We predict a novel selection phenomenon in nonlinear interface dynamics out
of equilibrium. Using a recently developed formalism based on the
Schottky-Klein prime functions, we extended the existing integrable theory from
a single interface to multiple moving interfaces. After applying this extended
theory to the two-dimensional Laplacian growth, we derive a new rich class of
exact (non-singular) solutions for the unsteady dynamics of an arbitrary
assembly of air bubbles within a layer of a viscous fluid in a Hele-Shaw cell.
These solutions demonstrate that all bubbles reach an asymptotic velocity, U,
which is {\it precisely twice} greater than the velocity, V, of the uniform
background flow, i.e., U=2V. The result does not depend on the number of
bubbles. It is worth to mention that contrary to common belief, the predicted
velocity selection does not require surface tension.Comment: 5 pages, 1 figure. Updated versio