Experimental Study on Capillary Microflows in High Porosity Open-Cell Metal Foams.

Metal foams have been widely used in heat pipes as wicking materials. The main issue with metal foams is the surface property capillary limit. In this paper, a chemical blackening process for creating a superhydrophilic surface on copper foams is studied with seven different NaOH and NaClO2 solution concentrations (1.5~4.5 mol/L), in which the microscopic morphology of the treated copper foam surface is analyzed by scanning electron microscopy. The capillary experiments are carried out to quantify the wicking characteristics of the treated copper foams and the results are compared with theoretical models. A the microscope is used to detect the flow stratification characteristics of the capillary rise process. The results show that the best wicking ability is obtained for the oxidation of copper foam using 3.5 mol/L of NaOH and NaClO2 solution. Gravity plays a major role in defining the permeability and effective pore radius, while the effect of evaporation can be ignored. The formation of a fluid stratified interface between the unsaturated and saturated zone results in capillary performance degradation. The current study is important for understanding the flow transport in porous materials.

Review and a Theoretical Approach on Pressure Drop Correlations of Flow through Open-Cell Metal Foam.

Due to their high porosity, high stiffness, light weight, large surface area-to-volume ratio, and excellent thermal properties, open-cell metal foams have been applied in a wide range of sectors and industries, including the energy, transportation, aviation, biomedical, and defense industries. Understanding the flow characteristics and pressure drop of the fluid flow in open-cell metal foams is critical for applying such materials in these scenarios. However, the state-of-the-art pressure drop correlations for open-cell foams show large deviations from experimental data. In this paper, the fundamental governing equations of fluid flow through open-cell metal foams and the determination of different foam geometry structures are first presented. A variety of published models for predicting the pressure drop through open-cell metal foams are then summarized and validated against experimental data. Finally, two empirical correlations of permeability are developed and recommended based on the model of Calmidi. Moreover, Calmidi's model is proposed to calculate the Forchheimer coefficient. These three equations together allow calculating the pressure drop through open-cell metal foam as a function of porosity and pore diameter (or strut diameter) in a wide range of porosities ε = 85.7-97.8% and pore densities of 10-100 PPI. The findings of this study greatly advance our understanding of the flow characteristics through open-cell metal foam and provide important guidance for the design of open-cell metal foam materials for different engineering applications.

Synthesis and Characterization of Molten Salt Nanofluids for Thermal Energy Storage Application in Concentrated Solar Power Plants-Mechanistic Understanding of Specific Heat Capacity Enhancement.

Molten salts mixed with nanoparticles have been shown as a promising candidate as the thermal energy storage (TES) material in concentrated solar power (CSP) plants. However, the conventional method used to prepare molten salt nanofluid suffers from a high material cost, intensive energy use, and laborious process. In this study, solar salt-Al2O3 nanofluids at three different concentrations are prepared by a one-step method in which the oxide nanoparticles are generated in the salt melt directly from precursors. The morphologies of the obtained nanomaterials are examined under scanning electron microscopy and the specific heat capacities are measured using the temperature history (T-history) method. A non-linear enhancement in the specific heat capacity of molten salt nanofluid is observed from the thermal characterization at a nanoparticle mass concentration of 0.5%, 1.0%, and 1.5%. In particular, a maximum enhancement of 38.7% in specific heat is found for the nanofluid sample prepared with a target nanoparticle mass fraction of 1.0%. Such an enhancement trend is attributed to the formation of secondary nanostructure between the alumina nanoparticles in the molten salt matrix following a locally-dispersed-parcel pattern. These findings provide new insights to understanding the enhanced energy storage capacity of molten salt nanofluids.

Analysis of Capillary Flow in a Parallel Microchannel-Based Wick Structure with Circular and Noncircular Geometries.

Capillary flow in porous media is of great significance to many different applications including microfluidics, chromatography, and passive thermal management. For example, heat pipe has been widely used in the thermal management of electronic system due to its high flexibility and low thermal resistance. However, the critical heat flux of heat pipe is often limited by the maximum capillary-driven liquid transport rate through the wicking material. A significant number of novel porous material with complex structures have been proposed in past studies to provide enhanced capillary-driven flow without substantial reduction in pore size and porosity. However, the increasing level of structural complexity often leads to a more tortuous flow path, which deprives the merits of enhanced capillarity. In this study, we examined the capillary performance of a porous material with simple geometric structures both analytically and numerically. Specifically, the capillary rate of rise of water in parallel hollow microchannels with different cross-sectional shapes is derived by solving the momentum transport equation. The relationships between the capillary flow rate and wicking height are further validated by two-phase flow simulation based on the conservative level-set method. The results demonstrate that parallel microchannel configuration, despite its geometric simplicity, provides superior capillary performance than most existing porous media in terms of both capillary flow rate and ultimate wicking height. In addition, design of noncircular cross section reduces the viscous drag and increases the packing density of the microchannels in the bulk solid without affecting the capillary pumping pressure. These features contribute to a further enhancement in the capillary performance by up to 32%. These results provide important guidance to the rational design of porous material with enhanced fluid transport property in a variety of microfluidic systems.

