Diffusiophoretic migration of colloidal particles in sucrose gradients

TL;DR

This study experimentally investigates diffusiophoresis of colloidal particles driven by sucrose concentration gradients in water, revealing significant particle migration and proposing a steric exclusion model to explain the observed velocities.

Contribution

It provides the first detailed experimental analysis of diffusiophoresis with molecular neutral solutes at high concentrations and introduces a steric exclusion model to explain particle migration.

Findings

Particles migrate toward low sucrose concentration regions at velocities up to a few μm/s.

A diffusion model with concentration-dependent interdiffusion coefficient describes sucrose profiles.

A steric exclusion length of approximately 5 Å explains the diffusiophoretic behavior.

Abstract

Diffusiophoresis (DP) refers to the migration of particles driven by a solute concentration gradient in a liquid. Observations in the case of molecular neutral solutes are rather scarce, due to the low drift velocities in dilute solutions, and the difficulty in distinguishing DP from other phenomena in concentrated solutions. We investigated experimentally DP of dispersed colloids driven by concentration gradients of sucrose in water at relatively high concentrations, $C \simeq 1$ mol L$^{-1}$. More precisely, we designed a microfluidic chip to impose a time-dependent sucrose gradient in dead-end microchannels with minimized parasitic flows. Significant migration of the particles toward the regions of low sucrose concentration has been observed, with velocities up to a few $\mu$m s$^{-1}$. Particle tracking and Raman confocal spectroscopy were used to measure individual trajectories and the unsteady sucrose concentration profile respectively. The latter is correctly described by a diffusion equation, but with an interdiffusion coefficient that significantly depends on $C$ in the range of concentrations investigated. We then showed that a model of DP based on a steric exclusion of sucrose molecules from the particle surface with an exclusion length $R_i = 5 \pm 0.9$ angstrom (close to the characteristic size of the sucrose molecule), accounts for the observed trajectories. Possible sources for the observed scattering of our experimental data are finally discussed: Brownian motion and advection of the particles by bulk flows driven by diffusioosmosis at the channel walls and buoyancy.