Spreading of triple line and dynamics of bubble growth inside nanoparticle dispersions on top of a substrate plate

This work investigates the feasibility of engineering surface wettability by using different nanoparticles. As an illustration, detailed formation of gas bubbles on top of a stainless steel substrate plate in a quiescent pool of aqueous gold and alumina nanofluids is studied. The presence of nanoparticles is shown to be able to modify the dynamics of triple line and bubble growth significantly. An early pinning of the bubble triple line is observed and a larger bubble contact angle is found for bubbles growing in a gold nanofluid, whereas an opposite phenomenon is observed for bubbles growing in an alumina nanofluid compared to those of pure water. Other bubble parameters such as departure volume, bubble frequency, and waiting time of bubble formation are also affected by the presence of nanoparticles. The variation of solid surface tensions due to the existence of nanoparticles and the resultant force at the triple line should be responsible for such differences. Such results illustrate the big potential of nanoparticle in engineering surface wettability of a solid-liquid-gas system.     

Molecular dynamics simulation of pressure isotherms for nanofluids

Pressure isotherms have been constructed by the molecular dynamics method for nanofluids based on argon and zinc, aluminum, and lithium nanoparticles. Nanoparticle volume concentration is varied from 1 to 10%. Nanoparticles have sizes of 1 or 2 nm. The equation of state has been shown to essentially depend on the volume concentration, size, and material of the particles. Depending on the density of a carrier fluid, the pressure of a nanofluid (at a preset density) may be either higher or lower than the pressure of the basic fluid. On the other hand, the partial pressure of a pseudogas of nanoparticles decreases rapidly with an increase in their sizes (inversely proportional to the cubed particle radius).     

Evaporative characteristics of sessile nanofluid droplet on micro‐structured heated surface

Micro‐structure patterned substrates attract our attention due to the special and programmable wettabilities. The interaction between the liquid and micro/nano structures gives rise to controllable spreading and thus evaporation. For exploration of the application versatility, the introduction of nanoparticles in liquid droplet results in interaction among particles, liquid and microstructures. In addition, temperature of the substrates strongly affects the spreading of the contact line and the evaporative property. The evaporation of sessile droplets of nanofluids on a micro‐grooved solid surface is investigated in terms of liquid and surface properties. The patterned nickel surface used in the experiments is designed and fabricated with circular and rectangular shaped pillars whose size ratios between interval and pillars is fixed at 5. The behavior is firstly compared between nanofluid and pure liquid on substrates at room temperature. For pure water droplet, the drying time is relatively longer due to the receding of contact line which slows down the liquid evaporation. Higher concentrations of nanoparticles tend to increase the total evaporation time. With varying concentrations of graphite at nano scale from 0.02% to 0.18% with an interval at 0.04% in water droplets and the heating temperature from 22 to 85°C, the wetting and evaporation of the sessile droplets are systematically studied with discussion on the impact parameters and the resulted liquid dynamics as well as the stain. The interaction among the phases together with the heating strongly affects the internal circulation inside the droplet, the evaporative rate and the pattern of particles deposition.     

Wetting and spreading of nanofluids on solid surfaces driven by the structural disjoining pressure: statics analysis and experiments

