Water Nanofluid Droplet

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Experimental investigation of TiO2/water nanofluid droplet impingement on nano-structured surfaces Mostafa Kahani, Robert Gordon Jackson, and Gary Rosengarten Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04465 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on January 29, 2016

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Industrial & Engineering Chemistry Research

Experimental investigation of TiO2/water nanofluid droplet impingement on nano-structured surfaces

Mostafa Kahani1*, Robert Gordon Jackson2, Gary Rosengarten2 1 2

Mechanical Engineering Faculty, Shahrood University of Technology, Shahrood, Iran

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Australia *

Email: [email protected] , Tel/fax: +982332300258

Abstract This paper presents the results of an experimental investigation of nanofluid droplets impacting on nano-structured surfaces. Nanofluids with required weight concentration of 0.25-0.75 wt.% were prepared by dispersing TiO2 (21 nm) nanoparticles and appropriate amounts of sodium dodecyl sulphate (SDS) surfactant in Milli-Q water. Superhydrophobic and superhydrophilic coatings were applied over a silicon substrate. The images obtained from the high speed camera clearly show that when a nanofluid droplet impacts a superhydrophobic surface at room temperature, it spreads, retracts, oscillates and continues to retract to approximately its initial size. In addition, the increase in nanofluid concentration leads to decrease in the maximum spreading and the height of droplets after impingement. Also, the use of nanofluids increases the temperature difference at the centre of the droplet impact region. TiO2 nanoparticles improve the cooling effectiveness of droplet on uncoated and superhydrophobic surface up to 33 and 214 percent respectively.

KEYWORDS: Spray and atomization; Nano/Micro scale measurement and simulation; Heat transfer enhancement; Droplet impingement; Nanofluid; Superhydrophobic 1. Introduction 1 Environment ACS Paragon Plus


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Droplet impact on solid surfaces is a key element of a wide variety of phenomena encountered in applications such as ink-jet printing and rapid spray cooling of hot surfaces (for example turbine blades, lasers, semiconductor chips, and electronic devices). It also can be used in other applications, such as fire suppression by sprinklers, internal combustion engines, incinerators, spray painting and coating, plasma spraying, and crop spraying1. The impingement of droplets on solid smooth surfaces has received considerable attention in the last few decades. Many studies have been carried out to determine the parameters influencing the behavior of a single drop impact, in order to characterize their respective influence. Various parameters such as impact velocity, droplet diameter, liquid viscosity, surface tension, and substrate wettability have been investigated to correlate the droplet impact with the spreading process2-5. Worthington6 was the first to photograph water droplets as they impinged on a solid surface, revealing the complex shapes that droplets assume as they spread and splash during impact. Some measurements have been reported on the spreading effect caused by surface roughness and porosity, and surface tension by adding surfactant to the droplet7-8. When a droplet impacts a solid surface at room temperature, it goes through a cycle of spreading and retracting then oscillating between these two modes. In extreme cases, the surface energy in the droplet can continue to retract it to its initial size9. In general, the impact of droplet on a surface is affected by three kinds of energy: the kinetic energy, the droplet surface energy, and the droplet internal energy. After the droplet touches the surface, it spreads radially outward, increasing the droplet area and thus the surface energy. For moderate initial energies, the surface tension will be able to absorb the initial kinetic energy and the restitution force will cause the droplet to recoil. For higher energies, the surface tension is not sufficient to stop the outward motion as the droplet spreads upon impact and can cause the droplet to splash and break away from the surface10. Chandra and Avedisian11 investigated the collision dynamics of a liquid droplet on a heated solid stainless steel surface using n-heptane droplets of 1.5 mm initial diameter with surface

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temperatures varied from 24 to 250 ℃. It was reported that the droplet spreading rate was independent of temperature directly after impact. Ko and Chung12 investigated n-decane droplet impact upon inclined heated surfaces and highlighted that the impinging velocity decrease with an increase in droplet diameter. One of the non dimensional numbers that need to be calculated on a water droplet impingement is the Weber number13 (

=

/ ). Where

droplet diameter before impact, and

is the fluid density,

is the impact velocity,

is the

is the surface tension. In 2007, Comeau et al.14 conduct research

on droplet's behaviour during impact to analyse the effect of Weber number on spread factor. They used superhydrophobic surfaces with a contact angle of 140o and Weber number range from 0-300 by varying the height and impact velocity. Using the experimental data, they also used a critical Weber number of 100 which is a predictor for water droplet splashing for their substrate. Also, the impact of

water droplets on heated surfaces with different roughness for a range of Weber numbers from 20 to 220 and surface temperatures from 100 ℃ to 280 ℃have been investigated by Bernandin et al.15. Mudawar and Estes16 observed that the critical heat flux was maximized when the spray impact area just inscribed the square heater surface, which has been accepted by most researchers. During the droplet impingement on a solid surface, heat transfer occurs by convection inside the drop and conduction in the substrate, and by evaporation at the liquid–gas interface and possibly at an imperfect thermal contact at the drop–substrate interface17-18. In the techniques currently used, the required cooling is achieved mainly by varying system parameters such as fluid mass flow rate, temperature, impinging velocity, droplet diameter and the type of liquid. Understanding all the variables during droplet impingement is important for optimizing heat transfer rate19. In 2010, Rosengarten et al.20 present a preliminary investigation into the effect of hydrophobicity on the heat transfer rate due to the impingement of a cold water droplet onto heated flat surfaces (76 μm thick copper foil and 56 μm brass foil). The flat surface had either a hydrophilic or superhydrophobic coating. The superhydrophobic surfaces resulted in significantly



