Liquid states in horizontal plate-inserted slits

Removing liquid from a channel is an important process. In a horizontal slit in the presence of a downward gravity field, two distinct liquid states were commonly observed: gravity-driven liquid non-occlusion and liquid plug (Parry et al. 2012 Phys. Rev. Lett. 108 , 246101). A wetting-driven non-occlusion at some contact angles was induced by insertion of a rod into a horizontal tube at an eccentric position (Tan et al. 2022 J. Fluid Mech. 946 , A7). Insertion of a plate into a horizontal slit may enhance the capacity of removing liquid. This situation is theoretically investigated, and the theoretical results are mutually verified by a computational fluid dynamics (CFD) numerical method that is first employed to determine the critical non-occlusion conditions. Four types of liquid states are observed. The effects of contact angles, plate position and Bond number (measured by downward gravitational force relative to surface tension force) on different types of liquid states are analysed. This paper additionally provides a CFD numerical method for understanding the conditions for the stability and existence of the liquid plugs in complex situations (e.g. considering the effect of the sidewalls, or when a rod or plate is inserted into a circular, elliptical or polygonal tube) in the future.