Abstract

A thin liquid film flowing down a vertical fiber is subject to several shape-change instabilities that result in a bead-on-fiber morphology with high surface area-to-volume ratio. This high-curvature fluid interface is desirable for numerous heat and mass transfer processes such as a novel low-power desalination process that can produce clean water for resource-constrained communities. The efficiency of this process is dependent upon the properties of the bead patterns with its maximum performance occurring at the point where the flow pattern transitions from an absolute to convective instability. Experiments are performed about this critical transition point and a new asymmetric instability is revealed which depends upon the liquid surface tension and fiber diameter and exhibits all the bead-on-fiber morphologies previously documented for the classical symmetric morphology. The bead dynamics are described by the bead spacing and bead velocity with the asymmetric morphology displaying more regular dynamics than the symmetric morphology. For the asymmetric morphology, the transition between absolute and convective instability agrees well with predications for a free viscous jet indicating a minimal effect between the thin film and fiber. In addition, the dimensionless bead velocity is shown to scale with the capillary number for all experimental data. These observations for the asymmetric bead dynamics can be used as a design tool for heat and mass transfer processes, including a new desalination technique that aims to alleviate global water scarcity.