Wetting on two parallel fibers: drop to column transitions

Abstract

While the shape and stability of drops on single cylindrical fibers have received vast attention, there are few studies that consider a drop sitting between two fibers, which is a first step toward understanding the wetting of larger fibrous networks. In this paper, we investigate experimentally the behavior of a finite volume of liquid on two parallel rigid fibers. The liquid wetting the fibers can adopt two distinct equilibrium shapes: a compact approximately hemispherical drop shape or a long liquid column of constant cross-section. These two morphologies depend on the inter-fiber distance, the liquid volume, the fiber radius and the liquid–fiber contact angle. We study the transitions between a drop shape and a column by incrementally varying the inter-fiber distance and find that the transition depends on the global geometry of the system as well as on the volume of liquid. For totally wetting drops, we identify the regions where the drops or columns prevail, and find that there is a region where both morphologies are stable, and the transitions from one state to the other are hysteretic. These switches in morphologies may be used to manipulate or transport liquid at small scales.