The wetting and spreading of nanofluids composed of liquid suspensions of nanoparticles have significant technological applications. Recent studies have revealed that, compared to the spreading of base liquids without nanoparticles, the spreading of wetting nanofluids on solid surfaces is enhanced by the structural disjoining pressure. Here, we present our experimental observations and the results of the statics analysis based on the augmented Laplace equation (which takes into account the contribution of the structural disjoining pressure) on the effects of the nanoparticle concentration, nanoparticle size, contact angle, and drop size (i.e., the capillary and hydrostatic pressure); we examined the effects on the displacement of the drop-meniscus profile and spontaneous spreading of a nanofluid as a film on a solid surface. Our analyses indicate that a suitable combination of the nanoparticle concentration, nanoparticle size, contact angle, and capillary pressure can result not only in the displacement of the three-phase contact line but also in the spontaneous spreading of the nanofluid as a film on a solid surface. We show here, for the first time, that the complete wetting and spontaneous spreading of the nanofluid as a film driven by the structural disjoining pressure gradient (arising due to the nanoparticle ordering in the confined wedge film) is possible by decreasing the nanoparticle size and the interfacial tension, even at a nonzero equilibrium contact angle. Experiments were conducted on the spreading of a nanofluid composed of 5, 10, 12.5, and 20 vol % silica suspensions of 20 nm (geometric diameter) particles. A drop of canola oil was placed underneath the glass surface surrounded by the nanofluid, and the spreading of the nanofluid was monitored using an advanced optical technique. The effect of an electrolyte, such as sodium chloride, on the nanofluid spreading phenomena was also explored. On the basis of the experimental results, we can conclude that a nanofluid with an effective particle size (including the electrical double layer) of about 40 nm, a low equilibrium contact angle (<3°), and a high effective volume concentration (>30 vol %) is desirable for the dynamic spreading of a nanofluid system with an interfacial tension of 0.5 mN/m. Our experimental observations also validate the major predications of our theoretical analysis.     

StarTube: a tube with reduced contact line for minimized gas bubble resistance

In this work we introduce a novel tubing design for multiphase flow that minimizes gas bubble resistance. The design termed "StarTube" has a lamella-like wall structure and was developed to prevent clogging by gas bubbles. This is performed by forcing gas bubbles into the center of the tube by capillary forces, allowing liquid to bypass in the outer grooves. It was found that the mobility of gas bubbles in such a tube is increased more than 1 order of magnitude. The reason is that the contact line perpendicular to the direction of flow is minimized, reducing resistant effects related to the contact line-in particular, contact angle hysteresis.     

Contact line motion and dynamic wetting of nanofluid solutions

The effect that nanoparticles play in the spreading of nanofluids dynamically wetting and dewetting solid substrates is investigated experimentally, using 'drop shape' analysis technique to analyse aluminium-ethanol contact lines advancing and receding over hydrophobic Teflon-AF coated substrates. Results obtained from the advancing/receding contact line analysis show that the nanoparticles in the vicinity of the three-phase contact line enhance the dynamic wetting behaviour of aluminium-ethanol nanofluids for concentrations up to approximately 1% concentration by weight. Two mechanisms were identified as a potential reason for the observed enhancement in spreading of nanofluids: structural disjoining pressure and friction reduction due to nanoparticle adsorption on the solid surface. The observed 'lubricating effect' that the nanoparticles seem to be inducing is similar to the 'superspreading' effect for surfactant solutions spreading on hydrophobic surfaces, up to a concentration (weight) of approximately 1%, could be a result of the predicted enhanced wetting behaviour. Indeed, Trokhymchuk et al. [Langmuir, 2001, 17, 4940] observed a solid-like ordering of nanoparticles in the vicinity of the three-phase contact line, leading to an increased pressure in the fluid 'wedge'. This increased pressure leads to a pressure gradient which causes the nanofluids to exhibit enhanced wetting characteristics. Another possible cause for the observed increase in advancing/receding contact line velocity could be deposition of nanoparticles on the solid surface in the vicinity of the three-phase contact line resulting in the nanofluid effectively advancing over aluminium rather than Teflon-AF, or the contact line 'rolling' over nanoparticles at the three-phase contact line due to sphericity of nanoparticles. For either of these to be the case, the nanoparticle effect at the three-phase contact line would have to be enhanced for the lower concentration in the same way that it would have to be for the increased pressure in the fluid 'wedge'.     