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lower instantaneous heat transfer rates and lower overall cooling effectiveness from droplet impingement compared to the hydrophilic surfaces. Recently the use a suspension of ultrafine solid particles in a fluid has been used to improve its thermal conductivity-so called nanofluids. Many types of particle, such as metallic, non-metallic and polymeric, can be added into fluids to form stable nanofluid to enhance the heat transfer rate in different thermal applications21. Compared to conventional solid/liquid suspensions used to intensify heat transfer, nanofluids possess the following advantages: (a) high specific surface area and therefore more heat transfer surface between particles and fluids; (b) high dispersion stability with predominant Brownian motion of particles; (c) reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification; (d) reduced particle clogging as compared to conventional slurries, thus promoting system miniaturization22. So nanofluids, as the new generation of heat transfer fluids, can be used in spray cooling to enhance the heat flux removal. Several researchers have applied nanofluids to the spray cooling of high-temperature bodies23-25. Duursma et al. 26 studied the effect of nanoparticles on droplet boil-off. They dispersed aluminium nanoparticles inside pure water, ethanol and dimethyl sulfoxide (DMSO) to evaluate the droplet impact on a copper block. They argue that there are differences in the behavior of nanofluid droplets and pure fluids as they boil off the surface. As well as this, they showed that increasing nanoparticle concentration decreases the receding droplet break up on rebound after impingement, and also appears to reduce the maximum spreading diameter of a droplet. Furthermore, their experimental measurements of heat fluxes associated with pure fluid and nanofluid droplets did not show significant enhancement, although there was noticeable improvement in the DMSO-based nanofluids. The boiling heat transfer during successive impacts of single nanofluid drops onto a hot stainless steel plate has been studied by Okawa et al.27. It was observed that colloidal dispersion of the nanoparticles degraded the heat transfer when the plate temperature was too high, and also the


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impact process revealed that droplet spreading area at low plate temperatures was wider for the nanofluid drops. Chang et al.28 conducted an experimental investigation into the effects of the particle volume fraction on the spray heat transfer performance of a de-ionized water and Al2O3 nanofluid and found that the optimal heat transfer performance is obtained using a particle volume fraction of 0.001%. The surface compositions of the sprayed samples were observed using scanning electron microscopy and X-ray spectrometry and their results showed that the surfaces sprayed with a nanofluid containing 0.025 Vol% or 0.05 Vol% of nanoparticles retained a thin deposition of Al particles after the droplet impact. This residual layer can increase or decrease the heat transfer rate. Mitra et al.29 investigated boiling heat transfer from heated horizontal steel surfaces using laminar jets comprised of TiO2/water and multiwalled carbon nanotube/water nanofluids. Their results clearly showed that cooling rate is enhanced by using nanofluids when compared to water over a range of flow rates. Recently, the formation of nano-adsorption layer and its subsequent effects on the spray heat transfer performance of a cooling system using Al2O3/water nanofluid has been reported by Chang30 experimentally. The results revealed that for all of the nanofluid, the nanoadsorption layer absorbs the nanofluid droplets under the effects of capillary forces, and therefore reduces the contact angle, which induces a hydrophilic surface property. As mentioned above, there is still a gap in the understanding of the hydrodynamics and thermal behavior of nanofluid droplets on nano-structured surfaces. Thus in this paper we use high speed and thermal images processing simultaneously to investigate the heat transfer and dynamics of impinging nanofluid droplets on horizontal superhydrophobic and superhydrophilic surfaces. 2. Apparatus, Material and methods 2.1 Nanofluids preparation

Nanometer-sized particles of Titanium dioxide (TiO2) (Sigma Aldrich, Australia) with 21nm nominal diameter and 99.5% purity were dispersed in Milli-Q water to form the TiO2/water nanofluid. To ensure no agglomeration, any one of the following methods suggested by Xuan and



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Li31 viz, using dispersants or using ultrasonic vibration can be followed. These methods are aimed at changing the surface properties of suspended particles and subsequently suppressing the formation of particle clusters. Sodium dodecyl sulfate (SDS) was used as a dispersant agent to ensure better dispersion of nanoparticles. The optimum weight ratio of SDS to TiO2 was obtained 1.0 wt.%. The appropriate weight of nanoparticles and surfactant were mixed with base fluid by a high-speed stirrer. The solutions were vibrated in an ultrasound device for 30 min in order to obtain uniformly dispersed nanofluid solutions of 0.25, 0.50 and 0.75 wt.%. A stable suspension was achieved, as no sedimentation at the bottom or clear layer at the top was observed after 8 hours. 2.2 Fabrication of nano-structured surfaces

