Nanofluid Droplets Research Articles

Disk-Ring Deposition in Drying a Sessile Nanofluid Droplet with Enhanced Marangoni Effect and Particle Surface Adsorption.

The present study is to explore the central particle deposition from drying a sessile nanofluid droplet experimentally and theoretically. Normally, a pinned colloidal droplet dries into a coffee-ring pattern as a result of moving the particles to a three-phase line by the radial direction capillary flow. However, the strong evaporation can generate the nonuniform temperature at the evaporating droplet interface and the droplet periphery temperature is higher than that close to the droplet centerline. The induced Marangoni flow would reversibly transport the particles at the periphery toward the centerline. We have thus designed the experiments to increase the droplet evaporation rate in vacuum conditions and accordingly to enhance the Marangoni effect. We have observed distinguishable disk deposition inside the outer coffee ring. A three-dimensional diffusion-limited cluster-cluster aggregation Monte Carlo model has been developed to simulate the deposition process. With modeling the Marangoni effect, particle adsorption at the liquid-air interface and particle aggregation behaviors, the formation of the disk pattern inside a coffee ring has been simulated. The qualitative agreement has been found in the comparison of local deposition distribution between the related experiment and simulation.

An experimental study of the evaporation rate of nanofluid droplets with SiO2 nanoparticles

In this work, experimental data are presented on the evaporation rate of suspended droplets of distilled water and distilled water with the addition of nano particles of silicon dioxide during evaporation in an air stream with a low blowing rate. The measurements were carried out at Reynolds numbers calculated from the relative flow velocity Re <10. The evaporation rate of suspended droplets of distilled water and nanofluid was compared.

Freezing of Nanofluid Droplets on Superhydrophobic Surfaces.

Droplet freezing on cold superhydrophobic surfaces has been studied extensively in recent years. However, previous works are mainly focused on studying water droplet freezing behavior; little work has been conducted to investigate the freezing dynamics of nanofluid droplets on superhydrophobic surfaces. In this work, freezing morphologies of water and nanofluid droplets on superhydrophobic surfaces with different roughnesses were compared and studied. The nanofluid droplets underwent a shape transition from spherical to flat plateau morphology, different from the frozen water droplets that exhibit a sharp cusp. The size of the flat plateau for the frozen nanofluid droplet increases with increasing nanoparticle concentration. The underlying mechanism of the morphology change during the freezing process was elucidated using COMSOL Multiphysics. Compared to the frozen water droplets, more air bubbles are trapped inside the frozen nanofluid droplets, which might be ascribed to the fast freezing speed of the nanofluid droplets. These results can provide important insights for many applications that require freezing of nanofluid droplets, such as material solidification, three-dimensional (3D) printing, as well as phase change enhancement.

Experimental studies on evaporation kinetics of gold nanofluid droplets: Influence of nanoparticle sizes and coating on thermal performance

In this work, a series of experiments investigate the evaporation of gold nanofluid sessile droplets on a perfluorodecyltrichlorosilane (PFTS) silicon substrate heated to 77 °C. The evaporation processes of different nanofluid droplets of the same initial volume, all for a 1% Cv volume concentration prepared from the 0.1 mM original suspension, are visualized to examine the size effect of gold nanoparticles (2.2, 5 and 10 nm) and the impact of surface coating (with and without Citrate capping in phosphate-buffered saline solvent) on enhancing heat transfer. This study open access to understand the size effect of gold nanoparticle in very small scale and the different types of surfactant on heat and mass transfer during the evaporation of droplet. Two methods are used to analyze the evaporation process, an optical one coupled to an infrared thermography method and an acoustic method. These complementary methods have the ability to investigate clearly the solid/liquid and liquid/vapor interfaces at the same time. From the optical observation, using a Drop Shape Analyzer (Kruss system), the evolution in time of the shape of the droplets (contact angle, base diameter and volume) are measured under controlled conditions (Humidity = 50%, Tatm = 23 °C). Then, the evaporation rate is deduced from the measurements of the evolution of volume in time. At the same time, an infrared camera is used to observe the droplet surface gradient temperature, air/liquid interface, due to thermal Marangoni flow. The acoustic method, based on a high- frequency echography principle, allowed to monitor the stability of nanoparticles inside the droplets (Au-water mixture) during the process of evaporation. From the kinetics of the reflection coefficient at the interface of the silicon substrate over which the nanofluids are deposited, the results show that the evaporation rate of 5 nm Au-water mixture (phosphate buffered saline solvent only) was faster than that of 10 nm Au-water mixture. While with citrate capping-PBS, the evaporation rate of the largest particle sizes (10 nm Au-water mixture) was the fastest than that of the smaller sizes (2.2 and 5 nm). Also, the 5 nm gold nanofluids with PBS showed the highest evaporation rate (+35%) than that of citrate capping gold nanofluids (+15%). As results, the addition of a surfactant (citrate capping) decreases the effectiveness of the heat transfer of gold nanofluids. This seems to be due to the absence of Marangoni cells in droplets made of Au-water mixtures (Citrate capping-PBS). Moreover, reducing in the thermal conductivity of gold nanofluid due to the surfactant coating by covering the gold nanoparticles surface and this eventually affected the nanofluid thermal properties. This is further supported by the acoustic method, which clearly shows worse stability of the nanoparticles in the latter case.

