Controlling the Motion of Interfaces in Capillary Channels with Non-uniform Surface Wettability

Abstract

The use of self-driven flows in microfluidic devices attracts many researchers as the external flow-driving mechanism is diminished or eliminated. One of the mechanisms providing such motions is generating a pressure difference across interfaces as in the case of the motion in capillary tubes. The capillarity, namely, the pressure difference across the interface due to its curvature drives the motion. This pressure depends on the interaction with the capillary walls and is controlled if one varies the surface energy of the walls. In this study, we search for the effects of surface energy on the motion of interfaces in capillary-driven flow. To this end, we model the motion of fluid particles in a capillary channel and integrate the governing equations using the binary lattice Boltzmann method for the two-phase flow. We, first, validate our solver for canonical static and dynamic problems. We, then, discuss two main contributions; we show how to deviate the interface speed from the ones moving in channels with uniform wall energies and discuss the conditions under which such an interface stagnates (like a passive valve in a channel). Tuning the wettability of the channel walls, we provide a simple condition for stopping the interface: the summation of the equilibrium contact angles interface make with the channel walls at the bottom and top wall need to satisfy $\theta_{eq}^{top}+\theta_{eq}^{bot} \geq \pi$. Configurations and wetting properties of different wettability regions play major roles together