Surfaces that exhibit a water contact angle greater than 150◦ are commonly called ‘superhydrophobic’. Conversely, surfaces exhibiting a water contact angle of less than 15° are termed ‘superhydrophilic’32-36. Typically, making a surface superhydrophobic or superhydrophilic requires engineering a water-attracting or repelling surface with severe roughness on the submillimeter scale; this decreases or increases the true contact area with water, respectively37. The superhydrophobic coatings were fabricated by first mixing a 10:1 weight ratio of 12 nm silica nanoparticles (Evonik), dimethylsiloxane polymer (Gelest). Appropriate amount of Methyltrimethoxysilane (Sigma-Aldrich) was used as the linking agent. The mixture was taken under sonication (Unisonics Pty Ltd.) at 40kHz for 30 minutes. The as-prepared mixture was then spun-coated (Laurell Technologies) onto silicon substrates at 1500 rpm for 25 seconds, and then cured at 150°C for 30 minutes. The superhydrophilic coating was prepared in a similar fashion except the linking agent used was tetraethoxysilane (TEOS) and an ethoxylated siloxane polymer. This ensures that the surface roughness between both coatings is consistent whilst only varying the surface chemistry. The Scanning Electron Microscope (SEM) images of prepare-nanostructured silicon surfaces are shown in figure (1).


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Static Milli-Q water and nanofluid droplets contact angle on silicon surface with no coating and a superhydrophobic and a superhydrophilic coating which done by a contact angle instrument (Data Physics OCA20) are shown in table (1). Since the nanofluid samples were diluted with SDS, they show lower contact angles compared to Milli-Q water. As the concentration of nanofluids increases, the contact angle shows more decrease which is due of surfactant effects. In addition, the contact angle measurements were taken from images of the droplets by the high-speed camera after the droplet had become sessile to satisfy the accuracy of our contact angle instrument. The image of the droplet was processed in MATLAB by firstly extracting the pixels representing the droplet from the surrounding image, then drawing a line tangent to the first five pixels that described the curvature of the top of the droplet. The maximum error, which was related to the line tangent to fit the first five pixels, is for contact angles close to 90°, where the error is ±5.5°. For superhydrophobic and superhydrophilic, the error is less than ±0.4°. 2.3 Experimental setup

A schematic and photo of the experimental setup is shown in figure (2). The apparatus consists of a heater, silicon surface, a light source (LED lamp), high speed and infrared cameras, droplet generation system and temperature measurement instrumentation. The heater for the silicon surface had to be designed such that it minimized the thermal gradient over the droplet impingement region while simultaneously allowing the thermal camera an unobstructed view of the bottom of the surface. In order to achieve this, a novel heater design was implemented; this essentially consisted of a 5W power supply connected to a nichrome heating element wrapped around an electricallyinsulated aluminium tube. The heater acts by warming the air within the cylinder, creating natural convection currents that rise to the top of the tube and heat the silicon wafer. The droplets had a constant diameter and were generated manually by a syringe (10 cc), formed at the tip of the needle (30 G size).The falling droplet was detected by an optical sensor inside the trigger box; this sent a signal to simultaneously trigger both of the cameras. The droplet entered the



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high speed camera’s field of view at a height of 6 mm above the substrate. Droplet impingement dynamics were imaged using a high speed camera (Phantom V1610) at 8700 frames per second (fps) using the full resolution available (1280 × 800 pixels). Also a Nikon lens (105 mm) was connected to the camera to control the focus of the images. The droplet’s shape and velocity was extracted using code written in MATLAB. Also, an infrared camera (Flir Titanium 560 M) at 870 fps and 160 x 128 pixels was used to capture the temperature change in the silicon surface over the impact period. A 0.125-mm-diameter T-type thermocouple (0.5℃ accuracy) was installed in-line between the syringe and needle to measure the temperature of the droplet immediately at the moment of dispensation. A second same thermocouple was hung near the apparatus to measure the ambient air temperature. A third thermocouple was attached to the bottom of the silicon wafer to measure the wafer temperature during IR camera calibration. A National Instruments NI9219 analogue input data logger recorded the temperature data at a rate of 10 kHz and passed this to a laptop running LabView. In order to observe black-body radiation in the thermal images, a thin layer of black paint was applied to the bottom of the silicon surfaces. The initial droplet diameter and impact height were fixed at 1.57 mm and 100 mm respectively and all the experiments were performed in atmospheric air. Weber number was calculated around 30 Milli-Q water and 60-65 for nanofluid droplets and the initial temperature difference between the surface and droplet was around 15 ℃. To increase the quality of images, a bright white light (LED lamp-7.5 W) was used with a diffuser. The paper seems not indicate what temperature difference is. 3. Data Analysis and experimental uncertainty In order to evaluate the ability of nanofluid droplets to enhance the heat transfer from the surfaces, the cooling effectiveness is defined as follow: = /

(1)


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By considering a cylindrical element of surface with thickness dr, the total cooling energy, Q, from the surface is calculated from the following equation: =

=

,

(2

)

,

=2

(2)

,

where ms, Cp,s, ρ and w are the mass, specific heat , density and thickness of silicon surface and r is radial distance of the spreading droplet from impingement point. The maximum possible cooling energy (if the droplet temperature equals the initial surface temperature) is measured as follows: =

,

(

,

−

,

)

(3)

in which Ti,s and Ti,d are the initial temperature of surface and droplet respectively. The equations are considered only for the heat transfer that occurs during droplet spreading until it reaches its maximum diameter. Equation (4) is a general relation for prediction of experimental errors38: X i P (4) U Xi P X i in which Xi is measurable parameter, P is calculated quantity from measurable parameter, UXi is U Pi 

measuring error and UPi is the maximum error of a parameter. The effect of all errors in calculation of goal function can be summarized as follows39: 0.5