Experimental investigation of nanoparticles distribution mechanisms and deposition patterns during nanofluid droplet evaporation

Droplet evaporation is fascinating, ubiquitous, and relevant for a wide spectrum of applications, such as printing, drug testing, coating, and biomedical diagnosis. In this paper, the effects of substrate temperature (30 °C, 47 °C, 64 °C, 81 °C) and 20 nm Al2O3 nanoparticles mass fraction (0.05%, 0.1%, 0.2%) on the nanoparticles distribution mechanisms and deposition patterns during the evaporation of Al2O3-H2O nanofluid droplet were experimentally investigated. An experimental device was designed and realized. The self-assembly and Marangoni forces were first highlighted and compared. Two coffee-ring and inner-ring patterns were then identified and analyzed. The evaporation process characteristics were finally determined and discussed.

Unraveling the Role of Excess Ligand in Nanoparticle Pattern Formation from an Evaporatively Dewetting Nanofluid Droplet

Nanoparticle (NP) patterning on a solid surface via nanofluid droplet evaporation is one of the most fascinating topics of research. Quite intriguingly, though a dose of excess ligand has been inva...

Heat Transfer and Fluid Dynamics of Nanofluid Droplets Impacting on a Smooth Heated Surface: Detailing Temporal Scale Effects by Using Time-Resolved Thermography

This study focuses on the experimental characterization of fluid dynamics and heat transfer processes occurring at the interaction between a nanofluid droplet and a heated smooth surface. Time resolved thermography is used to detail the understanding of the role of the particles in wetting modification that affects the heat transfer mechanisms within different characteristic time scales. Gold and silver nanoparticles are dissolved in distilled water in concentrations ranging between 0.1 and 1 wt%. Millimetric droplets with a fixed initial diameter of 3 mm are generated and impact on a smooth stainless steel surface with velocities varying between 0.8 and 2 ms−1. The surface is heated by Joule effect, from ambient temperature up to 120 °C. Results show that the heat transfer is enhanced by the presence of the nanoparticles, for low impact velocities of the order of 0.8 ms−1 and for earlier stages after impact. However, the heat transfer is deteriorated as the impact velocity is increased, as well as for later stages of impact, even though the spreading diameter of the nanofluid droplets is larger than that of the water droplet.

Effects of nanoparticles on the instability of liquid jets in a gaseous crossflow

Nanofluids have attracted interest for their high performance in enhancing heat transfer and improving combustion. However, few studies have investigated the influences of nanoparticles on spray processes. In this study, the surface waves, liquid column oscillation, and droplet size distribution of a nanofluid transverse jet were investigated at low gas Weber number. A new statistical image-processing method was proposed to identify the surface wavelength and jet oscillation amplitude. A new power index parameter was proposed to account for the effect of nanoparticles, and then introduced to correlate with the upwind wavelength and Sauter mean diameter (SMD) of nanofluid droplets in the far field. The results show that in bag breakup and multimode breakup, the wavelengths decrease along the jet flow direction while the amplitudes gradually increase in the initial stage. The characteristic wavelength and SMD diminish with the increase of nanoparticle concentration, and the oscillation angle of liquid column increases correspondingly. The nanoparticle additive at low concentration obviously enhances the flow instability and cavitation inside the jet. Therefore, the fineness and uniformity of nanofluid atomization in the far field are significantly improved.

