Probing fast kinetics in complex fluids by combined rapid mixing and small-angle X-ray scattering

Stopped-flow mixing coupled to small-angle X-ray scattering (SAXS) is an established technique for investigating structural kinetics in solution down to the millisecond range. More recently, the emphasis has shifted to the sub-millisecond range using continuous flow microfluidic mixing devices. The aim of this article is to review the present status and limitations when applying mixing techniques to a wide range of soft matter and biological systems. In the case of SAXS, special consideration of the mixing quality is necessary for a quantitative description of the scattered intensity. This is demonstrated through two representative examples involving protein refolding and micellar self-assembly.