Capillary-driven migration of droplets on conical fibers

TL;DR

This study investigates the capillary-driven movement of droplets on conical fibers through experiments and a dynamic model, revealing effects of fiber geometry, wetting state, and gravity on droplet migration and volume loss.

Contribution

It introduces a dynamic model that captures droplet migration on conical fibers, considering non-equilibrium effects, wetting conditions, and gravity, advancing beyond previous equilibrium-based laws.

Findings

Droplets accelerate then decelerate during migration.

More divergent fibers cause longer acceleration phases.

Dry fibers induce more friction and volume loss.

Abstract

A droplet placed on a hydrophilic conical fiber tends to move toward the end of larger radii due to capillary action. Experimental investigations are performed to explore the dynamics of droplets with varying viscosities and volumes on different fibers at the microscale. Droplets are found to accelerate initially and subsequently decelerate during migration. A dynamic model is developed to capture dynamics of the droplet migration, addressing the limitations of previous equilibrium-based scaling laws. Both experimental results and theoretical predictions indicate that droplets on more divergent fibers experience a longer acceleration phase. Additionally, gravitational effects are pronounced on fibers with small cone angles, exerting a substantial influence on droplet migration even below the capillary scale. Moreover, droplets move more slowly on dry fibers compared to those prewetted with the same liquid, primarily attributed to the increased friction. The experiments reveal the formation of a residual liquid film after droplet migration on dry fibers, leading to considerable volume loss in the droplets. To encompass the intricacies of migration on dry fibers, the model is refined to incorporate a higher friction coefficient and variable droplet volumes, providing a more comprehensive depiction of the underlying physics.