Synthesis of metal-based nanofluids and their thermo-hydraulic performance in compact heat exchanger with multi-louvered fins working under laminar conditions

Present experimental investigation incorporates characterization of Al nanopowder, synthesis of Al/water nanofluids, and effect of these nanofluids on thermal performance of compact heat exchanger. Al nanoparticles are characterized using TEM and XRD. Al/water nanofluid is prepared by dispersing metal basis aluminium nanoparticles of average 100 nm size into double distilled water at two different particle volume concentrations of 0.1 and 0.2%. The nanofluids are prepared by two-step method and cetyl trimethyl ammonium bromide surfactant is used to stabilize the nanofluid. Thermo-physical properties of nanofluids at two different concentrations and their variation with fluid temperature are measured experimentally. It is examined that thermal conductivity, viscosity, and density of the nanofluid increased with the increase of volume concentrations. Furthermore, by increasing the fluid temperature, thermal conductivity is intensified, while the viscosity and density are decreased. Heat transfer parameters are strong functions of these thermo-physical properties. Therefore, comprehensive findings on heat transfer coefficient, Nusselt number, colburn factor, friction factor, and effectiveness are determined experimentally for prepared nanofluids passing under laminar conditions through single-pass cross-flow compact heat exchanger attached with multi-louvered fins.

     

A model of nanofluids effective thermal conductivity based on dimensionless groups

Thermal conductivity is an important parameter in the field of nanofluid heat transfer. This article presents a novel model for the prediction of the effective thermal conductivity of nanofluids based on dimensionless groups. The model expresses the thermal conductivity of a nanofluid as a function of the thermal conductivity of the solid and liquid, their volume fractions, particle size and interfacial shell properties. According to this model, thermal conductivity changes nonlinearly with nanoparticle loading. The results are in good agreement with the experimental data of alumina-water and alumina-ethylene glycol based nanofluids.     

Synthesis and thermo-physical properties of water-based novel Ag/ZnO hybrid nanofluids

The comparative study on the thermo-physical properties of water-based ZnO nanofluids and Ag/ZnO hybrid nanofluids is reported in the present study. The outer surface of ZnO nanoparticles was modified with a thin coating of Ag nanoparticles by a wet chemical method for improved stability and heat transfer properties. The ZnO and Ag/ZnO nanofluids were prepared with varying volume concentration (??=?0.02–0.1%). The synthesized nanoparticles and nanofluids were characterized with different characterization methods viz., scanning electron microscopy, X-ray diffraction, dynamic light scattering, thermal conductivity measurement, and viscosity measurement. Results show that thermal conductivity of Ag/ZnO hybrid nanofluids is found to be significantly higher compared to ZnO nanofluids. The maximum thermal conductivity an enhancement for Ag/ZnO nanofluid (??=?0.1%) is found to 20% and 28% when it compared with ZnO nanofluid (??=?0.1%) and water, respectively.     

Microwave synthesis of silver nanofluids with polyvinylpyrrolidone (PVP) and their transport properties

Microwave synthesis has been applied to prepare stable silver nanofluids in ethanol by reduction of AgNO₃ with polyvinylpyrrolidone (PVP), used as stabilizing agent, having Ag concentrations of 1% by volume. The nanofluids were characterized by UV-vis spectroscopy, Fourier transform infrared, energy-dispersive X-ray spectroscopy, and transmission electron microscopy and systematically investigated for refractive index, electrical and thermal conductivity, and viscosity for different polymer concentrations. The size of nanoparticles was found to be in the range of 30–60 nm for two different salt-to-PVP ratios. For higher concentration of polymer in nanofluid, nanoparticles were 30 nm in size showing increase in thermal conductivity but a decrease in viscosity and refractive index, which is due to the polymer structure around nanoparticles. Thermal conductivity measurements of nanofluids show substantial increment in the thermal conductivity of nanofluid relative to the base fluid and nonlinear enhancement over the 283–323 K temperature range. Rheology of nanofluids was studied at room temperature showing effect of polymer on viscosity and confirming the Newtonian behavior of nanofluid.     

