, the spreading at the early stage is found only in the nanofluid droplets but not in the pure water droplet. It is sensitive to the graphite nanoparticle loading. Figure 8b demonstrates the initial spreading behavior of each droplet by plotting the increase of a droplet maximum baseline from its initial baseline. It can be found that the

Figure 8. (a) Baseline length change as a function of time in the waterbased nanofluids with graphite nanoparticle concentrations at 0, 0.5, 1.0, 1.5, and 2.0 g/L. (b) The maximum baseline spreading with nanofluid concentrations.

Figure 9. (a) Contact angle change as a function of time in waterbased droplets with graphite nanoparticle concentrations at 0, 0.5, 1.0, 1.5, and 2.0 g/L. (b) The relative contact angle change with time. The inset is the initial contact angle for droplets with various nanoparticle concentrations.

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Compared with the pure water droplet, the initial contact angle of the nanofluid droplets is apparently enlarged, as illustrated in the inset of Figure 9. However, the initial contact angle is not sensitive to the nanoparticle concentration. It recalls our discussion in the enhanced pinning effect of nanoparticles on the contact line. The deposition of nanoparticles on the substrate would modify and roughen the solid substrate surface, resulting in adsorption of the contact line to new sites on the substrate.13,14 It was clarified by Blake and De Coninck that the advancement of liquid on a solid surface was a successive-attaching process that liquid molecules were inclined to be absorbed on sites on the solid surface in a preferable orientation.32 Therefore, the deposited nanoparticles create new sites for liquid molecules to be attracted and thus to change the contact angle.14 The accumulated deposit at the contact line helps anchoring of the three-phase line on the substrate and prevents its macroscopic movement like the pure water droplet. Figure 10a demonstrates the change in droplet volume as a function of time at the particular nanoparticle concentrations

addition of nanoparticles generally enlarges the baseline length at the early evaporation stage. With the graphite nanoparticle concentration increasing from 0.0 to 0.5, 1.0, and 1.5 g/L, the maximum spreading of the baseline increases to 0.018, 0.023, and 0.046 mm. Therefore, the spreading is proportional to the increase of the nanoparticles within the range between 0.0 and 1.5 g/L. The nanoparticles arriving at the three-phase line tend to self-assemble into the ordered structures, leading to an excess energy in the wedge film, known as the structural disjoining pressure, which can drive the three-phase line into motion.28 With the increasing concentration of the nanoparticle, more available nanoparticles can access the thin wedge film and self-organize there; therefore, the baseline length is elongated from 0.0 to 1.5 g/L. However, as the nanoparticle concentration increases from 1.5 to 2.0 g/L, the initial extension of the baseline is smaller. As stated by Vafaei et al.,15 the structural disjoining pressure is more pronounced with smaller nanoparticles, and a larger number of nanoparticles would inhibit the film spreading due to a higher opportunity for them to agglomerate and to form clusters.30 The formed clusters (see Figure 7f) usually are unable to fit into the thin film; thus, the total number of nanoparticles capable of enhancing droplet spreading could be reduced. The nonunidirectional change of the spreading behavior with the nanoparticle concentration is determined by the two competing mechanisms, resulting in a peak of the baseline difference at 1.5 g/L of the nanoparticle concentration. The initial spreading by the nanoparticles might also help extend the baseline for the graphite−ethanol−water droplets (N2−N5), although it was not easy to be separated from the overlapping function by ethanol. During evaporation, the wettability development of droplets is expressed in both the baseline length in Figure 8a and the contact angle in Figure 9. As seen in Figure 8a, the baseline of the pure water droplet remains constant at the initial stage of evaporation. Afterward, at about 400 s, the baseline begins to reduce, and the shrinking is roughly constant until the evaporation completion. The corresponding contact angle in Figure 9a,b shows a quick and linear decrease until around 400 s, and from that moment, the decrease of the contact angle almost stops. The behaviors of the contact angle and baseline of the deionized water droplet suggest that the droplet goes through a constant contact radius (CCR) mode at first and then an approximate constant contact angle (CCA) mode on our silicon substrate. It is seen from the normalized contact angle curves in Figure 9b that the transition point occurs only in the pure water droplet. With the addition of 0.5, 1.0, 1.5, or 2.0 g/L graphite nanoparticles, the contact angle keeps reducing at an approximately linear rate before the final drying, and the change of contact angle as a function of time becomes faster as the nanoparticle concentration increases. In addition, the baseline length of the nanofluid droplets on the substrate remains similar during evaporation, although a small degree of spreading at the beginning stage has been observed. It is indicated that the nanofluid droplet drying goes through the CCR mode. The considerably enhanced pinning effect relates to the forces acting on the contact line. The local forces at the three-phase line include the depinning force responsible for the retracting behavior and the pinning force, which are found dependent on contact angle, roughness, and heterogeneity of the solid surface.31 The deposition of nanoparticles roughens the substrate and acts like an obstacle in the retracting path of the contact line.

