High-Resolution Numerical Simulation of Microfiltration of Oil-in-Water Emulsion Permeating through a Realistic Membrane Microporous Structure Generated by Focused Ion Beam Scanning Electron Microscopy Images.

Owing to the limitations of visualization techniques in experimental studies and low-resolution numerical models based on computational fluid dynamics (CFD), the detailed behavior of oil droplets during microfiltration is not well understood. Hence, a high-resolution CFD model based on an in-house direct numerical simulation (DNS) code was constructed in this study to analyze the detailed dynamics of an oil-in-water (O/W) emulsion using a microfiltration membrane. The realistic microporous structure of commercial ceramic microfiltration membranes (mullite and α-alumina membranes) was obtained using an image processing technique based on focused ion beam scanning electron microscopy (FIB-SEM). Numerical simulations of microfiltration of O/W emulsions on the membrane microstructure obtained by FIB-SEM were performed, and the effects of different parameters, including contact angle, transmembrane pressure, and membrane microporous structure, on filtration performance were studied. Droplet deformation had a strong impact on filtration behavior because coalesced droplets with diameters larger than the pore diameter permeated the membrane pores. The permeability, oil hold-up fraction inside the pores, and rejection were considerably influenced by the contact angle, while the transmembrane pressure had a little impact on the permeability and oil hold-up fraction. The membrane structure, especially the pore size distribution, also had a significant effect on the microfiltration behavior and performance.

Effect of liquid volume fraction and shear rate on rheological properties and microstructure formation in ternary particle/oil/water dispersion systems under shear flow: two-dimensional direct numerical simulation.

We numerically studied the rheological properties and microstructure formation under shear flow in a ternary particle/oil/water dispersion system. Our numerical simulation method was based on a phase-field model for capturing a free interface, the discrete element method for tracking particle motion, the immersed boundary method for calculating fluid-particle interactions, and a wetting model that assigns an order parameter to the solid surface according to the wettability. The effects of the water-phase volume fraction and shear rate on the microstructure and apparent viscosity were investigated. When the water-phase volume fraction was low, a pendular state was formed, and with an increase in the water-phase volume fraction, the state transitioned into a co-continuous state and a Pickering emulsion. This change in the microstructure state is qualitatively consistent with the results of previous experimental studies. In the pendular state, the viscosity increased with an increase in the water-phase volume fraction. This was due to the development of a network structure connected by liquid bridges, and the increase in the coordination number was quantitatively confirmed. In the case of the pendular state, significant shear thinning was observed, but in the case of the Pickering emulsion, no significant shear thinning was observed. It is concluded that this is due to the difference in the manner in which the microstructure changes with the shear rate. This is the first study to numerically demonstrate the microstructure formation of a ternary dispersion under shear flow and its correlation with the apparent viscosity.

In situ observation of meniscus shape deformation with colloidal stripe pattern formation in convective self-assembly.

Vertical convective self-assembly is capable of fabricating stripe-patterned structures of colloidal particles with well-ordered periodicity. To unveil the mechanism of the stripe pattern formation, in the present study, we focus on the meniscus shape and conduct in situ observations of shape deformation associated with particulate line evolution. The results reveal that the meniscus is elongated downward in a concave fashion toward the substrate in accordance with solvent evaporation, while the concave deformation is accelerated by solvent flow, resulting in the rupture of the liquid film at the thinnest point of the meniscus. The meniscus rupture triggers the meniscus to slide off from the particulate line, followed by the propagation of the sliding motion of the three-phase contact line, resulting in the formation of stripe spacing.

Permeation of concentrated oil-in-water emulsions through a membrane pore: numerical simulation using a coupled level set and the volume-of-fluid method.

In this paper, we investigated the demulsification behavior of oil-in-water (O/W) emulsions during membrane permeation in the oil-water separation process using a numerical simulation approach. To accurately deal with the large deformation of the oil-water interface by coalescence and wetting, and to estimate the volume of the coalesced oil droplet, the coupled level set and volume-of-fluid method was used as the interface capturing method. We applied the simulation model to the permeation of O/W emulsions through a membrane pore, and then investigated the effects of the wettability of the membrane surface, filtration flux, and pore size on the demulsification efficiency. The results showed that oil droplets were likely to coalesce on the outlet membrane surface. High wettability on the membrane surface and low fluid velocity inside the pore increased the demulsification efficiency. This is the first work to numerically simulate the demulsification behavior of emulsions through membranes in the oil-water separation process.

