Nanofluid Droplets Research Articles

The control of dry-out patterns using bubble-containing droplets

Hypothesis: For an evaporating nanofluid droplet that contains a bubble inside, we suspect the bubble boundary remains pinned during evaporation whereas the droplet perimeter recedes. Thus, the dry-out patterns are mainly determined by the presence of the bubble and their morphology can be tuned by the size and location of the added bubble.Experiments: Bubbles with varying base diameters and lifetimes are added into evaporating droplets that contain nanoparticles with different types, sizes, concentrations, shapes, and wettability. The geometric dimensions of the dry-out patterns are measured.Findings: For a droplet containing a long-lifetime bubble, a complete ring-like deposit forms, and its diameter and thickness increases and decreases with the bubble base diameter, respectively. The ring completeness, i.e., the ratio of actual ring length to its imaginary perimeter, decreases with the decrease in bubble lifetime. The pinning of droplet receding contact line by particles near the bubble perimeter has been found to be the key factor leading to ring-like deposits. This study introduces a strategy of producing ring-like deposit and allows a control of the ring morphology in a simple, cheap, and impurity-free fashion, which is applicable to various applications associated with evaporative self-assembly.

Evaporation of Suspended Nanofluid (SiO$$_{2}$$/Water) Droplets: Experimental Results and Modelling

The results of experimental studies and modelling of the evaporation of suspended water droplets containing silicon dioxide SiO $$_{2}$$ nanoparticles at mass fractions 0.02 and 0.07 are presented. The experimental results are analysed using the previously developed model for multicomponent droplet heating and evaporation. In this model droplets are assumed to be spherical and the analytical solutions to the heat transfer and species diffusion equations are incorporated into the numerical code. They are used at each timestep of the calculations. Silicon dioxide nanoparticles are considered to be a non-evaporating component. It is demonstrated that both experimental and predicted values of droplet diameters to the power 1.5 decrease almost linearly with time, except at the beginning and the final stages of the evaporation process, and are only weakly affected by the presence of nanoparticles. At the final point in this process, the effect of nanoparticles becomes dominant when their mass fraction at the droplet surface reaches about 40 % and a cenosphere-like structure is formed. Both predicted and observed droplet surface temperatures rapidly decrease during the initial stage of droplet evaporation. After about $$t=100$$ s the predicted surface temperature remains almost constant whilst its experimentally observed values increase with time. This might be related to a decrease in the temperature of ambient air in the vicinity of droplets, not taken into account in the model. Both observed and predicted values of the mass fraction of silicon dioxide at the droplet surfaces are shown to increase with time until they reach about 0.4.

The Experimental Study of Evaporation of Water and Nanofluid Droplets on the Surfaces of Materials with Different Thermal Conductivities

The Experimental Study of Evaporation of Water and Nanofluid Droplets on the Surfaces of Materials with Different Thermal Conductivities

Surface deposition characteristics of water-based SiO2 nanofluids on coal

Water-based SiO2 nanofluids, a type of wetting agent for water injection of coal seams, have been well recognized by many scholars due to being environment-friendly and having a stable modification effect. The key of the modification effect of water-based SiO2 nanofluids on coal seam is the deposition behavior of nanofluids on coal surface. Based on this issue, this research conducted real-time monitoring of the morphological evolution characteristics and deposition images of water-based SiO2 nanofluid droplets with different concentrations on coal surface. Moreover, the deposition behavior of water-based SiO2 nanofluids on coal surface was microscopically expounded. The results indicate that the initial contact angle between the nanofluid droplets and coal surface is small, and the spreading degree of water-based SiO2 nanofluids is better than that of deionized water. With the increase of time, the contact angles between water-based SiO2 nanofluid droplets and coal surface tend to linearly decrease. However, the relative length of contact line features flattening at first and then a sharp decline. As the nanoparticles are completely precipitated, water-based SiO2 nanofluid droplets form a deposition pattern with a coffee ring structure on coal surface. In addition, as the particle concentration increases, the relative width of the coffee ring increases stepwise. The particle aggregate on the middle of the deposition pattern gradually connects to coalesce and forms a relatively uniform and flat deposition layer. Based on the research results aforementioned, this research divided the deposition behavior of the water-based SiO2 nanofluid droplets on coal surface into initial stage, the contact angle dominance change, the contact angle and line co-dominance change and complete deposition. Among which, the duration of the angle dominance change stage is longest. With the increase of the particle concentration, the proportion of the angle dominance change stage increases. The research can fill the gap of the research on the deposition behavior of nanofluids on coal surface, which lays a foundation for further research of coal seam water injection using nanofluids.