Effect of Reynolds number on heat transfer and flow for multi-oxide nanofluids using numerical simulation

A numerical simulation model for laminar flow of nanofluids in a pipe with constant heat flux at the wall has been built to study the effect of Reynolds number on heat transfer and pressure loss. The investigation was performed for metallic oxide and multi-oxide nanoparticles suspended in water. The thermal conductivity and dynamic viscosity were measured for a range of temperature (10–60 °C) and volume fraction of multi-oxide nanofluid. Comparison of the thermal conductivity for monocular oxide and multi-oxide nanofluids reveals a new way to control the enhancement in nanofluid conductivity. The numerical results obtained were compared with existing well-established correlations. The predictions of the Nusselt number for nanofluids are in agreement with the Shah correlation, and the deviation in the results is less than 1 %. It is found that the pressure loss increases with the Reynolds number, nanoparticle density, and volume fraction for multi-oxide nanoparticles. However, the flow demonstrates enhancement in heat transfer which improves with increasing Reynolds number of the flow.     

Sliding variability of droplets on a hydrophobic incline due to surface entrained air bubbles

The ability of a liquid droplet to move on an incline has important ramifications in discrete volume fluidic devices. By taking advantage of the spontaneous and copious formation of visible air bubbles within water droplets on a polytetrafluoroethylene (PTFE) surface, we uncovered a direct correlation between their presence and the ability of droplets to slide down an incline. We forward two possible mechanisms to account for this behavior. The first is attributed to the air bubbles creating regions where additional solid-liquid-vapor phase interfaces are present; wherein due to the buoyancy force acting upwards, the orientation of the contact angles of each bubble (which should also be in hysteresis but in the opposite direction of the hysteresis at the droplet rim contact lines) dictate that the net force of the bubbles in the droplet act down an incline. We show here that this mechanism cannot fully account for the bubble enhanced sliding behavior. The second mechanism is based on the occurrence of the droplet front advancing first, causing the droplet to elongate and thus allowing the receding contact line to partially sweep inwards over the bubbles. This causes a series of point-wise disruptions on the contact line that permits the droplet to slide down more readily. The relatively short time of ～180s during which these micron sized bubbles decrease in size indicates a possibility of this mechanism contributing to a transient means to reduce the retention force of droplets that reside on hydrophobic surfaces.     

The thermal conductivity of alumina nanoparticles dispersed in ethylene glycol

The thermal conductivities of several nanofluids (dispersions of alumina nanoparticles in ethylene glycol) were measured at temperatures ranging from 298 to 411 K using a liquid metal transient hot wire apparatus. Our measurements span the widest range of temperatures that have been investigated to date for any nanofluid. A maximum in the thermal conductivity versus temperature behavior was observed at all mass fractions of nanoparticles, closely following the behavior of the base fluid (ethylene glycol). Our results confirm that additional temperature contributions inherent in Brownian motion models are not necessary to describe the temperature dependence of the thermal conductivity of nanofluids. Our results also show that the effect of mass or volume fraction of nanoparticles on the thermal conductivity of nanofluids can be correlated using the Hamilton and Crosser or Yu and Choi models with one adjustable parameter (the shape factor in the Hamilton and Crosser model, or the ordered liquid layer thickness in the Yu and Choi model).     

Nanoparticle flotation collectors: mechanisms behind a new technology

This is the first report describing a new technology where hydrophobic nanoparticles adsorb onto much larger, hydrophilic mineral particle surfaces to facilitate attachment to air bubbles in flotation. The adsorption of 46 nm cationic polystyrene nanoparticles onto 43 μm diameter glass beads, a mineral model, facilitates virtually complete removal of the beads by flotation. As little as 5% coverage of the bead surfaces with nanoparticles promotes high flotation efficiencies. The maximum force required to pull a glass bead from an air bubble interface into the aqueous phase was measured by micromechanics. The pull-off force was 1.9 μN for glass beads coated with nanoparticles, compared to 0.0086 μN for clean beads. The pull-off forces were modeled using Scheludko's classical expression. We propose that the bubble/bead contact area may not be dry (completely dewetted). Instead, for hydrophobic nanoparticles sitting on a hydrophilic surface, it is possible that only the nanoparticles penetrate the air/water interface to form a three-phase contact line. We present a new model for pull-off forces for such a wet contact patch between the bead and the air bubble. Contact angle measurements of both nanoparticle coated glass and smooth films from dissolved nanoparticles were performed to support the modeling.     

