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.