Morphology-induced entropic effects on colloidal adsorption

During the process of colloidal adsorption, colloids (physically or chemically) attach to the surface of a solid. This mechanism plays a crucial role in numerous applications, including water purification, materials synthesis, and catalyst formation. The adhesion of colloids is driven by intermolecular forces between the colloids and the surface, which are predominantly electrostatic. However, entropic forces can also be exploited to selectively induce a specific or preferred distribution of colloids in the vicinity of the surface of the wall. In this work, we report the entropic potential between walls and large particles immersed in a bath of small particles, with a focus on how varying wall geometries influence this entropic potential. Three types of wall geometries are examined: flat, concave, and convex. This wall-particle entropic potential is then used in Monte Carlo computer simulations to perform a more realistic analysis of the particle organization at finite colloidal concentration. For the wall, we particularly explore a rectangular microchannel-like configuration, which is created by the superposition of two step-edge walls separated by a given distance; we have chosen this kind of wall morphology, as it is the most popular configuration in microfluidic experiments. Our findings indicate that the strongest attraction on a colloidal particle occurs when it is near a concave surface. Furthermore, Monte Carlo simulations in a microchannel-like geometry reveal an optimal relationship between the channel gap and the colloid adsorption. More specifically, the adsorption coefficient exhibits a maximum at a well-defined channel gap. These findings are relevant for the design of microfluidic devices, where adsorption can be enhanced and controlled by tuning physical parameters or device geometry, enabling targeted particle localization in regions of high or low particle concentration.