Colloid recovery from porous structures under ambient flow: enhanced extraction via phoretic and osmotic mechanisms

Abstract

Chemical gradients are widely employed to enhance particle transport in porous media, such as laundry detergency and enhanced oil recovery. Diffusiophoresis and diffusioosmosis refer to the movement of colloid and movement of near-surface fluid in response to electrolyte gradients, respectively. These mechanisms play a crucial role in colloid and drug transport in constricted regions where bulk transport is infeasible. Our earlier work [Tiwari et al., Langmuir 41, 18583 (2025)] has shown that phoretic and osmotic transport in dead-end micro-pores can be controlled by orienting salt gradients into or out of the pores; however, the extent to which this orientation influences large-scale spatiotemporal patterns and colloid extraction is not thoroughly explored. In this work, we study the phoretic and osmotic colloidal extraction from porous structure exposed to an ambient flow. We characterize the impact of solute gradient orientation, such as solute-out (i.e., solute-emitting porous media) and solute-in (i.e., solute-consuming media) modes. The two-dimensional porous structure is made of a number of pillars/fibers arranged in a lattice ordered hexagonal packing with equal spacing. The results from finite-element simulations show that phoretic colloidal extraction exhibits a qualitatively distinct behavior in the two modes: in the solute-out mode, colloids are extracted from the peripheral region of the porous structure, whereas in the solute-in mode, extraction predominantly occurs from the stagnant core. Diffusioosmotic slip on the internal surface of pillars/fibres further amplifies extraction in both modes, with a relatively larger enhancement in the solute-in mode due to internal spatiotemporal flow patterns. Beyond demonstrating the sensitivity of osmotic transport in porous media, these insights can guide enhanced membrane filtration, laundry detergency, and enhanced oil recovery.