Molecular dynamics simulation of the wetting characteristics of a nanofluid droplet on rough substrate

Wettability of a nanofluid droplet (ND) on rough substrate with fence pattern was researched by molecular dynamics (MD) method. The Embedded Atom Method (EAM) potential was used to model the interatomic interaction between nanoparticle atoms, solid rough substrate atoms and between these two. The effects of the surface morphology of substrate, the surface wettability of the nanoparticle, and the nanoparticle volume fraction (NVF) on the wettability of ND were studied. The vapor-liquid surface tension and viscosity of nanofluid were also discussed in details. The results show that the initial equilibrium contact angle (θe) of ND on a rough substrate is larger than that on a smooth substrate. When the interaction parameter between the substrate atoms and oxygen atoms increases, θe decreases. For hydrophilic rough substrates, the larger the NVF of hydrophilic nanoparticles in the ND, the greater the equilibrium contact angle of the ND, and the worse the wetting properties of the ND, which determines that the main driving force for the dynamic spreading process of the ND is the vapor-liquid surface tension and viscosity of the ND.

Self-assembled nanoparticle-coated interfaces: Capillary pressure, shell formation and buckling

HypothesisParticle accumulation at liquid-liquid or liquid-gas interfaces can significantly alter capillary behavior and give rise to unusual interfacial phenomena including the asymmetric macroscopic mechanical response of the interface. ExperimentsThis study explores the accumulation of cetyltrimethylammonium bromide-modified nanoparticles at fluid interfaces and the subsequent mechanical response of nanoparticle-coated droplets during contraction and expansion. Droplet tests involve the simultaneous recording of the droplet shape and the capillary pressure. Complementary single-pore experiments examine the response of particle-coated interfaces as they traverse a pore constriction. FindingsInterfaces promote order. The time-dependent nanoparticle accumulation at the interface is diffusion-controlled. The nanoparticle coated droplets can sustain negative capillary pressure before they buckle. Buckling patterns strongly depend on the boundary conditions: non-slip boundary conditions lead to crumples while slip boundary conditions result in just a few depressions. The particle-coated interface exhibits asymmetric behavior in response to particle-level capillary forces: an “oil droplet in a nanofluid bath” withstands a significantly higher capillary pressure difference than a “nanofluid droplet in an oil bath”. A first-order equilibrium analysis of interaction forces explains the asymmetric response. Single-constriction experiments show that the formation of particle-coated interfaces has a pronounced effect on fluid displacement in porous media.

A comprehensive comparison between substrate heating and light heating induced nanofluid droplet evaporations

A comprehensive comparison between substrate heating and light plasmonic heating induced nanofluid droplet evaporations is presented. The substrate temperature was kept as 36.5 °C for substrate heating, but a 1400 W/m2 irradiation flux emitted by a xenon lamp was used for light heating. Both two heating modes yield constant contact diameter evaporation. Because light irradiation reorganizes nanoparticles to generate dynamically varied heat source in nanoscale due to the plasmonic effect, the two heating modes are found to display three differences. First, droplet surface temperature decreases monotonically from contact line to apex for substrate heating, but for light irradiation, droplet surface temperature displays nonmonotonic variation including a contact line region CLR and a bulk volume region BVR. Temperature gradient is significant in CLR but does not exist in BVR. Second, the substrate heating generates both radial flow and Marangoni flow in the whole droplet. Thus, coffee-ring and dispersed nanoparticles are observed on substrate. On the other hand, for light heating, the Marangoni flow is only confined in CLR due to the local temperature gradient there, creating most of nanoparticles deposition near the contact line. Third, because the droplet surface area decreases versus time, the evaporation rate reduces to behave the non-linear variation of droplet volume for substrate heating. The situation is changed for light heating induced droplet evaporation. The enhanced plasmonic heating in CLR due to nanoparticles deposition there offsets the effect of droplet surface area decreasing. Thus, light heating would give rise to the constant droplet evaporation rate, which is distinct to substrate heating. This work enhances the fundamental understanding of the droplet evaporation dynamics under the condition of light heating induced plasmonic heating effect.

