  We experimentally study the viscous fingering instability in a fluid-fluid
phase separated colloid-polymer mixture by means of laser scanning confocal
microscopy and microfluidics. We focus on three aspects of the instability. (i)
The interface between the two demixed phases has an ultralow surface tension,
such that we can address the role of thermal interface fluctuations. (ii) We
image the interface in three dimensions allowing us to study the interplay
between interface curvature and flow. (iii) The displacing fluid wets all walls
completely, in contrast to traditional viscous fingering experiments, in which
the displaced fluid wets the walls. We also perform lattice Boltzmann
simulations, which help to interpret the experimental observations.
