Nanofluid Film Research Articles

Thermocapillary instability in an evaporating two-dimensional thin layer film

A novel two-dimensional model for the thermocapillary instability of a thin liquid film was developed and validated in the paper. The model incorporates a novel solution for the unperturbed velocity components and the thin film thickness (based on the boundary layer approach and balance equation) subject to evaporation under the boundary conditions of either mechanical interaction between the liquid and vapor phases or stationary vapor above the interphase boundary. A novel model for thermocapillary instability in a thin film was developed in frames of the linear perturbation method, i.e. modified Orr–Sommerfeld equation, taking into account the surface and London–van der Waals forces. The critical Reynolds number was computed by considering the two-dimensional disturbances, which according to the Squire’s theorem are more dangerous than the three-dimensional disturbances. For constant surface tension at the interphase interface, the unperturbed velocity profile is parabolic, and maximum increases, while the critical Reynolds number decreases with the decreasing capillary number. If the surface tension at the interphase interface depends on the temperature, the maximum of the undisturbed velocity profile increases with the decreasing capillary number and increasing modified Marangoni number, which entails more rapid decrease in the critical Reynolds number. It was also shown that the flow is destabilized by the increase in the temperature difference between the wall and the vapor, by the decrease in the absolute pressure and by the increase in the thermal conductivity. To confirm existence of different flow regimes in the nanofluid film, numerical simulations of capillary flow was performed, which exhibit qualitative consistence with the stability theory.

An experimental investigation of a low temperature Al2O3-H2O nanofluid based direct absorption solar collector

The conventional tube-in-plate type flat plate solar collectors have low efficiency and higher heat losses due to surface based solar energy absorption and indirect transfer of heat from hot absorber surface to working fluid flowing through tubes. A full scale direct absorption solar collector having gross area of 1.4m2 and working on volumetric absorption principle was developed to perform experimental study using thin film of Al2O3-H2O nanofluid. Use of nanofluid as working fluid improves the optical and thermo physical properties that result into an increase in the efficiency of the collector. Experimentation was carried using four different volume fractions of 20nm Al2O3 nanoparticles, 0.001%, 0.005%, 0.01% and 0.05%. ASHRAE standard 93-86 was followed for calculation of instantaneous efficiency of solar collector. Improvement in efficiency of solar collector has been recorded in all four cases of using nanofluids in place of water. Instantaneous efficiency enhancement of 22.1%, 39.6%, 24.6% and 18.75% has been observed for 0.001%, 0.005%, 0.01% and 0.05% volume fraction respectively. The experimental results also indicated that collector efficiency peaked at certain volume fraction, and decreased for lower and higher values of volume fraction.

Analytical solutions of single and multi-phase models for the condensation of nanofluid film flow and heat transfer

Classical Nusselt’s condensate falling film theory is extended in this paper to the case when the base fluid is added ingredients of some frequently used popular nanoparticles. The resulting mixture, i.e, nanofluids, is analytically investigated either when the nanoparticles are uniformly distributed across the condensate boundary layer which is the most used model (single phase) in the literature, or when the concentration of nanoparticles through the film is allowed to vary from the wall to the outer edge of the condensate film in the light of modified Buongiorno’s nanofluid model (multi-phase) incorporating mechanisms of the Brownian and thermophoretic diffusion. In both theoretical cases, momentum and energy equations are solved analytically to deduce the flow and heat transport phenomena. As a result, the influences of employed nanofluids on the flow and heat of the condensate film are determined exactly. When the concentration of nanoparticles is assumed constant both models are shown to coincide. Otherwise, effects of nanofluids as compared to the regular fluid on the velocity profiles, the mass flow rate, the thickness of the condensate film and the Nusselt number are easy to conceive from both single and multi-phase models. In particular, the theoretical treatment in both models enables us to understand the heat transfer enhancement feature of the nanofluids models. When the diffusion parameter is increased in the multi-phase model, more enhancement in the rate of heat transfer is observed. In agreement with the experimental evidences, the water-based nanofluid with nanoparticles Ag is the best heat transferring mixture.

Electrowetting of partially wetting thin nanofluid films.

