Hysteresis in spreading and retraction of liquid droplets on parallel fiber rails
Wetting and spreading of liquids on fibers occurs in many natural and artificial processes. Unlike on a planar substrate, a droplet attached to one or more fibers can assume several different shapes depending on geometrical parameters such as liquid volume and fiber size and distance. This paper presents lattice Boltzmann simulations of the morphology of liquid droplets on two parallel cylindrical fibers. We investigate the final shapes resulting from spreading of an initially spherical droplet deposited on the fibers and from retraction of an initial liquid column deposited between the fibers. We observe three possible equilibrium configurations: barrel-shaped droplet, droplet bridges, and liquid columns. We determine the complete morphology diagram for varying inter-fiber spacing and liquid volume and find a region of bistability that spans both the column regime and the droplet regime. We further present a simulation protocol that allows to probe the hysteresis of transitions between different shapes. The results provide insights into energies and forces associated with shape transformations of droplets on fibers that can be used to develop fiber-based materials and microfluidic systems for manipulation of liquids at small scale.