Figure 10. (a) Droplet volume as a function of time for droplets with 0, 0.5, 1.0, 1.5, and 2.0 g/L graphite nanoparticle concentrations. (b) The average evaporation rate of the nanofluid droplets with different graphite nanoparticle concentrations. The data are compared with those from the pure water experiments.21,22

for experiments N1 and N6−N9. The volume change of the deionized water droplet as a function of time is much slower, and the lifetime is longer compared to that of the droplets suspended with the graphite nanoparticles, implying that the pure water droplet has a lower average evaporation rate. The steeper trend lines of the other nanofluid droplets suggest that the evaporation is enhanced with a higher concentration of graphite nanoparticles. Figure 10b presents the average evaporation rates of the droplets in the series of experiments. The average evaporation rate of N6 is comparable with those of the pure water droplets.21,22 The evaporation rate basically increases linearly with the graphite concentration ranging from H

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0.0 (N6) to 1.5 g/L (N8). However, the evaporation increasing trend at 2.0 g/L (N9) is slowed down but still higher than the water droplet. As illustrated by Figure 8a, the depinning behavior of the pure water droplet cannot be seen in the nanofluid droplets due to the greatly enhanced pinning effect by nanoparticles. Evaporation is faster in those nanofluid droplets with a nearly pinned baseline. Furthermore, as the graphite nanoparticles have a higher thermal conductivity compared to water, the high loading of graphite nanoparticles can enhance heat conduction for phase change and lead to the evaporation enhancement consequently. However, adding nanoparticles into base fluids leads to a higher fluid viscosity,30 which could inhibit the outward radial flow serving for the replenishment of the high evaporation loss at the droplet edge. More particle aggregation at the surface or in the vicinity of the contact line can also negatively influence the evaporation. The combination of the reasons determines that the increase of the evaporation rate reduces as the nanoparticle concentration reaches 2.0 g/L.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +65 67905510. Fax: +65 67924062. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of A*Star Public Sector Funding (1121202010).

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REFERENCES

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CONCLUSION

We provide direct observation and analysis of the effects of the ethanol concentration in the binary fluids and the nanoparticle concentration in water-based droplets on the evaporation dynamics and the deposition patterns in graphite nanofluid droplets. The nanofluid droplet with a higher ethanol ratio has a higher evaporation rate, and the evaporation rate at the initial stage is more rapid than that at the late stage, revealing a deviation from a constant evaporation rate. This deviation is more intensified with a higher ethanol concentration. The experiments demonstrate that nanoparticles are carried to the interface and form aggregate at the droplet surface only in droplets containing ethanol, revealing a strong fluid circulation that is not observed in the water-based droplets. The formed drying pattern is sensitive to the ethanol concentration in the graphite−ethanol−water nanofluid droplets. In comparison with the pure ethanol−water droplet, the graphite particles enhance the pinning effect and block the contact angle increase and contact line shrinkage during the ethanol-dominated fast evaporation stage. The nanoparticle pinning effect reinforces a high average evaporation rate with a high ethanol ratio in the nanofluid droplets. Moreover, we present an initial baseline spreading in the sessile graphite−water droplets. The initial spreading is intensified first and then attenuated with the loading of graphite nanoparticles. In the drying process, the droplet with the graphite nanoparticles basically shows a stronger pinning effect and an enlarged initial contact angle in comparison with the pure water droplet. The intensified evaporation is mainly due to a prolonged pinning stage and higher thermal conduction for phase change with more nanoparticles. However, the graphite nanoparticles with the concentration at 2.0 g/L attenuate the evaporation rate increasing trend. Evaporation of a nanofluid droplet is a complex process. The pining and depinning behaviors are affected by the base fluids and nanoparticles. The complexity would pose a challenge for the prediction of droplet evaporation from a simple model,3 and further modeling would help to explicate the phenomena. In addition, further detailed flow tracking would improve our understanding of the role of convection on nanofluid droplet evaporation. I

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