Sharp interfaces in two dimensional free boundary problems: Efficient interface calculation via matched conformal maps

Abstract

We use conformal maps to study a free boundary problem for a two-fluid electromechanical system, where the interface between the fluids is determined by the combined effects of electrostatic forces, gravity and surface tension. The free boundary in our system develops sharp corners/singularities in certain parameter regimes, and this is an impediment to using existing "single-scale" numerical conformal mapping methods. The difficulty is due to the phenomenon of crowding, i.e. the tendency of nodes in the preimage plane to concentrate near the sharp regions of the boundary, leaving the smooth regions of the boundary poorly resolved. A natural idea is to exploit the scale separation between the sharp regions and smooth regions to solve for each region separately, and then stitch the solutions together. However, this is not straightforward as conformal maps are rigid "global" objects, and it is not obvious how one would patch two conformal maps together to obtain a new conformal map. We develop a "multi-scale" (i.e. adaptive) conformal mapping method that allows us to carry out this program of stitching conformal maps on different scales together. We successfully apply our method to the electromechanical model problem and discuss how it generalizes to other situations.