Diffusiophoretic migration of colloidal particles in sucrose gradients

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 $μ$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.

💡 Research Summary

This paper presents a comprehensive experimental and theoretical investigation of diffusiophoretic (DP) transport of colloidal particles driven by concentration gradients of sucrose, a neutral molecular solute, at relatively high concentrations (≈ 1 mol L⁻¹). The authors designed a microfluidic chip containing an array of dead‑end pores (length ≈ 1 mm, width ≈ 50 µm, height ≈ 9 µm for particle experiments) connected to a wider main channel. By initially filling the dead‑end channels with water and dispersed polystyrene (PS) particles and then imposing a step change to a sucrose solution in the main channel, a time‑dependent sucrose gradient develops inside the dead‑end pores while net flow in the pores remains essentially zero. This geometry minimizes hydrodynamic coupling and allows the pure effect of the solute gradient on particle motion to be observed.

Physical properties of the sucrose solutions were carefully characterized. The specific volume varies linearly with mass fraction, indicating negligible volume change on mixing. Density follows ρ = ρ_w(1 + βC) with β ≈ 0.133 L mol⁻¹. Viscosity increases three‑fold at 1 mol L⁻¹ (η ≈ 3.2 η_w). Importantly, the osmotic pressure deviates strongly from the van’t Hoff law; an empirical fit Π = RT C(1 + εC) with ε ≈ 0.4 L mol⁻¹ reproduces literature data. Raman confocal microscopy was used to map the sucrose concentration field in real time; the field obeys the diffusion equation ∂C/∂t = ∇·