  Front-propagating systems provide some of the most fundamental physical
examples of interfacial instability and pattern formation. However, their
nonlinear dynamics are rarely addressed. Here, we present an experimental study
of air displacing a viscous fluid within a collapsed, compliant channel - a
model system for pulmonary airway reopening. We show that compliance induces
fingering instabilities absent in the rigid channel and we present the first
experimental observations of the counter-intuitive 'pushing' behaviour
previously predicted numerically, for which a reduction in air pressure results
in faster flow. We find that pushing modes are unstable and moreover, that the
dynamics of the air-fluid front involves a host of transient finger shapes over
a significant range of experimental parameters.