It is observed that the presence of negatively charged, suspended nanoparticles significantly changes the electric-field-induced spreading and contact line dynamics of partially wetting liquid films. Image-analyzing interferometry is used to accurately measure the meniscus profile, including the spatial change in the meniscus curvature. The nanoparticle-containing meniscus exhibits enhanced spreading with an increase in the particle size and weight fraction. The instantaneous contact line velocities are measured using video microscopy and a frame-by-frame analysis of the extracted images. The effects of electric field polarity reversal on the flow toward the contact line are explored as well. The movement of the meniscus is analyzed taking into account the capillary forces and Maxwell-stress-induced flows. An analytical model based on the Young-Laplace equation is used to analyze the electric-field-induced contact line motion, and the model-predicted velocities are compared to the experiments.

The dynamics of capillary-driven two-phase flow: The role of nanofluid structural forces

Capillary-driven flows are fundamental phenomena and are involved in many key technological processes, such as oil recovery through porous rocks, ink-jet printing, the bubble dynamics in a capillary, microfluidic devices and labs on chips. Here, we discuss and propose a model for the oil displacement dynamics from the capillary by the nanofluid (which is composed of a liquid suspension of nanoparticles); we elucidate the physics of the novelty of the phenomenon and its application. The oil displacement by the nanofluid flow is a multi-stage phenomenon, first leading to the oil film formation on the capillary wall, its break-up, and retraction over the capillary wall; this lead to the formation of the oil double concave meniscus. With time, the process repeats itself, leading to the formation of a regular “necklace” of oil droplets inside the capillary. Finally, the oil droplets are separated by the nanofluid film from the capillary wall. The light reflected differential interferometry technique is applied to investigate the nanofluid interactions with the glass wall. We find nanoparticles tend to self-structure into multiple layers close to the solid wall, which cause the structural forces to arise that lead to the oil displacement from the capillary. This research is expected to benefit the understanding of nanofluid phenomena in a capillary and promote their use in technological applications.

Optimizing Vanadium Pentoxide Thin Films and Multilayers from Dip-Coated Nanofluid Precursors

Using an alkoxide-based precursor, a strategy for producing highly uniform thin films and multilayers of V2O5 is demonstrated using dip coating. Defect-free and smooth films of V2O5 on different surfaces can be deposited from liquid precursors. We show how pinholes are formed due to heterogeneous nucleation during hydrolysis as the precursor forms a nanofluid. Using knowledge of instability formation often found in composite nanofluid films and the influence of cluster formation on the stability of these films, we show how polymer-precursor mixtures provide optimum uniformity and very low surface roughness in amorphous V2O5 and also orthorhombic V2O5 after crystallization by heating. Pinhole and roughness instability formation during the liquid stage of the nanofluid on gold and ITO substrates is suppressed giving a uniform coating. Practically, understanding evolution pathways that involve dewetting processes, nucleation, decomposition, or hydrolysis in complex nanofluids provides a route for improved uniformity of thin films. The method could be extended to improve the consistency in sequential or iterative multilayer deposits of a range of liquid precursors for functional materials and coatings.

Dynamic Spreading of Nanofluids on Solids Part II: Modeling

Recent studies on the spreading phenomena of liquid dispersions of nanoparticles (nanofluids) have revealed that the self-layering and two-dimensional structuring of nanoparticles in the three-phase contact region exert structural disjoining pressure, which drives the spreading of nanofluids by forming a continuous wedge film between the liquid (e.g., oil) and solid surface. Motivated by the practical applications of the phenomenon and experimental results reported in Part I of this two-part series, we thoroughly investigated the spreading dynamics of nanofluids against an oil drop on a solid surface. With the Laplace equation as a starting point, the spreading process is modeled by Navier-Stokes equations through the lubrication approach, which considers the structural disjoining pressure, gravity, and van der Waals force. The temporal interface profile and advancing inner contact line velocity of nanofluidic films are analyzed through varying the effective nanoparticle concentration, the outer contact angle, the effective nanoparticle size, and capillary pressure. It is found that a fast and spontaneous advance of the inner contact line movement can be obtained by increasing the nanoparticle concentration, decreasing the nanoparticle size, and/or decreasing the interfacial tension. Once the nanofluidic film is formed, the advancing inner contact line movement reaches a constant velocity, which is independent of the outer contact angle if the interfacial tension is held constant.