2 2  X P 2  X P    X i P  1 2 U Xi   U 2   ...   U1    MaxU P     P X 1   P X 2     P X i The uncertainty of the experimental data may have resulted from measuring errors of

(5)

parameters such as temperature difference, density and specific heat of silicon wafer and also radial distance of the spreading droplet from impingement point. It can be calculated using equation (5) for maximum cooling energy and cooling effectiveness: = ±[ ∆

+(

) +(

) +(

) +

]


+

(6)


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= ±[

+ −

]

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(7)

The uncertainty of the temperature difference and radial distance are calculated 0.34% and 0.23% respectively. Also, the accuracy of wafer’ thickness which measured by a micrometer device is around 1.00%.

are 2329 kg/m3 and 710 J/(kgK) which directly extracted from Perry’

handbook40 assuming have negligible uncertainty. The maximum uncertainty of the maximum cooling energy and cooling effectiveness, taking into account the above considerations are obtained 1.10% and 2.21% respectively. 4. Results and discussions 4.1 Nanofluid properties

The addition of nanoparticles to pure water changes the thermal and mass transport properties of the fluid. The change is dependent on the properties of the nanoparticles and its concentration in the nanofluid. The properties of the nanofluid can be predicted by the equations provided by Kahani et al.22, these values are presented in Table (2) below for a nanoparticle concentration of 0.50% by volume. The most significant impact of adding nanofluids to water is the ~35% decrease in the specific heat capacity, CP, due to the low heat capacity of the TiO2. This means that the maximum amount of energy absorbed by the nanofluid droplets is less than that absorbed by the pure water droplets. The theoretical mass transport properties, ρ and μ, both increase by a little over 1%. As a result of this, the Reynolds number (

=

/ ) of initial momentum-dominated spreading phase of

the droplet impingement flow will not change significantly between the water and the nanofluid. This means the two droplet types should show very similar flow patterns, such as radial velocity and, to some extent, spreading diameter. The thermal conductivity of the nanofluid does not increase significantly above that of water, (approximately 1.3%). The combined effects of increased thermal conductivity and decreased specific heat significantly enhance the thermal diffusivity,( =


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), of the nanofluid. Thermal diffusivity measures the ability of nanofluid droplet to conduct thermal energy relative to its ability to store thermal energy. The heat that the droplet absorbs from the plate should theoretically travel through the entire droplet faster than it would with water; this means that it will absorb its full thermal energy faster. This should mean that, for very short impingement durations (30 millisecond) the nanofluid will absorb a greater amount of energy as it is able to distribute it throughout the entire droplet more quickly. 4.2 High speed and infra-red camera images

The comparative results of Milli-Q water and 0.5 wt.% nanofluid droplet impingement on silicon, captured by high speed and infrared cameras are depicted in figure (3(a-c)), illustrating the large difference in droplet shape and temperature change associated with the different surfaces. In addition, the effect of nanoparticle presence on the impact behavior of droplets has also been assessed visually. The first set of images (figure (3a)) shows droplet impingement on to a superhydrophobic surface. The spreading and retracting phases of droplet impingement, reported in9, are present in both the water and nanofluid droplets. The nanofluid droplet, however, does not retract to the same extent as the water droplet; this is a combination of the higher viscosity of the nanofluid (see table (2)) and the decreased surface tension resulting from the use of surfactants to suspend the nanoparticles. The thermal images indicate that the nanofluid droplet cools a larger area of the surface and reduces the local surface temperature by a greater amount than the water droplet (about 5°C for the nanofluid compared to about 3°C for the water). There are two reasons for this, firstly the higher thermal diffusivity of the nanofluid droplet causes it to absorb more thermal energy in the same amount of time, secondly, the water droplet recoils so much that it completely loses contact with the surface, unlike the nanofluid droplet which remains in contact. As a result of this, the nanofluid is in contact with a larger area of the surface for a longer period of time, which allows it to conduct more heat away.



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In figure (3b), it can be seen that the bouncing and oscillating phases do not occur on the superhydrophilic surface for either kind of droplet; instead both droplets impact the surface, spread and then stop. This result in the droplet area remaining constant after the maximum spreading diameter has been reached. This is because the adhesive forces between the liquid and the surface are stronger than the surface tension force. Also, The temperature difference caused by the nanofluid droplet is greater than the temperature difference caused by the water droplet at each point in time. It clearly indicates that the nanofluid droplet removed more heat from the surface than the water droplet. Both droplet types cool a larger area than was cooled by the droplets on the superhydrophobic surface. The forces that usually cause the droplet to rebound act in a direction normal to the surface and when there is a high surface contact angle, some part of this force acts radially towards the centre of the droplet, when the surface contact angle is low, however, most of the force acts to flatten the droplet instead. On the uncoated surface, shown in figure (3c), the water and nanofluid droplets display the same hydrodynamic behaviour up to about 5.7 ms, this is the initial, momentum-dominated ‘spreading’ phase of the droplet impingement. It was predicted that, due to the fact that the density and dynamic viscosity of the nanofluid was not significantly different to that of pure water, the Reynolds number, and hence the flow properties of the initial phase, would be very similar for the two fluid types. This can also be observed for the two other surface types (superhydrophobic and superhydrophilic); in both instances (figure (3a) and figure (3b)) the shape droplets are very similar up to the end of the spreading phase at around 5 ms. After this point, surface tension forces begin to dominate, and the effect of the surfactant in the nanofluid can be observed in the lesser extent to which the nanofluid droplet consistently retracts. After 9 ms, the water droplet has finished rebounding and the droplet has formed a peak of fluid in the centre of the impact site, this is the result of the kinetic energy of the rebounding flow being converted into potential (height) energy. In the time following this point, the droplet enters an ‘oscillating’ phase, during which it spreads out,


