Printing wet-on-wet: attraction and repulsion of drops on a viscous film

TL;DR

This study investigates how drops on a viscous film interact via capillary forces, revealing that interactions can be attractive or repulsive depending on their separation and film thickness, driven by visco-capillary waves.

Contribution

The paper uncovers the non-monotonic attraction-repulsion behavior of drops on thin viscous films and links it to visco-capillary wave dynamics using the thin-film equation.

Findings

Interaction switches from attraction to repulsion based on distance.

The transition distance depends on film thickness and increases over time.

Visco-capillary waves govern the interaction range.

Abstract

Wet-on-wet printing is frequently used in inkjet printing for graphical and industrial applications, where substrates can be coated with a thin liquid film prior to ink drop deposition. Two drops placed close together are expected to interact via deformations of the thin viscous film, but the nature of these capillary interactions is unknown. Here we show that the interaction can be attractive or repulsive depending on the distance separating the two drops. The distance at which the interaction changes from attraction to repulsion is found to depend on the thickness of the film, and increases over time. We reveal the origin of the non-monotonic interactions, which lies in the appearance of a visco-capillary wave on the thin film induced by the drops. Using the thin-film equation we identify the scaling law for the spreading of the waves, and demonstrate that this governs the range over which interaction is observed.