Photochemically assisted patterning: An interfacial hydrodynamic model perspective
Photoresponsive organic liquids and polymers may undergo reversible photochemical reactions with accompanying change in chemical structure upon exposure to a suitable wavelength of light. Spatial variations in chemical structure can cause surface tension gradients along thin films of such materials, resulting in Marangoni flows when the material is in its fluid state. Consequently the fluid film can self-organize into topographical patterns. Here, we provide a hydrodynamic model perspective of photochemically assisted patterning in thin fluid films using principles of momentum and mass transfer. Photochemical reaction occuring simultaneously along with hydrodynamic flow is modelled. Dynamical variation of the photoproduct concentration, accounting for first order chemical reaction kinetics, is considered. Our simulations highlight the counteracting effects of reaction kinetics and Marangoni flow as the mechanism responsible for pattern evolution. We capture various pillar and hole array morphologies obtained by controlling the direction of Marangoni flow and reaction-induced mass transfer. The resolution and timescales of pattern formation are computed as function of experimental control parameters such as the reaction rate coefficient (K0), Marangoni number (M), and distance between the film and light source. A process map comparing feature sizes on a photomask (w) with those of the patterns evolved in the material is developed. It reveals optimum w − M and w − K0 combinations required for faithful reproduction of photomask feature sizes and deviation from ideally templated patterns.
Interplay of substrate wettability and surfactant distributions in controlling interfacial instability
The current work investigates the combined effects of substrate wettability patterns and preset surfactant distributions on the dynamics and rupture of thin liquid films. The configuration considered here is a liquid film with a free surface above, and bounded below by a solid substrate with alternate zones of less and more wettable areas. This wettability-based patterning triggers an instability of the liquid film, caused by the outward flow from less to more wettable regions at the liquid–substrate interface. Introducing surfactants in the liquid film also affects the interfacial dynamics through Marangoni flow at the air–liquid interface. A variety of distribution functions are used as initial conditions for the surfactant concentration, to suitably modify the final configuration of a flat film. A set of two evolution equations for the film height and surfactant concentration are developed as partial differential equations to be solved numerically and simultaneously. Results indicate that the interplay between substrate wettability and surfactant distributions may significantly alter the ruptured film configurations. Typical scenarios include multiple near-rupture events in place of a single rupture, asymmetry in the ruptured film profile, and reversal of flow leading to film rupture on a more wettable region of the substrate. The results also provide insights into the dependence of rupture time scales on the inherent length scales in the problem, such as the pattern feature widths and the size of the surfactant-depleted zones.