Making Photonic Crystals via Evaporation of Nanoparticle-Laden Droplets on Superhydrophobic Microstructures.

We employed a convenient evaporation approach to fabricate photonic crystals by naturally drying droplets laden with nanoparticles on a superhydrophobic surface. The final drying morphology could be controlled by the concentration of nanoparticles. A dilute droplet resulted in a torus, whereas a quasi-spherical cap with a bottom cavity was made from a concentrated droplet. Remarkably, the nanofluid droplets maintained high contact angles (≳120°) during the entire evaporation process because of inhomogeneous surface wetting. Bottom-view snapshots revealed that during evaporation the color of the contact area changed sequentially from white to red, orange, yellow, and eventually to green. Scanning electron microscopy and Voronoi analysis demonstrated that nanoparticles were self-assembled to a hexagonal pattern. Finally, based on the effects of particle size, material, and volume concentration on the reflected wavelengths, a model has been developed to successfully predict the reflected wavelength peaks from the contact area of evaporating colloidal droplets. Our model can be easily adopted as a manufacturing guide for functional photonic crystals to predict the optimal reflected color made by evaporation-driven self-assembly of photonic crystals.

Evaporation and wetting behavior of silver-graphene hybrid nanofluid droplet on its porous residue surface for various mixing ratios

Droplet evaporation offers high heat rejection rates and is widely used in the form of spray cooling or dropwise cooling of various heat dissipating devices. However, due to the limiting heat flux removal capacity of conventional fluids, such as water, these cannot be used in thermal management of high heat flux devices. In this research, the evaporation of silver (Ag)-graphene (GNP) hybrid nanofluid droplet and its residue effects on the evaporation of following Ag-GNP hybrid nanofluid droplet, due to its synergistic thermal properties, is experimentally investigated for various mixing ratios, from MR-1 (0.1(Ag):0.9(GNP)) to MR-5 (0.9(Ag):0.1(GNP)), and different residue sizes. A theoretical model is also proposed for hybrid nanofluid droplet evaporation and semi-empirical relations are developed to estimate the hybrid nanofluid droplet spreading over its residue surface. The results show a substantial increase in the droplet evaporation rate with increasing residue size and decreasing mixing ratio. MR-1 hybrid nanofluid droplet gives the highest evaporation rate (up to 370%) on its highly wetted residue surface, while the evaporation rate significantly drops moving from MR-2 to MR-5 hybrid nanofluid droplets on their partially wetted residue surfaces. Moreover, the evaporation rate substantially increases (up to 240%) with increasing residue size for MR-1 hybrid nanofluid droplet resting on its residue surface, however, the effect of residue size on droplet evaporation rate considerably diminishes moving from MR-2 to MR-5 hybrid nanofluid droplets resting on their respective residues.

An experimental study on the effect of nanoparticle shape on the dynamics of Leidenfrost droplet impingement

Extensive investigations have been carried out on the thermo-hydrodynamics of nanofluid droplet interaction with heated and non-heated flat surfaces. However, the influence of shape of nanoparticles on the dynamics of droplet impingement on heated flat surfaces is yet to be explored in detail. In this study, hydrodynamics of nanofluid droplet impingement process on heated and mechanically polished aluminum substrate was studied using dissolved Al2O3 nanoparticles having spherical as well as cylindrical shapes. Nanofluids of 0.3% volume fractions were prepared from spherical Al2O3 particles of mean size less than 50 nm and from cylindrical Al2O3 particles of 2–6 nm in diameter and 200–400 nm in length. It was observed that, the impact dynamics is different from that of base pure fluid owing to the presence of nanoparticles. Leidenfrost temperatures of both nanofluids were dropped drastically in comparison with pure liquid. Further, the residence time, spread factor as well as retraction height also exhibit a different behavior against the base fluid. Detailed investigations were carried out for different Weber numbers (We = 18–159) and surface superheat and resul ts obtained were compared with de-ionized (DI) water.

