Instabilities of layers of deposited molecules on chemically stripe patterned substrates: Ridges vs. drops

TL;DR

This paper uses a mesoscopic continuum model to analyze the stability and formation of ridges and droplets during organic molecule deposition on striped substrates, revealing two main instability modes and the effects of different transport mechanisms.

Contribution

It introduces a detailed stability analysis of ridge formations on patterned substrates and examines how various transport processes influence morphology evolution.

Findings

Identified two instability modes leading to bulges and droplets.

Confirmed predictions through nonlinear numerical simulations.

Transport mechanisms affect process time scales but not stability thresholds.

Abstract

A mesoscopic continuum model is employed to analyse the transport mechanisms and structure formation during the redistribution stage of deposition experiments where organic molecules are deposited on a solid substrate with periodic stripe-like wettability patterns. Transversally invariant ridges located on the more wettable stripes are identified as very important transient states and their linear stability is analysed. It is found that there exist two different instability modes that result (i) at large ridge volume in the formation of bulges that spill from the more wettable stripes onto the less wettable bare substrate and (ii) at small ridge volume in the formation of small droplets located on the more wettable stripes. These predictions are confirmed by direct numerical simulations of the fully nonlinear evolution equation for two-dimensional substrates. In addition, the influence of different transport mechanisms during redistribution is investigated focusing on the cases of convective transport with no-slip at the substrate, transport via diffusion in the film bulk and via diffusion at the film surface. In particular, it is shown that the transport process does neither influence the linear stability thresholds nor the sequence of morphologies observed in the time simulation, but only the ratio of the time scales of the different process phases.