Controlling the diffusiophoretic motion of colloidal particles with photoreactive walls

Colloidal particles experience directed motion when placed in an imposed solute concentration gradient, a phenomenon known as diffusiophoresis. Recent advances in microfluidic techniques helped uncover many of the fundamental aspects of these out-of-equilibrium, electrokinetic flows, as well as propose a variety of potential applications for them, in particular in the fields of particle separation, concentration, or screening. [1] Microfluidic chips with continuously reactive side walls have been shown to constitute interesting platforms for water purification through diffusiophoretic exclusion, but the stationary nature of the reaction limits their versatility. [2] In this work, we realize reactive walls by embedding photocatalytic particles in a hydrogel matrix. [3] Thus, we can switch on or off the reaction through the shining of UV light, creating gradients of solutes at the walls at will, and controlling the diffusiophoretic motion of the particles nearly instantaneously. We take advantage of this new technique to generate temporally- and spatially-dependent particle concentration profiles in linear and circular channel geometries.

[1]        S. Shin, Phys. Fluids 32, 101302 (2020).

[2]        H. Lee, J. Kim, J. Yang, S. W. Seo, and S. J. Kim, Lab Chip 18, 1713 (2018).

[3]        H.-J. Koo and O. D. Velev, J. Mater. Chem. A 1, 11106 (2013).