X-ray photon correlation spectroscopy under flow

TL;DR

This study demonstrates how X-ray photon correlation spectroscopy combined with microfluidics can measure colloidal particle diffusion in flow, revealing geometry-dependent effects of shear flow on relaxation times.

Contribution

It introduces a method to measure Brownian diffusion in flowing colloidal suspensions using X-ray photon correlation spectroscopy with microfluidics, highlighting geometry effects.

Findings

Relaxation times are flow-independent in transverse geometry at low shear.

Flow significantly affects relaxation times in longitudinal geometry.

Higher q values facilitate easier measurement of diffusion in flow.

Abstract

X-ray photon correlation spectroscopy was used to probe the diffusive dynamics of colloidal particles in a shear flow. Combining X-ray techniques with microfluidics is an experimental strategy that reduces the risk of x-ray induced beam damage and also allows time-resolved studies of processes taking place in flowcells. The experimental results and theoretical predictions presented here, show that in the low shear limit, for a ``transverse flow'' scattering geometry (scattering wave vector q perpendicular to the direction of flow) the measured relaxation times are independent of the flow rate and determined only by the diffusive motion of the particles. This is not generally valid and in particular, for a ``longitudinal flow'' (q || flow) scattering geometry, the relaxation times are strongly affected by the flow-induced motion of the particles. Our results show that the Brownian diffusion of colloidal particles can be measured in a flowing sample and that, up to flux limitations, the experimental conditions under which this is possible are easier to achieve at higher values of q.