Obstacles to progress: Probing interactions between moving fronts and rigid boundaries in 2D systems

The interaction between a moving boundary and stationary obstacles constitutes a recurring theme in soft condensed matter, ranging from dislocation-defect pinning to Hele-shaw patterns in fluids. In this work, we investigate the spatio-temporal dynamics of a controlled 2D solidification front interacting with pre-engineered obstacles in a fluid. We first demonstrate a sequence of instabilities that results in different interface morphologies with distinct geometric properties. Various single and arrays of obstacles are then introduced in the path of the moving front using high-precision lithography techniques. The use of insitu optical microscopy enables quantification of the resulting dynamics. A complex interplay between interface curvature and obstacle geometry is observed, including stable and transient patterns, systematic front pinning and detachment. We also present theoretical calculations to evaluate the underlying thermal fields and hence explain the observed interface patterns. Some of these observations are shown to be directly analogous to those reported via postmortem analyses during rapid solidification of metallic alloys.