MORPHOLOGICAL EVOLUTION OF Al2O3 AND CNT NANOFLUID DROPLETS DURING SOLIDIFICATION

Nanofluid is an emerging heat transfer fluid with good heat transfer and thermal conductivity properties. It is important to investigate the phase change properties and morphological evolution during the freezing of nanofluid droplets to understand their practical applications. The effect of dynamic wettability on the deformation of a single droplet of aluminum trioxide (Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O) and graphene (CNT-H<sub>2</sub>O) nanofluids at different mass concentrations and substrate temperatures was investigated by visualizing the droplet freezing. The formation of solid-like and freezing front motions inside the droplet during the freezing process of these droplets was investigated. The solidification process was strongly influenced by the temperature gradient perpendicular to the cold surface and the change in the solid–liquid interface wettability during the phase change, resulting in volume redistribution at the top of the droplet. The freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O nanodroplets resembled a "moon crater," and the influence of wettability decreased with increasing concentration, leading to a relative increase in the aperture of the top platform. The fully frozen state of the nanofluid droplet had an increasingly pointed tip, with a strong relationship between the substrate temperature and solidification time when the CNT-H<sub>2</sub>O concentration was 5 times higher and showed no change in the freezing droplet deformation rate under the experimental conditions. The contact angle of the two nanofluid droplets did not fluctuate significantly with increasing concentration, while that of the 1% nanofluid droplets remained at an average value of 85° during freezing. Under different freezing conditions, the freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O droplets tended to increase in diameter as the subcooling temperature decreased, with the final deformation rate of 1% Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O being twice that at 5% concentration, while the contact angle of the same mass concentration of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O decreased by 1° as the subcooling temperature dropped. The CNT-H<sub>2</sub>O droplet became sharper at the tip as the subcooling temperature increased, and its contact angle did not change with temperature.

THE EXPERIMENTAL STUDY OF EVAPORATION OF WATER AND NANOFLUID DROPLETS ON THE SURFACES OF MATERIALS WITH DIFFERENT THERMAL CONDUCTIVITIES

The article presents the results of experimental studying evaporation of water and nanofluid droplets on the surfaces of various materials. Plates made of materials with significantly different thermal conductivity coefficients have been used as substrates: copper (λ = 401 W/m °С), Teflon (λ = 0.25 W/m °С), and extruded foamed polystyrene (λ = 0.03 W/m °С). In the experiments, the evaporation of water and nanofluid droplets with a volume of 5 μL has been considered at a constant temperature and humidity of the ambient air. A nanofluid (a mixture of water with gold nanoparticles) has been prepared by laser ablation. The concentration of nanoparticles in the nanofluid is about 0.1 wt %. Infrared thermography has been employed to determine the average temperatures of evaporating droplet surfaces. The results obtained have shown that, for all studied materials, the surface temperature of evaporating water droplets is higher than the temperature of adiabatic evaporation. Therewith, the lower the thermal conductivity coefficient of a substrate material, the lower the surface temperature of the droplet and the longer the time of its evaporation. The performed experiments have shown that the minimum temperature of nanofluid droplets is lower than that of water droplets, and the evaporation time of nanofluid droplets is longer than that of water droplets on the corresponding surfaces.

Role of Nanoparticles in Nanofluid Droplet Impact on Solid Surfaces.

Splashing of a liquid droplet onto a substrate, while ubiquitous, sits at the intersection of several key fluid mechanical regions. Typically, this problem is often simplified to the transition between spreading and splashing, even for splashing on complex surfaces. Recently, there has been increased interest in using not just pure liquids but also nanofluids in applications such as spray cooling. While the addition of a few percent of nanoparticles to a Newtonian fluid does not change its apparent viscosity, the influence of the nanoparticles on the splashing transition is pronounced. We often view splashing in terms of fluid mechanics where a simple material is subjected to a complex flow and the fluid can be simply characterized by a Newtonian viscosity. For nanofluids, we have an apparently simple material in a complex flow, but the results show that the impact of the particles is nontrivial. This implies that we must now combine some of the insights we obtain from studying the rheological properties of nanosuspensions with this already complex problem.

Counter-Intuitive Evaporation in Nanofluids Droplets due to Stick-Slip Nature.

