Local and global force balance for diffusiophoretic transport

TL;DR

This paper investigates the local and global force balances in diffusiophoresis, revealing that local forces do not vanish despite zero net force, and clarifies osmotic balance debates with analytic predictions for various particle types.

Contribution

It provides a detailed analysis of local force distributions in diffusiophoresis, contrasting with electrophoresis, and offers new insights into osmotic balance and particle deformation.

Findings

Local force on particles does not vanish in diffusiophoresis.

Global force remains zero, consistent with force-free motion.

Analytic predictions for different particle interactions and structures.

Abstract

Electro- and diffusio- phoresis of particles correspond respectively to the transport of particles under electric field and solute concentration gradients. Such interfacial transport phenomena take their origin in a diffuse layer close to the particle surface, and the motion of the particle is force-free. In the case of electrophoresis, it is further expected that the stress acting on the moving particle vanishes locally as a consequence of local electroneutrality. But the argument does not apply to diffusiophoresis, which takes its origin in solute concentration gradients. In this paper we investigate further the local and global force balance on a particle undergoing diffusiophoresis. We calculate the local tension applied on the particle surface and show that, counter-intuitively, the local force on the particle does not vanish for diffusiophoresis, in spite of the global force being zero as expected. Incidentally, our description allows to clarify the osmotic balance in diffusiophoresis, which has been a source of debates in the recent years. We explore various cases, including hard and soft interactions, as well as porous particles, and provide analytic predictions for the local force balance in these various systems. The existence of local stresses may induce deformation of soft particles undergoing diffusiophoresis, hence suggesting applications in terms of particle separation based on capillary diffusiophoresis.