Controlling droplet spreading with topography

TL;DR

This paper introduces an experimental system and a predictive model to study and control droplet spreading on topographically varied substrates, with applications in display manufacturing.

Contribution

It presents a novel experimental setup and a combined thin-film and spreading law model to predict droplet behavior on complex topographies.

Findings

The model accurately predicts droplet morphologies during spreading.

Topography gradients can either enhance or hinder spreading.

The Cox-Voinov law effectively describes most of the spreading dynamics.

Abstract

We present a novel experimental system that can be used to study the dynamics of picoliter droplet spreading over substrates with topographic variations. We concentrate on spreading of a droplet within a recessed stadium-shaped pixel, with applications to the manufacture of POLED displays, and find that the sloping side wall of the pixel can either locally enhance or hinder spreading depending on whether the topography gradient ahead of the contact line is positive or negative. Locally enhanced spreading occurs via the formation of thin pointed rivulets along the side walls of the pixel through a mechanism similar to capillary rise in sharp corners. We demonstrate that a thin-film model combined with an experimentally measured spreading law, which relates the speed of the contact line to the contact angle, provides excellent predictions of the evolving liquid morphologies. We also show that the spreading can be adequately described by a Cox-Voinov law for the majority of the evolution. The model does not include viscous effects and hence, the timescales for the propagation of the thin pointed rivulets are not captured. Nonetheless, this simple model can be used very effectively to predict the areas covered by the liquid and may serve as a useful design tool for systems that require precise control of liquid on substrates.