We experimentally investigate the evaporation characteristics of a sessile ethanol droplet containing Al2O3 and Cu nanoparticles of sizes 25 and 75 nm on a heated substrate using shadowgraphy and infrared imaging techniques. Our results demonstrate that the droplet contact line dynamics resulting from the presence of various nanoparticles plays a dominant role in the evaporation process. This is in contrast to the widely held assumption that the enhanced evaporation rate observed in sessile nanofluid droplets is due to the higher thermal conductivity of the added nanoparticles. We observe that even though the thermal conductivity of Al2O3 is an order of magnitude lower than that of Cu, droplets containing 25-nm-sized Al2O3 exhibit pinned contact line dynamics and evaporate much more rapidly than droplets containing Cu nanoparticles of both sizes and 75 nm Al2O3 nanoparticles that exhibit stick-slip behavior. We also found that the droplets with different nanoparticles display distinct thermal patterns due to the difference in contact line behavior, which alters the heat transfer inside the droplets. We establish this counter-intuitive observation by analyzing the temporal variations of the perimeter, free surface area, and deposition patterns on the substrate.

Evaporation investigation of nanofluid droplet affected by electrical field with periodically changed direction

The droplet evaporation phenomenon is very important for the development of droplet microfluidic technology. The existing studies have shown that high frequency and high electrical field strength have great effects on droplet evaporation and wetting characteristics respectively. However, the effects of high frequency together with high voltage on nanofluid droplet evaporation and wetting characteristics have not been studied and the mechanism is not clear. Therefore, the evaporation characteristics of nanofluid droplets at high voltage and high frequency were experimentally investigated to improve its evaporation rate and reveal its evaporation mechanism. The results show that the evaporation rates increase first and then decrease with the increase of voltage frequency (0–2 kHz), and the frequency at the high voltage obtained the maximum evaporation rate increases (90 Hz) when compared with the low voltage condition. Besides, the effect of electrical field on nanoparticles using the deposition surface was also explored, and the results show that the motion of the particles manipulated by the electrical field forces together with the oscillatory fluctuations of the droplets result in the enhancement of the evaporation rate of the nanofluid. • Frequency obtained maximum evaporation rate increases when compared to low voltage. • Evaporation rate of nanofluids increases with improvement of concentration and voltage. • Oscillatory fluctuations combined nanoparticle motion result in evaporation enhancement.

Coalescence dynamics of nanofluid droplets in T-junction microchannel

The effect of nanoparticles on droplet coalescence in microchannel was investigated experimentally. The interaction between acid-functionalized nanoparticles and polymers with complementary terminal functional groups facilitates the adsorption of particles at the oil–water interface. Three flow patterns were observed: squeezing coalescence, decompression coalescence and non-coalescence. The presence of nanoparticles significantly reduces the droplet coalescence percentage, and stable droplets could be obtained when the particles concentration increased to 1%. Furthermore, it was found that the film drainage time of squeezing coalescence was independent of the nanoparticles concentration. However, the film drainage time of decompression coalescence increases with nanoparticles concentration. Taking the influence of nanoparticles on the film drainage time into consideration, a new prediction model was proposed. Coupling the droplet contact time with the film drainage time, the critical capillary number for droplet coalescence was obtained. This work could provide important data and technical support for the application of nanoparticle-stabilized emulsions.

Light-driven mixing strategy inside a nanofluid droplet by asymmetrical Marangoni flow

PurposeThe purpose of this study is to establish an effective numerical simulation method to describe the flow pattern and optimize the strategy of noncontact mixing induced by alternating Gaussian light inside a nanofluid droplet and analyzing the influencing factors and flow mechanism of fluid mixing inside a droplet.Design/methodology/approachFirst, the heat converted by the alternating incident Gaussian light acting on the nanoparticles was considered as the bulk heat source distribution, and the equilibrium equation between the surface tension and the viscous force at the upper boundary force was established; then, the numerical simulation methods for multiple-physical-field coupling was established, and the mixing index was used to quantify the mixing degree inside a droplet. The effects of the incident position of alternating Gaussian light and the height of the droplet on the mixing characteristics inside a droplet were studied. Finally, the nondimensional Marangoni number was used to reveal the flow mechanism of the internal mixing of the droplet.FindingsNoncontact alternating Gaussian light can induce asymmetric vortex motion inside a nanofluid droplet. The incident position of alternating Gaussian light is a significant factor affecting the mixing degree in the droplet. In addition, the heat transfer caused by the surface tension gradient promotes the convection effect, which significantly enhances the mixing of the fluid in the droplet.Originality/valueThis study demonstrates the possibility of the chaotic mixing phenomenon induced by noncontact Gaussian light that occurs within a tiny droplet and provides a feasible method to achieve efficient mixing inside droplets at the microscale.

