Coating and printing on chemically patterned surfaces.

Abstract

A number of emerging applications like flexible electronic devices and displays and patterned microfluidic devices require selective deposition of material on micro- and nanoscale patterns. At these length scales, mathematical models with appropriate simplifying assumptions would prove handy to understand liquid dewetting mechanisms in coating and printing processes. For example, the liquid films in many coating and printing processes may be assumed to be thin enough so that intermolecular forces are important and the lubrication approximation can be invoked. Using a combination of nonlinear simulations and linear stability analysis, three important problems pertaining to coating and printing on chemically patterned surfaces are examined. The first problem is concerned with the liquid displacement phenomenon that occurs in lithographic printing processes. The model allows us to obtain physical insights into and numerical estimates of the smallest and largest feature sizes that can be printed, as well as the minimum spacing between feature sizes that can be tolerated. In addition, the model provides insights into experimental observations on a closely related process, wire-wound rod coating on chemically patterned surfaces. Next, the model is used to examine the effect of shear on the liquid displacement process. Linear theory reveals that the growth rate of interfacial perturbations has an imaginary component, indicating the existence of traveling waves. Nonlinear simulations show that shear delays interfacial rupture, and suppression of rupture occurs beyond a critical shear rate. Propagation of traveling waves along the interface, and subsequent weakening of van-der-Waals-driven dewetting, is found to be the cause of the rupture delay. Finally, the dewetting of a solitary liquid film resting on a chemically patterned surface, under the combined action of thermally induced Marangoni effects and the intermolecular forces is explored. The model results suggest that combined localized heating and cooling may be used to modify the film rupture dynamics induced by the disjoining forces and cause rupture at desired locations. This physical phenomenon provides a handle to deposit liquids on surfaces whose wettability is difficult to control. The work presented here has special practical relevance for manipulating liquid flow in industrial applications like templating, coating and printing processes, and microfluidics.