Spontaneous formation of cluster array of gold particles by convective self-assembly.

Cluster arrays composed of metal nanoparticles are promising for application in sensing devices because of their interesting surface plasmon characteristics. Herein, we report the spontaneous formation of cluster arrays of gold colloids on flat substrates by vertical-deposition convective self-assembly. In this technique, under controlled temperature, a hydrophilic substrate is vertically immersed in a colloid suspension. Cluster arrays form when the particle concentration is extremely low (in the order of 10(-6)-10(-8) v/v). These arrays are arranged in a hierarchically ordered structure, where the particles form clusters that are deposited at a certain separation distance from each other, to form "dotted" lines that are in turn aligned with a constant spacing. The size of the cluster can be controlled by varying the particle concentration and temperature while an equal separation distance is maintained between the lines formed by the clusters. Our technique thus demonstrates a one-step, template-free fabrication method for cluster arrays. In addition, through the direct observation of the assembly process, the spacing between the dotted lines is found to result from the "stick-and-slip" behavior of the meniscus tip, which is entirely different from the formation processes observed for the striped patterns, which we reported previously at higher particle concentrations. The difference in the meniscus behavior possibly comes from the difference in colloidal morphology at the meniscus tip. These results demonstrate the self-regulating characteristics of the convective self-assembly process to produce colloidal patterns, whose structure depends on particle concentration and temperature.

Lattice Boltzmann model for capillary interactions between particles at a liquid-vapor interface under gravity.

A computational technique based on the lattice Boltzmann method (LBM) is developed to simulate the wettable particles adsorbed to a liquid-vapor interface under gravity. The proposed technique combines the improved smoothed-profile LBM for the treatment of moving solid particles in a fluid and the free-energy LBM for the description of a liquid-vapor system. Five benchmark two-dimensional problems are examined: (A) a stationary liquid drop in the vapor phase; a wettable particle adsorbed to a liquid-vapor interface in (B) the absence and (C) the presence of gravity; (D) two freely moving particles at a liquid-vapor interface in the presence of gravity (i.e., capillary flotation forces); and (E) two vertically constrained particles at a liquid-vapor interface (i.e., capillary immersion forces). The simulation results are in good quantitative agreement with theoretical estimations, demonstrating that the proposed technique can reproduce the capillary interactions between wettable particles at a liquid-vapor interface under gravity.

Fabrication of colloidal grid network by two-step convective self-assembly.

We explored a "template-free" approach to arranging colloidal particles into a network pattern by a convective self-assembly technique. In this approach, which we call "two-step convective self-assembly," a stripe pattern of colloidal particles is first prepared on a substrate by immersing it in a suspension. The substrate with the stripes is then rotated by 90° and again immersed in the suspension to produce stripes perpendicular to the first ones, resulting in a grid-pattern network of colloidal arrays. The width of the colloidal grid lines can be controlled by changing the particle concentration while maintaining an almost constant spacing between the lines. On the basis of these results, we propose a mechanism for grid pattern formation. Our method is applicable to various types of particles. In addition, the wide applicability of this method was employed to create a hybrid grid pattern.

Simulation on Pore Formation from Polymer Solution at Surface in Contact with Solid Substrate via Thermally Induced Phase Separation.

The formation of porous structures from polymer solutions at the surface in contact with various solid surfaces via a thermally-induced phase separation (TIPS) process is investigated. The pore formation process at the bulk and the surface of the poly(methyl methacrylate)/cyclohexanol solution is simulated with a model based on the phase field method. When the compatibilities between the polymer-rich phase formed by the phase separation and the solid surface are high or low, surface porosity decreases. In contrast, for the solid surface having similar compatibilities with the polymer and solvent, high surface porosity is achieved. This indicates that the compatibility between the solid surface and polymer solution is important, and that optimal compatibility results in high surface porosity. The knowledge obtained in this work is useful to design the coagulation bath component in the membrane preparation process by TIPS for achieving high surface porosity.