Nanospheres in phase-separating multicomponent fluids: a three-dimensional dissipative particle dynamics simulation

The dynamics of phase separation of three-dimensional fluids containing nanospheres, which interact preferentially with one of the two fluids, is studied by means of large-scale dissipative particle dynamics simulations. We systematically investigated the effect of volume fraction, radius, and mass of the nanoparticles on both kinetics and morphology of the binary mixture. We found that nanospheres lead to a reduction of domain growth which is intensified as their volume fraction is increased for a given radius of nanoparticles, or as the nanoparticles radius is decreased for a given volume fraction. Up to moderate volume fractions of nanoparticles, the growth law, however, is found to be identical to that pure binary fluids, i.e., R(t) approximately t(n), with n=1. For relatively high volume fractions of nanoparticles, a diffusive growth regime was detected. The crossover to the slower growth regime as the nanoparticles volume fraction is increased or their radius is decreased is associated with the crystallization of the nanospheres within the preferred component. These results are qualitatively in good agreement with previous two-dimensional simulations using molecular dynamics [M. Laradji and G. MacNevin, J. Chem. Phys. 119, 2275 (2003)] and a time-dependent Ginzburg-Landau model [M. Laradji, J. Chem. Phys. 120, 9330 (2004)], as well as recent experiments.     

Thermal conductivity modeling of MgO/EG nanofluids using experimental data and artificial neural network

The application of nanofluids in energy systems is developing day by day. Before using a nanofluid in an energy system, it is necessary to measure the properties of nanofluids. In this paper, first the results of experiments on the thermal conductivity of MgO/ethylene glycol (EG) nanofluids in a temperature range of 25–55 °C and volume concentrations up to 5 % are presented. Different sizes of MgO nanoparticles are selected to disperse in EG, including 20, 40, 50, and 60 nm. Based on the results, an empirical correlation is presented as a function of temperature, volume fraction, and nanoparticle size. Next, the model of thermal conductivity enhancement in terms of volume fraction, particle size, and temperature was developed via neural network based on the measured data. It is observed that neural network can be used as a powerful tool to predict the thermal conductivity of nanofluids.     

Determination of nanolayer thickness and effective thermal conductivity of nanofluids

Nanofluids having high thermal conductivity enhancement relative to conventional pure fluids are fluids engineered by suspending solid nanoparticles into base fluids. In the present study, calculating the Van der Waals interaction energy between a nanoparticle and an ordered liquid nanolayer around it, the nanolayer thickness was determined, the average velocity of the Brownian motion of nanoparticles in a fluid was estimated, and by taking into account both the aggregation of nanoparticles and the presence of a nanolayer a new thermal conductivity model for nanofluids was proposed. It has been shown that the nanolayer thickness in nanofluids is independent on the radius of nanoparticles when the radius of the nanoparticles is much greater than the nanolayer thickness and determines by the specific interaction of the given liquid and solid nanoparticle through the Hamaker constant, the surface tension and the wetting angle. It was proved that the frequency of heat exchange by fluid molecules is two orders of magnitude higher than the frequency of heat transfer by nanoparticles, so that the contribution due to the Brownian motion of nanoparticles in the thermal conductivity of nanofluids can be neglected. The predictions of the proposed model of thermal conductivity were compared with the experimental data and a good correlation was achieved.     

