arXiv subjects

Kapil Upadhyaya

Publications and source records attributed to Kapil Upadhyaya.

2 recordsLinked to original sources

arXiv2026

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

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.

cond-mat.soft↗

arXiv2025

On the influence of electrolytic gradient orientation on phoretic transport in dead-end pores

Electrolytic diffusiophoresis refers to directional migration of colloids due to interfacial forces that develop in response to local electrolytic concentration ($c$) gradients. This physicochemical transport provides an efficient alternative in numerous microscale applications where advection-induced transport is infeasible. Phoretic withdrawal and injection in dead-end pores can be controlled by orienting salt gradients into or out of the pore; however, the extent to which this orientation influences spatiotemporal transport patterns is not thoroughly explored. In this study, we find that it has a significant influence: colloidal withdrawal in solute-out mode ($β=c_\infty/c_{\text{pore}}<1$) is faster and shallower, whereas the solute-in mode enables deeper withdrawal. Similarly, solute-out injection features rapidly propagating wavefronts, whereas the solute-in mode ($β>1$) promotes uniform and gradual injection. Each mode's transport is found to evolve and persist over different time scales. We characterize the performance of these modes and find that while persistence of the solute-out mode strengthens with a growing electrolytic gradient [$\sim \ln(β^{-0.4})$], solute-in mode diminishes and eventually its persistence is insensitive to $β$. We also incorporate the variable mobility model to examine the impact of large zeta potentials, which intensifies the transport of solute-out mode further and weakens the solute-in mode. Additionally, we investigate how osmotic flows of the two modes affect injection and withdrawal patterns. We find that osmosis-induced mixing can counterintuitively inhibit injection effectiveness in solute-out mode. These insights bring attention to the distinctions between different phoretic transport modes and contribute to the rational design and setup of electrolytic gradients in numerous microscale applications.

cond-mat.soft↗