Lattice Boltzmann method for simulation of wettable particles at a fluid-fluid interface under gravity.

A computational technique was developed to simulate wettable particles trapped at a fluid-fluid interface under gravity. The proposed technique combines the improved smoothed profile-lattice Boltzmann method (iSP-LBM) for the treatment of moving solid-fluid boundaries and the free-energy LBM for the description of isodensity immiscible two-phase flows. We considered five benchmark problems in two-dimensional systems, including a stationary drop, a wettable particle trapped at a fluid-fluid interface in the absence or presence of gravity, two freely moving particles at a fluid-fluid interface in the presence of gravity (i.e., capillary floatation forces), and two vertically constrained particles at a fluid-fluid interface (i.e., capillary immersion forces). The simulation results agreed well with theoretical estimations, demonstrating the efficacy of the proposed technique.

Effect of internal mass in the lattice Boltzmann simulation of moving solid bodies by the smoothed-profile method.

A computational method for the simulation of particulate flows that can efficiently treat the particle-fluid boundary in systems containing many particles was developed based on the smoothed-profile lattice Boltzmann method (SPLBM). In our proposed method, which we call the improved SPLBM (iSPLBM), for an accurate and stable simulation of particulate flows, the hydrodynamic force on a moving solid particle is exactly formulated with consideration of the effect of internal fluid mass. To validate the accuracy and stability of iSPLBM, we conducted numerical simulations of several particulate flow systems and compared our results with those of other simulations and some experiments. In addition, we performed simulations on flotation of many lightweight particles with a wide range of particle size distribution, the results of which demonstrated the effectiveness of iSPLBM. Our proposed model is a promising method to accurately and stably simulate extensive particulate flows.

Formation of regular stripes of chemically converted graphene on hydrophilic substrates.

Chemically converted graphene (CCG), from a chemistry point of view, is a giant molecule with a unique two-dimensional (2D) configuration. The availability of CCG dispersion provides a range of scalable methods to assemble graphene-based materials but brings the challenge of understanding and control of the CCG morphology in solution processing. In this study, we found that, similar to conventional colloidal systems (e.g., spherical particles or polymers), a 2D sheet of CCG can be transferred from its aqueous dispersion to solid substrates in the form of highly regular stripe patterns by evaporation-driven deposition. The width and spacing can be defined by the concentration of the CCG dispersion and the properties of the substrate (e.g., roughness and surface charge). Furthermore, the high resolution AFM images illustrate that both 2D flattened and highly wrinkled CCG can be formed in each individual stripe, depending on the location across the stripe. The in situ optical observation of the stripe formation indicates that the morphological change of CCG may occur in the crowded meniscus of the drying front.

Colloidal stripe pattern with controlled periodicity by convective self-assembly with liquid-level manipulation.

We describe a template-free technique for arranging colloidal particles into a stripe pattern on a large scale. A simple liquid-level manipulation system was incorporated into the vertical-deposition convective self-assembly (CSA) technique. By periodically pumping a colloidal dispersion out of or into a reservoir to manipulate the liquid level, we successfully fabricated stripe patterns with various periodicities (i.e., line widths and spacings) that are unachievable with the normal CSA technique. We developed a simple model to predict the periodicity of the resultant colloidal stripes that enables the tailored fabrication of colloidal stripes with the desirable periodicity for a practical application. This technique has the advantages of versatility and scalability. By combining this technique with the two-step CSA technique (Mino et al., Langmuir2011, 27(9), 5290-5295), we fabricated a large-sized colloidal grid network pattern of silver nanoparticles.

Calculating the gravitational self-force in Schwarzschild spacetime.

We present a practical method for calculating the local gravitational self-force (often called "radiation-reaction force") for a pointlike particle orbiting a Schwarzschild black hole. This is an implementation of the method of mode-sum regularization, in which one first calculates the (finite) contribution to the force due to each individual multipole mode of the perturbation, and then applies a certain regularization procedure to the mode sum. Here we give the values of all the "regularization parameters" required for implementing this regularization procedure, for any geodesic orbit in Schwarzschild spacetime.