Motion of Nanofluid Droplets through Immiscible Quiescent Liquid: An Experimental Study

Motion of droplets in liquid –liquid extraction column is of importance for many industrial processes. This study aims at evaluation of hydrodynamic behavior of a nanofluid particle moving through an immiscible liquid. Toluene based nanofluids were used as referral systems in this study. Nanoparticles of ZnO, Al2O3, SiO2 and TiO2 have been used to synthesize stable nanofluids. The experiments were carried out using different concentrations of nanoparticles to study the variation in terminal velocity, drag coefficient and eccentricity of the nanofluid droplet. Observations show enhancement in terminal velocity and reduction in drag coefficient of the droplets, for all nanofluids up to optimum concentration of nanoparticles. With increase in concentration of nanofluid beyond the optimum concentration, the droplet terminal velocity decreases and drag coefficient increases due to deformation in shape and an increase in oscillation that could be due to nanoparticle aggregation in nanofluid. The eccentricity of...

Thermal Property Measurement of Nanofluid Droplets with Temperature Gradients

In this study, the 3ω method was used to determine the thermal conductivity of nanofluids (ethylene glycol containing multi-walled carbon nanotubes (MWCNTs)) with temperature gradients. The thermal modeling of the traditional 3ω method was modified to measure the spatial variation of thermal conductivity within a droplet of nanofluid. A direct current (DC) heater was used to generate a temperature gradient inside a sample fluid. A DC heating power of 14 mW was used to provide a temperature gradient of 5000 K/m inside the sample fluid. The thermal conductivity was monitored at hot- and cold-side 3ω heaters with a spacing of 0.3 mm. Regarding the measurement results for the hot and cold 3ω heaters, when the temperature gradient was applied, the maximum thermal conductivity difference was determined to be 3% of the original value. By assuming that the thermo-diffusion of MWCNTs was entirely responsible for this difference, the Soret coefficient of the MWCNTs in the ethylene glycol was calculated to be −0.749 K−1.

Enhancement of radiation characteristics and reduction of NOx emission in natural gas flame through silver-water nanofluid injection

The aim of the present work is to investigate the effect of low-flow-rate injection of silver-water nanofluid on the enhancement of thermal and radiative characteristics and reduction of NOx emission in natural gas flame. In our method, 0.009 Lit/min of 100-ppm silver-water nanofluid is directly injected into the natural gas flame at the equivalence ratio of 0.65. An SBG01 water cooled heat flux sensor is used to measure the total radiation of flame and IR photography method and luminosity measurement are used to determine the spectral emissivity of flame in infrared and visible wavelengths. Furthermore, a KIGAS 310 gas analyzer measures the concentrations of pollutant emissions. The results indicate that nanofluid injection enhances the IR radiation of natural gas flame and creates a high-value and uniform radiative heat flux distribution for flame. Solid nanoparticles, as foreign surfaces, enhance the nucleation rate and surface growth of soot particles, which are the best radiative species in flame. The other parameters which affect the flame radiation are increase in H2O concentration, as another radiative species, and increased number of tiny high temperature dark nanoparticles, as highly emissive bodies. Furthermore, although the nanofluid injection does not significantly change the concentration of carbonaceous pollutant emission, it decreases the NOx emission as much as 22.6%. The reduction of NOx emission is caused by decrease in the maximum flame temperature, due to absorption of heat by nanofluid droplets, and increase in the concentration of OH radicals, due to dissociation of H2O at the high temperatures of flame reaction zone.