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then retracts to form a (smaller) peak and continues to do this until all of the kinetic energy has been dissipated by viscous forces within the fluid. The thermal images show that the nanofluid droplet cools a larger area of the surface at a faster rate than the water droplet. This is the result of the nanofluid having a higher thermal diffusivity and a lower surface contact angle than the water. The lower contact angle means that the nanofluid has a larger spreading diameter for the same volume of fluid as the water droplet, so the nanofluid film on the surface is thinner than the water film resulting in a higher cooling energy rate. 4.3 Normalized droplet diameter

The normalized droplet diameter, which defined as the droplet diameter to initial droplet size (D/Di) was measured using MATLAB image processing, the results of which are presented in figure (4 (a-d)) for the first 10 ms after the droplet impact. Figure 4(a) shows that the normalized diameter of a Milli-Q droplet on superhydrophilic surface remains stable after its maximum spreading, not recoiling at all; this means that the superhydrophilic surface results in a larger wetted area than the other surface types. The uncoated surface does not allow droplets to spread as far as the hydrophilic surface, although the maximum diameter does remain constant, only recoiling slightly, after maximum spreading has taken place. Droplets’ impinging on the superhydrophobic surface, on the other hand, follows a similar trend of diameter vs. time in the initial spreading phase, but do not spread as far as on either the uncoated or hydrophilic surfaces. Furthermore, the recoil phase of the droplet impact brings the normalized diameter back to unity as the droplet completely separates from the surface. Based on the figure (4(b-d)) it can be observed that nanofluid droplets on superhydrophilic surface generally show the highest normalized diameter among the other surfaces, which is similar to the trends shown by the Milli-Q water droplets. When the concentration of nanofluid increases from 0.25 to 0.75 wt.%, however, the maximum normalized diameter decreases from 3.5 to 3. This may be due to the increased nanoparticle concentration increasing the dynamic viscosity of the



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nanofluid. In addition to this, the nanofluid droplets do not recoil as completely as the milli-Q water droplet, retaining complete attachment with the surface; this can be seen in figure (4(b-d)) as the normalized diameters of the droplets asymptote during the recoil phase before reaching unity. This also means that the recoiling phase for nanofluid droplet takes more time: where milli-Q water droplets reach their minimum diameter at 7.5 ms, the nanofluid droplet diameter is still decreasing after 10ms. Furthermore, when the nanofluid concentration increases, the slope of curves at initial (spreading) stage also increases for all surface types. Since Brownian motion, dispersions and fluctuations of nanoparticles lead to an amplification of the momentum and exchange rate between the particles and surface, so the spreading velocity increases. The description of normalized droplet diameter changes of TiO2 nanofluid at a concentration of 0.75 wt.% that dropped from 10 cm onto an uncoated silicon surface are shown in figure (5), briefly. 4.4 Cooling energy and cooling effectiveness The cooling energy versus radial distance of the spreading droplet from impingement point for

milli-Q water droplets are shown in figure (6). It can be observed that for the milli-Q droplet, the cooling energy is higher on the superhydrophilic surface, compared to the other surfaces. Higher friction and adhesive forces as well as a larger wetted area on this surface type, as compared to the superhydrophobic and the uncoated surfaces, lead to an increased the heat transfer between the liquid and surface. In addition, the uncoated surface shows better thermal performance than the superhydrophobic one for milli-Q droplet. This is due to the smaller wetted area and shorter contact time of the droplet on the superhydrophobic surface. The variation of cooling effectiveness of milli-Q and nanofluid droplets on uncoated and nanostructured silicon surfaces is shown in figure (7). When the nanofluid concentration increases, the cooling effectiveness on the uncoated and superhydrophobic surface increases due to the higher thermal conductivity of the nanofluid and the lower surface tension. Brownian motion of the


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nanoparticles, particle migration and the reduction of boundary layer thickness as reported in previous literature41 are the other significant parameters that enhance the thermal ability of nanofluid droplets. In addition, the dispersion effects and chaotic movement of the nanoparticles intensifies the mixing fluctuations and flattens the temperature profile, causing an increase in the heat transfer to the nanofluid droplet. This means that increasing the concentration of nanoparticles amplifies the mechanisms responsible for enhanced heat transfer. Therefore, in general, nanofluid droplets with higher concentration have generally higher cooling effectiveness. It can be observed from figure (4) that nanofluid droplets impinging onto a surface with a superhydrophilic coating resulted in a lower cooling energy. In order to find the cause of this phenomenon, SEM images were taken before and after 2 days of 0.25 wt.% nanofluid droplet impingement on superhydrophilic coating (figure (8)). The images clearly show that the initial structure of the coating is uniform and monolithic. However, after the nanofluid droplet impacts the surface, significant changes are observed in its structure. It appears that the nanoparticles have destroyed the coating on the surface, and some large tracks are observed on it. Further investigations are necessary to fabricate more suitable and adamant nanostructure coatings to increase the lifetime of them. 5. Conclusion

