IMA Journal of Applied Mathematics Advance Access originally published online on September 9, 2008
IMA Journal of Applied Mathematics 2009 74(1):107-127; doi:10.1093/imamat/hxn029
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On travelling-wave solutions for a moving boundary problem of Hele–Shaw type
Mathematisches Institut, Universität Leipzig, Johannisgasse 26, 04103 Leipzig, Germany

Department of Mathematics and Computer Science, Technische Universiteit Eindhoven, PO Box 513, 5600 MB Eindhoven, The Netherlands
Email: g.prokert{at}tue.nl
Received on November 28, 2007; Revision received June 20, 2008. Accepted on July 31, 2008
We discuss a 2D moving boundary problem for the Laplacian with Robin boundary conditions in an exterior domain. It arises as a model for Hele–Shaw flow of a bubble with kinetic undercooling regularization and is also discussed in the context of models for electrical streamer discharges. The corresponding evolution equation is given by a degenerate, non-linear transport problem with non-local lower-order dependence. We identify the local structure of the set of travelling-wave solutions in the vicinity of trivial (circular) ones. We find that there is a unique non-trivial travelling wave for each velocity near the trivial one. Therefore, the trivial solutions are unstable in a comoving frame. The degeneracy of our problem is reflected in a loss of regularity in the estimates for the linearization. Moreover, there is an upper bound for the regularity of its solutions. To prove our results, we use a quasi-linearization by differentiation, index results for degenerate ordinary differential operators on the circle and perturbation arguments for unbounded Fredholm operators.
Keywords: degenerate transport equation; moving boundary problem; Hele–Shaw flow; kinetic undercooling; electrical streamer discharges.