Film coating by directional droplet spreading on fibers

TL;DR

This study investigates how droplets moving along conical fibers deposit a fluid film, revealing that film thickness varies greatly with droplet size and fiber radius, with implications for biological and industrial coating processes.

Contribution

The paper introduces an asymptotic analysis combined with experiments and simulations to understand film deposition by droplets on slender fibers, highlighting finite size effects.

Findings

Film thickness depends on fiber radius and droplet size.

Changing droplet size can alter film thickness by over an order of magnitude.

Self-propelled droplets can passively create coated structures.

Abstract

Plants and insects use slender conical structures to transport and collect small droplets, which are propelled along the conical structures due to capillary action. These droplets can deposit a fluid film during their motion, but despite its importance to many biological systems and industrial applications the properties of the deposited film are unknown. We characterise the film deposition by developing an asymptotic analysis together with experimental measurements and numerical simulations based on the lubrication equation. We show that the deposited film thickness depends significantly on both the fiber radius and the droplet size, highlighting that the coating is affected by finite size effects relevant to film deposition on fibres of any slender geometry. We demonstrate that by changing the droplet size, while the mean fiber radius and the Capillary number are fixed, the thickness of the deposited film can change by an order of magnitude or more. We show that self-propelled droplets have significant potential to create passively coated structures.