The hydrodynamics and heat transfer effects of a single TiO2/water nanofluid droplet impinging onto a nano-structured surface were investigated experimentally. Superior heat transfer was observed on superhydrophilic surfaces for milli-Q water droplets; this is due to a larger wetted area. Both nanofluid and milli-Q water droplets impinging on to a superhydrophobic surface were observed to have heat transfer inferior to an uncoated surface; this is due to the smaller wetted area and shorter contact time. Increasing the concentration of nanoparticles in the nanofluid led to a decrease in the maximum normalized diameter. Also, Poor wettability of surface results in reduction of cooling energy. The colloidal dispersion of TiO2 nanoparticles in water droplets



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improved the heat transfer on uncoated and superhydrophobic surfaces over that of pure water. No obvious trends were observed on the superhydrophilic surface. Nomenclature Cp: Specific heat (J kg-1 K-1) D: droplet diameter (m) k: Thermal conductivity (W/m.K) m: mass (kg) : Cooling energy (J) r: Radial distance of the spreading droplet from impingement point (m) T: Temperature (K) w: Thickness of silicon surface (m) We: Weber number(-) Greek letters α: Thermal diffusivity (m2/s)

: Cooling effectiveness (dimensionless) μ: Dynamic viscosity (pa.s)

: Static contact angle (degree) : Impact velocity (m.s-1)

ρ: Density (kg m-3) : Surface tension (N.m-1) Subscripts

d: Droplet f: Fluid i: Initial : Silicon wafer


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max: Maximum nf: Nanofluid

Acknowledgment The authors acknowledge the Australian Microscopy & Microanalysis Research Facility at the RMIT university. Special thanks to Dr. Alex Wu and Professor Robert Lamb at department of chemistry, Melbourne university for providing us the material that we used for the research (superhydrophobic and superhydrophilic surfaces).

References (1) Yarin, A. L. Drop Impact Dynamics: Splashing, Spreading, Receding and Bouncing. Annu. Rev. Fluid. Mech. 2006, 38, 159. (2) Yu, Z.; Ge, Y.; Fan, L. S. Multi-scale Simulation of Oblique Collisions of a Droplet on a Surface in the Leidenfrost Regime. Chem. Eng. Sci. 2007, 62, 3462. (3) Werner, S. R.; Jones, J. R.; Paterson, A. H.; Archer, R. H.; Pearce, D. L. Droplet Impact and Spreading: Droplet Formation Effects. Chem. Eng. Sci. 2007, 62, 2336. (4) Gu, Y. Drop Size Dependence of Contact Angles of Oil Drops on a Solid Surface in Water. Colloids Surf. A: Physicochem. Eng. Asp. 2001, 181, 215. (5) Joung, Y. S.; Buie, C. R. Scaling Laws for Drop Impingement on Porous Films and Papers. Phys. Rev. E 2014, 89, 013015. (6) Worthington, A. M. Study of Splashes; Longmans & Green: London, 1908. (7) Reis, N. C.; Griffiths, R. F.; Santos, J. M. Parametric Study of Liquid Droplets Impinging on Porous Surfaces. Appl. Math. Model. 2008, 32, 341. (8) Sivakumar, D.; Katagiri, K.; Sato, T.; Nishiyama, H. Spreading Behavior of an Impacting Drop on a Structured Rough Surface. Phys. Fluids. 2005, 17,100608.



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(9) Xu, Q.; Li, Z.; Wang, J.; Wang, R. Characteristics of Single Droplet Impact on Cold Plate Surfaces. Drying Technol. 2012, 30, 1756. (10) Durickovic, B.; Varland, K. Between Bouncing and Splashing: Water Drops on a Solid Surface. Technical Report; University of Arizona, 2005. (11) Chandra, S.; Avedisian, C. T. On the Collision of a Droplet with a Solid Surface. Proc. R. Soc. London A. 1991, 432, 14. (12) Ko, Y. S.; Chung, S. H. An Experiment on the Breakup of Impinging Droplets on a Hot Surface. Expt. Fluids. 1996, 21, 118. (13) Roberson, J. A.; Crowe, C. T. Engineering fluid mechanics; John Wiley and Sons, Inc: New York, 1997. (14) Comeau, D. The effect of Weber number and spread factor of a water droplet impinging on a superhydrophobic substrate. Master Thesis, The University of Arizona, USA, 2007. (15) Bernandin, J. D.; Stebbins, C. J.; Mudawar, I. Effect of Surface Roughness on Water Droplet History and Heat Transfer Regimes. Int. J. Heat Mass Transfer 1997, 40, 73. (16) Mudawar, I.; Estes, K. A. Optimizing and Predicting CHF in Spray Cooling of a Square Surface. J. Heat Transf. 1996, 118, 672. (17) Bhardwaj, R.; Longtin, J. P.; Attinger, D. Interfacial Temperature Measurements, High-Speed Visualization and Finite-Element Simulations of Droplet Impact and Evaporation on a Solid Surface. Int. J. Heat Mass Transfer 2010, 53, 3733. (18) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827. (19) Fukuda, S.; Kohno, M.; Tagashira, K.; Ishihara, N.; Hidaka, S., Takata, Y. Behavior of Small Droplet Impinging on a Hot Surface. Heat Transfer Eng. 2014, 35, 204.


