Surface-Sensitive Fractioning of Flowing Colloidal Suspensions Sedimented at a Photochemically Active Wall

A colloidal particle exposed to nonequilibrium inhomogeneities in the chemical-composition of the solution responds by phoresis with a magnitude dictated by its surface properties (adsorption of solutes relative to the solvent) and the strength of the inhomogeneity gradient. Fractioning by surface-properties for a suspension of sedimented colloidal particles flowing within a microfluidic device can then be achieved by exposing it to chemical-composition gradients normal to the wall. One such method is developed by employing a wall imbibed with a photo-sensitive surfactant; under suitable illumination, it undergoes reversible isomerization and turns the wall into a source of chemical inhomogeneities by isomers-exchange with the solution. Proof-of-concept experiments, complemented by theoretical modeling, demonstrate the feasibility of the proposed method in several examples of particles mixtures. The results highlight its potential in terms of high performance, simple technical requirements, and suitability for dealing with equally-sized-but surface different-particles.