Improving the surface chemistry-based separation of microparticles - a modified gravitational field flow...

The separation of particles by characteristics such as size, morphology, or material identity is critical in applications such as particle separation, bioanalytics, and particle characterization. However, the separation of particles based solely on subtle differences in surface properties or surface/bulk morphology remains still a complex challenge up to now. Modern methods target the surface properties via induced chemical activity, for instances in combination with microfluidics and light induced local self-phoresis/osmosis. This technique generates upon light illumination unique particle drift motions depending on surface property difference between two distinct particle fractions, where the separation is possible by elution times differences, akin to chromatographic and field flow techniques. In this paper we discuss the three essential key parameters - flow rate, intensity and wavelength of applied light illumination - for maximizing the velocity difference between the two distinct particle fractions, i.e. improving the separation performance via elution time principle suitable for microparticles of equal size but differences in surface property (e.g. porosity, surface functionalization, particle material). We show that the increase in flow rate always increases the velocity difference, where intensity and wavelength of applied light illumination is a complex behavior of light induced activity of one particle relative to the other. This complex behavior arises from a constant flux of expelled photo surfactant isomers combined with the superposition of a particle relative to the bottom wall. While the flux remains steady and scales linearly with the light intensity and applied wavelength, the resulting upstream particle velocity decreases as the levitation height increases. This reduction occurs because the growing distance from the reflective boundary diminishes until the upstream velocity is completely counterbalanced by the sedimentation velocity. Furthermore, changes in applied wavelength changes simultaneously multiple parameters yielding into a peak activity (∼highest levitation) at an optimum wavelength and thus into optimum separation performance.