Evaporative Deposition of Surfactant-Laden Nanofluid Droplets over a Silicon Surface.

Morphologies of evaporative deposition, which has been widely applied in potential fields, were induced by the competition between internal flows inside evaporating droplets. Controlling the pattern of deposition and suppressing the coffee-ring effect are essential issues of intense interest in the aspects of industrial technologies and scientific applications. Here, evaporative deposition of surfactant-laden nanofluid droplets over silicon was experimentally investigated. A ring-like deposition was formed after complete evaporation of sodium dodecyl sulfate (SDS)-laden nanofluid droplets with an initial SDS concentration ranging from 0 to 1.5 CMC. In the case of initial SDS concentrations above 1.3 CMC, no cracks were observed in the ring-like deposition, indicating that the deposition patterns of nanofluid droplets could be completely changed and cracks could be eliminated by sufficient addition of SDS. With the increase of the initial concentration of hexadecyl trimethylammonium bromide (CTAB), the width of the deposition ring gradually decreased until no ring-like structure was formed. On the contrary, with the increase of the initial Triton X-100 (TX-100) concentration, the width of the deposition ring gradually increased until a uniform deposition was generated. Moreover, when the initial TX-100 concentration was high, a "tree-ring-like" pattern was discovered. Besides, morphologies of evaporative pattern due to the addition of surfacants were qualitatively analyzed.

Flow mechanism of Gaussian light-induced vortex motion inside a nanofluid droplet

PurposeThe purpose of this study is to use a weak light source with spatial distribution to realize light-driven fluid by adding high-absorbing nanoparticles to the droplets, thereby replacing a highly focused strong linear light source acting on pure droplets.Design/methodology/approachFirst, Fe3O4 nanoparticles with high light response characteristics were added to the droplets to prepare nanofluid droplets, and through the Gaussian light-driven flow experiment, the Marangoni effect inside a nanofluid droplet was studied, which can produce the surface tension gradient on the air/liquid interface and induce the vortex motion inside a droplet. Then, the numerical simulation method of multiphysics field coupling was used to study the effects of droplet height and Gaussian light distribution on the flow characteristics inside a droplet.FindingsNanoparticles can significantly enhance the light absorption, so that the Gaussian light is enough to drive the flow, and the formation of vortex can be regulated by light distribution. The multiphysics field coupling model can accurately describe this problem.Originality/valueThis study is helpful to understand the flow behavior and heat transfer phenomenon in optical microfluidic systems, and provides a feasible way to construct the rapid flow inside a tiny droplet by light.

Copper-alumina hybrid nanofluid droplet phase change dynamics over heated plain copper and porous residue surfaces

Droplet phase change is the key phenomenon for high heat transfer rates in spray or drop-wise cooling applications. Despite high cooling efficiency of the spray cooling technology, conventional fluids, such as water, cannot be used for thermal management of modern high heat flux devices due to their immense power density, resulting in early device failures. To address this issue, in this research, we experimentally study the evaporation performance for various volumes of the copper-alumina hybrid nanofluid (CAHF) droplet on a plain copper substrate and compare it with water (H2O) droplet in sub-boiling and boiling regimes (i.e., for substrate temperatures of 25–170 °C). We also numerically investigate and compare the internal velocity and thermal fields of CAHF and H2O droplets on a heated plain copper substrate. Besides the plain copper surface, we examine the phase change behaviour of the subsequent CAHF droplet over a heated residue surface that was obtained from the phase transition of the first CAHF droplet on a heated plain copper substrate. Our results demonstrate that the evaporation rate of CAHF droplets on a plain copper surface is up to 24% and an order of magnitude higher than water droplets in sub-boiling and nucleate boiling regimes, respectively. Moreover, the evaporation rate of the CAHF droplet on a residue surface increases up to 141% and 800% compared to that on a plain copper surface in sub-boiling and nucleate boiling regimes, respectively. Furthermore, the latent heat flux up to 10 times can be achieved using the CAHF droplet compared to H2O droplet on a plain copper substrate in the nucleate boiling region, making the CAHF a potential fluid for high heat flux cooling applications.

