Molecular dynamics simulation of phase separating binary liquids in cylindrical Couette flow

Abstract

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Spinodal decomposition of asymmetric binary fluids in a micro-Couette geometry simulated with molecular dynamics

“…At the polymer-solvent interface, for r Ϸ 52 , the velocity profile shows a steep drop, indicative of a significant degree of slippage. Rod-solvent and even solvent-solvent 34 systems show similar hallmarks of slippage at their interfaces. It might be argued that the partial nonstick boundary condition is exacerbated by the purely repulsive interactions between unlike particles, which weakens the mutual grip of the two fluids.…”

“…Theoretical expressions for the interfacial shape in a Couette cell have been derived in Ref. 34, where it was found that the surface tensions consistently enter the expressions in the form ⌫ = ͑␥ AW − ␥ BW ͒ / ␥ AB , with A, B, and W denoting the two fluids and the wall, respectively. For a -stack, for instance, the interface is described by…”

“…These corrections are readily implemented in a Cartesiancoordinate based simulation code, 34 the only minor complication is that any calculation involving a boundary crossing term now involves a rotation over Ϯ⌰ t around the central axis of the Couette cell. Regular periodic boundary conditions apply along the axial direction.…”

“…Several factors may contribute to the deviations, which are more abundant here than for previous simulations of a fluid-fluid mixture in a micro-Couette cell. 34 The assumption that the surface tensions dominate the free energy, ignoring all other contributions, is approximate by nature, and bending contributions to the interfacial free energy are neglected. The surface tensions have been calculated for flat interfaces and are perhaps less suited for the current curved interfaces, as exemplified by the interfacial profile in Fig.…”

“…30,31 Deviations from the ideal case also occur in the presence of walls and other fixed obstacles, which limit the attainable domain size, interfere with the buildup of a hydrodynamic flow field, and may have a preference for wetting by either constituent of the mixture. [32][33][34] Another interesting spinodal decomposition process has been reported for rodlike colloids, where the nematic ͑dis͒ordering enslaves the spatial ordering into dense and less dense phases. 35,36 Experimental investigations of spinodal decomposition are traditionally aimed at studying one of the above decomposition processes by specifically designing the equipment and conditions such as to eliminate or minimize perturbing side effects.…”