Simultaneous spreading and evaporation: Recent developments

The recent progress in theoretical and experimental studies of simultaneous spreading and evaporation of liquid droplets on solid substrates is discussed for pure liquids including nanodroplets, nanosuspensions of inorganic particles (nanofluids) and surfactant solutions. Evaporation of both complete wetting and partial wetting liquids into a nonsaturated vapour atmosphere are considered. However, the main attention is paid to the case of partial wetting when the hysteresis of static contact angle takes place. In the case of complete wetting the spreading/evaporation process proceeds in two stages. A theory was suggested for this case and a good agreement with available experimental data was achieved. In the case of partial wetting the spreading/evaporation of a sessile droplet of pure liquid goes through four subsequent stages: (i) the initial stage, spreading, is relatively short (1–2 min) and therefore evaporation can be neglected during this stage; during the initial stage the contact angle reaches the value of advancing contact angle and the radius of the droplet base reaches its maximum value, (ii) the first stage of evaporation is characterised by the constant value of the radius of the droplet base; the value of the contact angle during the first stage decreases from static advancing to static receding contact angle; (iii) during the second stage of evaporation the contact angle remains constant and equal to its receding value, while the radius of the droplet base decreases; and (iv) at the third stage of evaporation both the contact angle and the radius of the droplet base decrease until the drop completely disappears. It has been shown theoretically and confirmed experimentally that during the first and second stages of evaporation the volume of droplet to power 2/3 decreases linearly with time. The universal dependence of the contact angle during the first stage and of the radius of the droplet base during the second stage on the reduced time has been derived theoretically and confirmed experimentally. The theory developed for pure liquids is applicable also to nanofluids, where a good agreement with the available experimental data has been found. However, in the case of evaporation of surfactant solutions the process deviates from the theoretical predictions for pure liquids at concentration below critical wetting concentration and is in agreement with the theoretical predictions at concentrations above it.     

Simulation of pool boiling of nanofluids by using Eulerian multiphase model

In the present work, a new simulation of nanofluid/vapor two-phase flow inside the 2-D rectangular boiling chamber was numerically investigated. The Eulerian–Eulerian approach used to predict the boiling curve and the interaction between two phases. The surface modification during pool boiling of silica nanofluid represented by surface roughness and wettability is put into the account in this simulation. New closure correlations regarding the nucleation sites density and bubble departure diameter during boiling of silica nanofluid were inserted to extend the boiling model in this work. Besides, the bubble waiting time coefficient which involved in quenching heat flux under heat flux partitioning HFP model was corrected to improve the results of this study. The numerical results validated with experimental works in the literature, and they revealed good agreements for both pure water and nanofluids. The results found that when improving the heat flux partitioning model HFP by considering the surface modification of nucleate pool boiling parameters, it will give more mechanistic sights compared to the classical model, which is used for predicting of boiling heat transfer of pure liquid.

     

Bubble Consumption Dynamics in Electrochemical Oxygen Reduction

Electrochemical oxygen reduction reaction(ORR) is crucial for fuel cells and metal-air batteries, while the oxygen consumption dynamics study during ORR, which affects the ORR efficiency, is not as concerned as catalysts design does. Herein the consumption behavior of an individual oxygen bubble on Pt foils with different wettabilities during ORR was tracked by a real-time approach to reveal whether the surface wettability of electrode can influence the consumption dynamics and determine the reaction reactive zones of oxygen bubble consumption. The oxygen bubble underwent a "constant contact angle" dominant consumption model on aerophobic Pt foil, while an initial "constant radius" and the subsequent "constant contact angle" oxygen consumption models were observed on aerophilic Pt foil. Results here demonstrated that the current was proportional to the bottom contact area, rather than the three-phase contact line of the bubbles according to the fitting curves between individual bubble current and the consumption behavior parameters. This study highlights the important role of the gas-solid interface in influencing the efficiency of gas consumption electrochemical reactions, which shall benefit the understanding of reaction kinetics and the rational design of electrocatalysts.