Controlled Micro–Nano-Scale Droplet Generation via Spin Dewetting

Abstract

A combined theoretical and experimental study is presented to investigate the interplay of forces in the spin-dewetting process in order to achieve enhanced control over droplet generation. In this regard, toluene–polystyrene (PS) film is spin dewetted on a solid substrate to generate an array of droplets. The underlying mechanisms of the spin dewetting of the films into the droplets are explained with the help of a theoretical model followed by a long-wave linear stability analysis (LWLSA). Stabilizing forces like solution viscosity and surface tension play essential roles. The study uncovers that the centripetal force stretches the film radially outward, before it becomes ultrathin and undergoes dewetting under the influence of van der Waals forces, while the surface tension force acts as a stabilizing influence. On the other hand, the viscous force kinetically stabilizes the system to expedite or delay drop formation on the substrate. An imbalance of these factors ultimately decides the droplet spacing, which leads to interesting morphologies such as singlet, doublet, triplet, and clusters of droplets at specific PS concentrations in the range 0.0001–0.0005%, with a ~10–14 nm average droplet height. The experimental data revealed that, at ~3000 rpm, PS (0.01–0.1%) results in critical droplet spacings of λmax~98–172 μm, leading to immediate dewetting and uniform droplet formation. Our theoretical predictions are in close agreement with the experimental results, validating the present model. The insights gained in this work provide a foundation by presenting a robust framework for controlled droplet generation by optimizing process parameters to achieve the desired droplet size, distribution, and uniformity. The findings have broad applications in material science, biomedical engineering, and related disciplines.