Dynamic Spreading of Nanofluids on Solids. Part I: Experimental

Nanofluids have enhanced thermophysical properties compared to fluids without nanoparticles. Recent experiments have clearly shown that the presence of nanoparticles enhances the spreading of nanofluids. We report here the results of our experiments on the spreading of nanofluids comprising 5, 10, and 20 vol % silica suspensions of 19 nm particles displacing a sessile drop placed on a glass surface. The contact line position is observed from both the top and side views simultaneously using an advanced optical technique. It is found that the nanofluid spreads, forming a thin nanofluid film between the oil drop and the solid surface, which is seen as a bright inner contact line distinct from the conventional three-phase outer contact line. For the first time, the rate of the nanofluidic film spreading is experimentally observed as a function of the nanoparticle concentration and the oil drop volume. The speed of the inner contact line is seen to increase with an increase in the nanoparticle concentration and decrease with a decrease in the drop volume, that is, with an increase in the capillary pressure. Interestingly, the formation of the inner contact line is not seen in fluids without nanoparticles.

Nanoparticle Self-Structuring in a Nanofluid Film Spreading on a Solid Surface

Liquids containing nanoparticles (nanofluids) exhibit different spreading or thinning behaviors on solids than liquids without nanoparticles. Previous experiments and theoretical investigations have demonstrated that the spreading of nanofluids on solid surfaces is enhanced compared to the spreading of base fluids without nanoparticles. However, the mechanisms for the observed enhancement in the spreading of nanofluids on solid substrates are not well understood. The complex nature of the interactions between the particles in the nanofluid and with the solid substrate alters the spreading dynamics [Wasan, D. T.; Nikolov, A. D. Nature 2003, 423, 156]. Here, we report, for the first time, the results of an experimental observation of nanoparticles self-structuring in a nanofluid film formed between an oil drop and a solid surface. Using a silica-nanoparticle aqueous suspension (with a nominal diameter of 19 nm and 10 vol %) and reflected light interferometry, we show the nanoparticle layering (i.e., stratification) phenomenon during film thinning on a smooth hydrophilic glass surface. Our experiments revealed that the film thickness stability on a solid substrate depends on the film size (i.e., the drop size). A film formed from a small drop (with a high capillary pressure) is thicker and contains more particle layers than a film formed from a large drop (with a lower capillary pressure). The data for the film-meniscus contact angle verses film thickness (corresponding to the different number of particle layers) were obtained and used to calculate the film structural energy isotherm. These results may provide a better understanding of the complex phenomena involved in the enhanced spreading of nanofluids on solid surfaces.

Predicted Efficiency of a Low-Temperature Nanofluid-Based Direct Absorption Solar Collector

Due to its renewable and nonpolluting nature, solar energy is often used in applications such as electricity generation, thermal heating, and chemical processing. The most cost-effective solar heaters are of the “flat-plate” type, but these suffer from relatively low efficiency and outlet temperatures. The present study theoretically investigates the feasibility of using a nonconcentrating direct absorption solar collector (DAC) and compares its performance with that of a typical flat-plate collector. Here a nanofluid—a mixture of water and aluminum nanoparticles—is used as the absorbing medium. A two-dimensional heat transfer analysis was developed in which direct sunlight was incident on a thin flowing film of nanofluid. The effects of absorption and scattering within the nanofluid were accounted for. In order to evaluate the temperature profile and intensity distribution within the nanofluid, the energy balance equation and heat transport equation were solved numerically. It was observed that the presence of nanoparticles increases the absorption of incident radiation by more than nine times over that of pure water. According to the results obtained from this study, under similar operating conditions, the efficiency of a DAC using nanofluid as the working fluid is found to be up to 10% higher (on an absolute basis) than that of a flat-plate collector. Generally a DAC using nanofluids as the working fluid performs better than a flat-plate collector, however, much better designed flat-plate collectors might be able to match or outperform a nanofluids based DAC under certain conditions.