  The viscous fingering instability, which forms when a less-viscous fluid
invades a more-viscous one within a confined geometry, is an iconic system for
studying pattern formation. For both miscible and immiscible fluid pairs the
growth dynamics change after the initial instability onset and the global
structures, typical of late-time growth, are governed by the viscosity ratio.
Here we introduce an experimental technique to measure flow throughout the
inner and outer fluids. This probes the existence of a new length scale
associated with the local pressure gradients around the interface and allows us
to compare our results to the predictions of a previously proposed model for
late-time finger growth.
