Controlling the Motion of Capillary Driven Interfaces in Channels with Chemical Heterogeneity

The use of self-driven fluids (e.g., droplets, capillary flows) attracts many researchers as the external driving mechanisms are diminished or eliminated. The contact angle hysteresis generates a driving force (a pressure difference across interfaces). This pressure depends on the interaction with the solid substrates and is controlled if one varies the surface energy of the walls. Self-transport and manipulation of interfaces play an important role in the development of microfluidic devices, self-cleaning, water harvesting and heat transfer enhancement. In this study, we search for the effects of surface energy on the motion of interfaces. To this end, we model the motion of fluid particles and integrate the governing equations using the D2Q9 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; The first one is, for capillary driven flows, we show how to deviate the interface speed from the ones moving in channels with uniform wall energies, the conditions under which the interface stagnates (like a passive valve in a channel). Tuning the wettability of the channel walls, we provide a simple criteria 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 𝜃𝑒𝑞 𝑏𝑜𝑡 + 𝜃𝑒𝑞 𝑡𝑜𝑝 ≥ 𝜋. The second contribution is that, by varying the surface energy and fluid viscosities, we systematically study the behavior of single droplets on surfaces, their merging mechanism and equilibrium shapes and motions within confinements.