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(20) Rosengarten, G.; Tschaut, R. Effect of Superhydrophobicity on Impinging Droplet Heat Transfer. In Proceedings of the 14th International Heat Transfer Conference; ASME, Washington DC, 2010. (21) Navaei, A. S.; Mohammed, H. A.; Munisamy, K. M.; Yarmand, H.; Gharehkhanic, S. Heat Transfer Enhancement of Turbulent Nanofluid Flow Over Various Types of Internally Corrugated Channels. Powder Technol. 2015, 286, 332. (22) Kahani, M.; Zeinali Heris, S.; Mousavi, S. M. Comparative Study between Metal Oxide Nanopowders on Thermal Characteristics of Nanofluid Flow through Helical Coils. Powder Technol. 2013, 246, 82. (23) Hsieh, S. S.; Leu, H. Y.; Liu, H. H. Spray Cooling Characteristics of Nanofluids for Electronic Power Devices. Nanoscale Res. Lett. 2015, 10, 139. (24) Ravikumar, S.V.; Haldar, K.; Jha, J. M.; Chakraborty, S.; Sarkar, I; Pal S. K.; Chakrabortya, S. Heat Transfer Enhancement Using Air-Atomized Spray Cooling with Water – Al2O3 Nanofluid. Int. J. Therm Sci. 2015, 96, 85. (25) Bellerová, H.; Tseng, A. A.; Pohanka, M.; Raudensky, M. Spray Cooling by Solid Jet Nozzles Using Alumina/water Nanofluids. Int. J. Therm Sci. 2012, 62, 127. (26) Duursma, G.; Sefiane, K.; Kennedy, A. Experimental Studies of Nanofluid Droplets in Spray Cooling. Heat Transfer Eng. 2009, 30, 1108. (27) Okawa, T.; Nagano, K.; Hirano, T. Boiling Heat Transfer during Single Nanofluid Drop Impacts onto a Hot Wall. Exp. Therm, Fluid Sci. 2012, 36, 78. (28) Chang, T. B.; Syu, S.C.; Yang, Y. K. Effects of Particle Volume Fraction on Spray Heat Transfer Performance of Al2O3–water Nanofluid. Int. J. Heat Mass Transfer 2012, 55, 1014. (29) Mitra, S.; Saha, S. K.; Chakraborty, S.; Das, S. Study on Boiling Heat Transfer of Water -TiO2 and Water- MWCNT Nanofluids based Laminar Jet Impingement on Heated Steel Surface. Appl. Therm. Eng. 2012, 37, 353.



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(30) Chang, T. B. Formation of Nano-Adsorption Layer and Its Effects on Nanofluid Spray Heat Transfer Performance. J. Heat Transfer 2015, 137, 021901. (31) Xuan, Y.; Li, Q. Heat Transfer Enhancement of Nanofluids. Int. J. Heat Fluid Fl. 2000, 21, 58. (32) Shi, F.; Niu, J.; Liu, J.; Liu, F.; Wang, Z.; Feng, X.; Zhang, X. Towards Understanding Why a Superhydrophobic Coating is needed by water striders. Adv. Mater 2007, 19, 2257. (33) Li, W.; Amirfazli, A. Microtextured Superhydrophobic Surfaces: a Thermodynamic Analysis. Adv. Colloid Interface Sci. 2007, 132, 51. (34) Li, W.; Amirfazli, A. Hierarchical Structures for Natural Superhydrophobic Surfaces. Soft Matter 2008, 4, 462. (35) Xue, Z. X.; Liu, M. J.; Jiang, L. Recent Developments in Polymeric Superoleophobic Surfaces. J. Polym. Sci. Part B: Polym. Phys. 2012, 50, 1209. (36) Bhushan, B.; Jung, Y. C. Natural and Biomimetic Artificial Surfaces for Superhydrophocity, Self-Cleaning, Low Adhesion, and Drag Reduction. Prog. Mater. Sci. 2011, 56, 1. (37) Betz, A. R.; Jenkins, J.; Kim, C. J.; Attinger, D. Boiling Heat Transfer on Superhydrophilic, Superhydrophobic, and Superbiphilic Surfaces. Int. J. Heat Mass Transfer 2013, 57, 733. (38) Young, H.D. Statistical Treatment of Experimental Data; McGraw-Hill: New York, 1962. (39) Holman, J.D. Experimental Methods for Engineers; McGraw-Hill: New York, 1986. (40) Green, D. W.; Perry, R. H. Perry’s Chemical Engineers’ Handbook; McGraw-Hill: New York, 2008. (41) Kahani, M.; Zeinali Heris, S.; Mousavi, S. M. Multiwalled Carbon Nanotube/Water Nanofluid or Helical Coiling Technique, Which of Them Is More Effective?. Ind. Eng. Chem. Res. 2013, 52, 13183.