Experiments and analytical solutions of light driven flow in nanofluid droplets

Adding nanoparticles with high light response characteristics to a light-transmitting fluid medium can form a light-driven nanofluid and achieve efficient use of light energy. This paper conducts the experimental observation and theoretical analysis of the light driven nanofluid flow behavior, which is the theoretical basis for achieving the precise control of optical drive nanofluid. To realize the efficient conversion of light energy into kinetic energy, here, the motion of Fe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; particles with a diameter of 300 nm in droplets induced by the Marangoni effect is studied under different light sources by using the particle image velocimetry (PIV). The experimental results show that when the number density of particles is higher than the critical value, the vertical vortices with symmetrical structure can be induced. At the bottom of the droplet, the particles move from the periphery to the center of droplet, and at the top of the droplet, the particles move from the center to the periphery of droplet. In addition, the frequency of light source and the number density of particles are the dominant factors in this process. Subsequently, for the light driven nanofluid experiment in this paper, the analytical solution of the flow field distribution is achieved by using the Stokes equation and the surface tension gradient boundary condition. The analytical solution of the flow field distribution obtained here is consistent with the experimental results, confirming the validity of the quantitative theory. Finally, the correlation between various driving modes, including surface tension at the top surface, surface pressure at the bottom surface or concentrated light radiation force in bulk phase, is discussed. This research provides theoretical support for the precise regulation of flow behavior and efficient conversion of light energy in the optical microfluidic system.

Influence of Nanoparticles on the Evaporation Behavior of Nanofluid Droplets: A Dh Law and Underlying Mechanism.

The evaporation behaviors of droplet containing nanoparticles play an important role in nanofluid combustion, spray drying, and so on. The average evaporation rate of a nanofluid droplet will decrease sharply at the end stage of droplet evaporation because the aggregation of nanoparticles on the surface of the droplet results in a shell. To illustrate the microscopic mechanism for the variation of the average evaporation rate and surface tension caused by the copper nanoparticles on the surface of the water droplet, numerical simulations based on the Brownian dynamics are conducted to study the effects of nanoparticle behaviors on the average evaporation rate and surface tension for various initial volume fractions and various distributions of nanoparticles. The results show us that the nanoparticles' distribution and the initial volume fraction of nanoparticles will greatly affect the average evaporation rate of the nanofluid droplet. Therefore, a Dh law can be expected due to the effects of added copper nanoparticles on the evaporation behavior where h will vary with the initial volume fraction and distribution of nanoparticles. Comparisons to the published results indicate that the exponent h = 2.0 for pure liquid, h < 2.0 when lyophilic nanoparticles are added, and h > 2.0 when lyophobic nanoparticles are added. In general, the most important factors to affect the evaporation rate are the volume fraction, distribution, and lyophilic nature of nanoparticles.

Nanoparticle deposition from nanofluid droplets during Leidenfrost phenomenon and consequent rise in transition temperature

Termination of film boiling at an elevated Leidenfrost temperature can have important implications in applications which require rapid cooling. Here we report the Leidenfrost phenomenon of nanofluid (nano-TiO2 dispersed in water) droplets exposed to a substrate heated to 300 °C and evaporation lifetime of water droplets dispensed over a copper substrate heated to 300 °C as a benchmark for comparison. Unlike water or sparsely dispersed nanofluid droplets, the abundantly dispersed nanofluid droplets exhibit deposition and boiling at 300 °C accompanied by violent agitation and fragmentation followed by continuous ejection of small droplets. As a result, the Leidenfrost phenomenon ceases to occur and the droplet evaporation lifetime is reduced by an order of magnitude in comparison to water. We anticipate that the vapor generated beneath the droplet for a higher concentration of nanoparticles is unable to levitate the droplet (mass) as deposition of a nano-particles generating a random pattern on the hot substrate disrupts the Leidenfrost state. We confirm the evidence of nanostructured porous coating from microscopy and topography analysis which reveals micro-pits and protuberances as high as several micrometers formed by clustered nanoparticles. In the presence of such random nanometric surface features, the interplay between the capillary pressure and vapor pressure dictates the existence of Leidenfrost condition. At a substrate temperature of 300 °C, the capillary pressure dominates the vapor pressure for the highly concentrated nanofluid droplets thereby, causing deposition of nanoparticles and ejection of small droplets. However, the same droplets exhibit levitation over the vapor layer when the substrate temperature is 400 °C. Thus, we conclude that the rise in Leidenfrost temperature for nanofluid droplets is governed by the concentration of dispersed nanoparticles, whose deposition generates a unique porous structure on the substrate and causes capillary wicking.