Evaluation of three methods of static contact angle measurements for TiO2 nanofluid droplets during evaporation

This article discusses the evaluation of three methods of static contact angle measurements of de-ionized (DI) water and TiO2 nanofluid droplets with three concentrations (0.5%, 1%, and 5%) that evaporate on the substrate of three different temperatures. To compare the discussed approximation methods, the differences between the results of the mean contact angles were identified; the highest standard deviation was recorded for the Young–Laplace and circle methods, such as for the nanofluid TiO2–DI with 1% at 50 °C, it is 3.21°, while the lowest value was obtained for TiO2–DI with 5% at Tenv, SD = 0.04°, by comparing the contact angle results using the ellipse and circle methods. Research shows that circular shape approximation is not to be excluded for contact angle determination in the angular range of 0°–75°.

Effects of nanoparticle concentration and peclet number on nanofluid droplet evaporation behavior

Effects of droplet parameters are reported using model simulations on the evaporation characteristics of droplets containing insoluble nanoparticles and related to the formation of shells of particles. The droplet parameters are initial droplet diameter (do), initial particle mass fraction (Yvo), and the Peclet number (Pe) based on the fluid evaporation rate constant (K) to particle diffusivity in the fluid (Dpl). Time-dependent phenomena of the instantaneous droplet size (d), d2 vs time (for example, following the d2-law or the deviation from it), instantaneous evaporation efficiency (i.e., effective area of evaporation due to particle inclusion on the droplet surface compared to a pure fluid droplet) are reported. Analytical expression of shell parameters at shell formation such as time to shell formation, shell size, and evaporation efficiency are derived using the initial droplet parameters and compared with the computational results. It is found that the evaporation behavior of droplets containing insoluble nanoparticles, including the deviation from d2-law, is independent of do but is strongly influenced by the product of Pe·Yvo.

Evaporation of Suspended Nanofluid (Sio2/Water) Droplets: Experimental Results and Modelling

Evaporation of Suspended Nanofluid (Sio2/Water) Droplets: Experimental Results and Modelling

Evaporation of Suspended Nanofluid (Water/Sio2) Droplets: Experimental Results and Modelling

Evaporation of Suspended Nanofluid (Water/Sio2) Droplets: Experimental Results and Modelling

Dynamic wetting behavior of nanofluid droplet on a vertically vibrating surface: A molecular dynamics study

Nanofluidic devices have been rapidly developed in recent years for a series of biological applications. Therefore, it is critical to understand the liquid characteristics that might be different from the ones at a larger scale. We try to establish a general system to describe the wetting behavior of nanoparticle droplets on vertically vibrating surfaces in this study. The results show that the contact states of nanofluids with the substrate can be changed by changing the vibration amplitude while the period remains constant. The effects of volume fraction and hydrophilicity of nanoparticles on the behavior of nanofluids on the vibrating surface are also studied. It is found that the volume fraction has a greater impact on dynamic wetting behavior of nanofluid droplet than hydrophilicity of the nanoparticles, but the influence of the volume fraction and hydrophilicity of nanoparticles on the nanofluids is concentrated in the initial stage of vibration.

Experience in registration of evaporation of liquid drops on a substrate by the capacitive method

This paper presents the results of an experimental study of the dynamics of evaporation of nanofluid droplets based on distilled water with a mass concentration of SiO2 nanoparticles of 0.1%, 0.5%, and 7% lying on a metal surface. The drop height was changed over time using original equipment, which is based on an integrated approach to the combined use of capacitive and optical recording methods. The experimental results show that the change in the height of nanofluid droplets with concentrations of 0.1%, 0.5%, and 7% is linear over the main part of the evaporation time interval. A deviation from the linear law is observed at the final stage, at the time interval of complete evaporation. The time for complete evaporation of droplets of nanofluids with a concentration of 0.1% increases by 20%, for droplets with a concentration of 0.5%, it increased by 28% in comparison with the evaporation of droplets of the base liquid. The particle concentration of 7% does not lead to an increase in the evaporation time of droplets in comparison with the evaporation of low concentration droplets. Before the formation of a jelly-like residue of nanoparticles, the evaporation rate of droplets with a particle concentration of 7% is comparable to the evaporation rate of droplets with a concentration of 0.1%.