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Figure captions and tables: Figure 1. SEM images of prepared (a) superhydrophilic (b) superhydrophobic silicon wafers Figure 2. Experimental apparatus for analysis of droplet impact and temperature distribution on surface Figure 3. Simultaneous high speed and infra-red camera images on (a) superhydrophobic (b) superhydrophilic (c) uncoated surface Figure 4. Normalized droplet diameter versus time for (a) Milli-Q water (b) 0.25 wt.% (c) 0.5 wt.% (d) 0.75 wt.% TiO2 nanofluid Figure 5. The description of normalized droplet diameter changes of TiO2 nanofluid at a concentration of 0.75 wt.% Figure 6. The Cooling energy versus radial distance of impingement point for milli-Q water droplets Figure 7. Cooling effectiveness for milli-Q water and nanofluid droplets on different surfaces Figure 8. SEM images of superhydrophilic surface (a) before (b) after nanofluid droplet impingement Table 1. Static Milli-Q water and nanofluid droplets contact angle on silicon surface with no coating and a Superhydrophobic and a Superhydrophilic coating. Table 2. Thermal and mass transport properties of TiO2, water and TiO2/water nanofluid for a concentration of 0.50 wt.%



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

Fig. 1 SEM images of prepared (a) superhydrophilic (b) superhydrophobic silicon wafers


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Fig.2 Experimental apparatus for analysis of droplet impact and temperature distribution on surface



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(a) Superhydrophobic Surface Time (ms)

Milli-Q Water

0.5 wt.% Nanofluid

0

1.1

3.4

5.7

6.9

9.2


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(b)Superhydrophilic Surface Time (ms)

Milli-Q Water

0.5 wt.% Nanofluid

0

1.1

3.4

5.7

9.2

30



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(c) Uncoated Surface Time (ms)

Milli-Q Water

0.5wt.% Nanofluid

0

1.1

3.4

5.7

9.2

30

Fig. 3 Simultaneous high speed and infra-red camera images on (a) superhydrophobic (b) superhydrophilic (c) uncoated surface


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3.5

Normalized Droplet Diameter

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)Milli-Q Water

3 2.5 2 1.5 1 Uncoated

0.5

Superhydrophilic Superhydrophobic

0 0

0.002

0.004

0.006

Time (s)


0.008

0.01


4

(b) 0.25 wt.% TiO2

3.5


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3 2.5 2 1.5 1 Uncoated Superhydrophilic

0.5

Superhydrophobic

0 0

0.002

0.004

0.006

Time (s)


0.008

0.01

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3.5

(c) 0.5 wt.% TiO2

3


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2.5 2 1.5 1 Uncoated

0.5

Superhydrophilic Superhydrophobic

0 0

0.002

0.004

0.006

Time (s)


0.008

0.01


3.5

(d) 0.75 wt.% TiO2

3


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2.5 2 1.5 1

Uncoated Superhydrophilic

0.5

Superhydrophobic

0 0

0.002

0.004

Time (s)

0.006

0.008

0.01

Fig. 4 Normalized droplet diameter versus time for (a) Milli-Q water (b) 0.25 wt.% (c) 0.5 wt.% (d) 0.75 wt.% TiO2 nanofluid


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Normalized droplet diameter

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4

D

E

3.5

F

3

C

2.5

A

2

B

1.5 1 0.5 0 0

0.005

0.01

0.015

0.02

0.025

0.03

Time (s)

(A) Droplet falling towards silicon surface

(B) Initial contact between droplet and surface

(C) Droplet spreading outwards

(D) Droplet reaches maximum spreading diameter and begins to rebound

(E) Droplet rebound reaches maximum height

(F) Droplet reaches minimum rebound diameter

Fig. 5 The description of normalized droplet diameter changes of TiO2 nanofluid at a concentration of 0.75 wt.%



0.09 0.08

Cooling Energy (J)

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0.07 0.06 0.05

Uncoated

0.04

Superhydrophilic

0.03

Superhydrophobic

0.02 0.01 0 0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Radial Distance (m) Fig. 6 The Cooling energy versus radial distance of impingement point for milli-Q water droplets


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Fig. 7 Cooling effectiveness for milli-Q water and nanofluid droplets on different surfaces



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(a)

(b)

Fig. 8 SEM images of superhydrophilic surface (a) before (b) after nanofluid droplet impingement


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Table 1. Static Milli-Q water and nanofluid droplets contact angle on silicon surface with no coating and a Superhydrophobic and a Superhydrophilic coating Uncoated Superhydrophobic Superhydrophilic

Milli-Q Water

57.2°

169.3°

12.7°

36.6°

115.0°

7.7°

29.7°

109.2°

7.3°

10.6°

103.4°

6.8°

0.25 wt.% TiO2/Water





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Table 2. Thermal and mass transport properties of TiO2, water and TiO2/water nanofluid for a concentration of 0.50 vol.% TiO2

Water

Nanofluid

% change

Units

ρ

3900

996.999

1011.51

+ 1.456

kg/m3

Cp

0.71

4.17997

2.73748

-34.510

kJ/kg.K

μ

-

0.00089

0.00090

+ 1.261

Pa.s

k

13.7

0.60963

0.61769

+ 1.322

W/m.K

α

4.948×10-6

1.46×10-7

2.231×10-7

+ 52.492

m2/s

36 Environment ACS Paragon Plus

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Water Nanofluid Droplet

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