Does the contact angle hysteresis control the droplet shapes on cylindrical fibers?

Abstract

HypothesisEquilibrium droplets on cylindrical fibers are divided into two classes: barreled and clamshell droplets. In the barreled droplet, the liquid body fully envelops the fiber, and the drop forms two boundary contact lines. In the clamshell droplets, some fiber surface under the liquid body remains dry and only one boundary contact line exists. So far, the transition from one shape to the other was predicted by ignoring the contact angle hysteresis. A new series of experiments using drop-on-demand technology were conducted to analyze the shape of droplets. The existing theory cannot explain the obtained data. We hypothesized that the morphological clamshell-barrel transition of droplets significantly depends on the method of drop formation and is largely controlled by the contact angle hysteresis. ExperimentTo test this hypothesis, we investigated two scenarios of drop formation. In the first scenario, the hexadecane drop was growing on the fiber by printing smaller drops on it. In the second scenario, the drop was formed spontaneously from a coating glycerol film due to the Plateau-Rayleigh instability. To obtain a range of contact angles, a set of different silanes were adsorbed on the fiber surfaces. FindingsThe results showed that for small contact angles (<40°) and small contact angle hysteresis, both methods of drop formation led to the same theoretically explainable conditions for clamshell-barrel transition. As the contact angle increases and hysteresis becomes appreciable, the conditions for clamshell-barrel transition become significantly dependent on the method of drop formation. We discovered that no barreled droplets exist for contact angles greater than 60°. The experimental results were documented in a set of diagrams describing the clamshell-barrel transitions. These diagrams can be used in different engineering applications.