Evolution of Microdroplet Morphology Confined on Asymmetric Micropillar Structures.

The design of topological features to control the spreading of liquid has been widely investigated. Micropillar structures, for example, can retain stable droplets on the tip by inhibiting the contact line from advancing over a sharp solid edge. The pinning behavior of droplets on noncircular pillars, however, has received little attention. In this study, we analyze the retention of microdroplets with high and low surface tensions on axisymmetric and asymmetric porous micropillar structures. Circular, square, and triangular structures fabricated on silicon substrates are used to characterize the dynamic behavior of droplets before and after bursting. The critical pinning conditions are based on the visualization and pressure measurements of droplets. A theoretical model is developed based on a free energy analysis for predicting the change in pressure as the working fluid advances on the micropillar. For high surface tension liquids (e.g., water), the maximum pressure occurs when the contact line is pinned along the edge of the inner pore. For low surface tension liquids (e.g., Isopropanol and Novec 7500), the maximum pressure occurs when the contact line is pinned along the outer edge of the structure. The theoretical and experimental results demonstrate how a droplet pinned atop a triangular micropillar exhibits the smallest critical volume at the bursting moment. When using IPA solution (γ = 23 mN/m) and Novec 7500 (γ = 16 mN/m) as the working fluids, a change in the micropillar shape from circle to triangle, respectively, yields a 83% and 76% reduction in the critical burst volume. Meanwhile, the bursting pressure increases from 172 to 300 Pa and from 127 to 216 Pa for IPA and Novec 7500, respectively. These findings provide new insights to the rational design of surface micro/nanoengineered structures for tuning the surface wetting characteristics in scientific and engineering applications.

Investigation of the confinement effect on the evaporation behavior of a droplet pinned on a micropillar structure.

Evaporation of sessile droplet suffers from reduced evaporation rate due to the confinement of vapor diffusion imposed by the bottom substrate. However, it is possible to change the evaporation behavior of a droplet by suspending it from the bottom substrate, in particular, supporting the droplet on a micropillar. This is expected to enable diffusion transport in the downward direction that will subsequently enhance evaporative transport. In this study, we investigate the diffusion confinement effect imposed by the bottom substrate and the side wall of the micropillar through numerical simulations and experimental investigation. The approximate solutions for total evaporation rate and local evaporative flux were subsequently derived from the total evaporation rate predicted by the simulation results. The simulation results, agreeing within 5% with the experimental measurements, show that increasing the micropillar height enhances the total evaporation rate from the suspended hemispherical droplet. This enhancement is due to a dramatic improvement of the local evaporation rate near the contact line region as micropillar heights increase. The micropillar heights examined for maximum evaporation rates were observed under substrate temperatures from 60-98 °C. The increasing pillar height leads to smaller vapor diffusion resistance but greater conduction resistance.

Porous micropillar structures for retaining low surface tension liquids.

The ability to manipulate fluid interfaces, e.g., to retain liquid behind or within porous structures, can be beneficial in multiple applications, including microfluidics, biochemical analysis, and the thermal management of electronic systems. While there are a variety of strategies for controlling the disposition of liquid water via capillarity, such as the use of chemically modified porous adhesive structures and capillary stop valves or surface geometric features, methods that work well for low surface tension liquids are far more difficult to implement. This study demonstrates the microfabrication of a silicon membrane that can retain exceptionally low surface tension fluorinated liquids against a significant pressure difference across the membrane via an array of porous micropillar structures. The membrane uses capillary forces along the triple phase contact line to maintain stable liquid menisci that yield positive working Laplace pressures. The micropillars have inner diameters and thicknesses of 1.5-3 µm and ∼1 µm, respectively, sustaining Laplace pressures up to 39 kPa for water and 9 kPa for Fluorinert™ (FC-40). A theoretical model for predicting the change in pressure as the liquid advances along the porous micropillar structure is derived based on a free energy analysis of the liquid meniscus with capped spherical geometry. The theoretical prediction was found to overestimate the burst pressure compared with the experimental measurements. To elucidate this deviation, transient numerical simulations based on the Volume of Fluid (VOF) were performed to explore the liquid pressure and evolution of meniscus shape under different flow rates (i.e., Capillary numbers). The results from VOF simulations reveal strong dynamic effects where the anisotropic expansion of liquid along the outer micropillar edge leads to an irregular meniscus shape before the liquid spills along the micropillar edge. These findings suggest that the analytical prediction of burst Laplace pressure obtained under quasi-static condition (i.e., equilibrium thermodynamic analysis under low capillary number) is not applicable to highly dynamic flow conditions, where the liquid meniscus shape deformation by flow perturbation cannot be restored by surface tension force instantaneously. Therefore, the critical burst pressure is dependent on the liquid velocity and viscosity under dynamic flow conditions. A numerical simulation using Surface Evolver also predicts that surface defects along the outer micropillar edge can yield up to 50% lower Laplace pressures than those predicted with ideal feature geometries. The liquid retention strategy developed here can facilitate the routing and phase management of dielectric working fluids for application in heat exchangers. Further improvements in the retention performance can be realized by optimizing the fabrication process to reduce surface defects.