Directional self-propelled transport of coalesced droplets on a superhydrophilic cylindrical wire

TL;DR

This study reveals a unique droplet coalescence behavior on superhydrophilic wires where larger droplets are attracted to smaller ones, driven by non-intuitive viscous friction effects, supported by a predictive MSD model.

Contribution

It introduces a novel coalescence pattern on cylindrical wires and develops a dimensional analysis model that accurately predicts various coalescence behaviors.

Findings

Larger droplets can be pulled toward smaller ones on cylindrical wires.

Viscous friction on a wire is not proportional to droplet size.

The MSD model predicts coalescence patterns matching experiments.

Abstract

Droplets coalescing on flat surfaces tend to end up with the smaller droplet migrating into the larger one. We report a counter-intuitive droplet coalescence pattern on a superhydrophilic cylindrical wire, where the larger droplet is pulled toward the smaller one. Consequently, the center of the combined mass significantly moves toward, often beyond, the original location of the smaller droplet. This phenomenon occurs primarily because the viscous friction that a droplet experiences on a cylindrical wire is not positively correlated with the size of the droplet, unlike the droplets coalescing on flat surfaces. We conducted a dimensional analysis based on a mass-spring-damper (MSD) model. Our model predicts a variety of coalescence patterns as a function of the ratio between droplet sizes, and the prediction matches the experimental observation.