  We study the partitioning of motile bacteria into different thermodynamic
states of a phase-separated dextran (DEX)/polyethylene glycol (PEG) aqueous
mixture. While non-motile bacteria partition exclusively into the DEX-rich
phase in all conditions tested, we observed that motile bacteria transverse the
soft DEX/PEG interface and partition variably among the two phases. For our
model organism \textit{Bacillus subtilis}, the fraction of motile bacteria in
the DEX-rich phase increased from 0.58 to 1 as we increased DEX composition
within the two-phase region. We hypothesized that the chemical affinity between
dextran and the bacteria cell wall acts to weakly confine the bacteria within
the DEX-rich phase; however, motility can generate sufficient mechanical forces
to overcome the soft confinement and propel the bacteria into the PEG-rich
phase. Using optical tweezers to drag a bacterium across the DEX/PEG interface,
we developed a model to predict the overall phase partitioning based on a
competition between the interfacial forces and bacterial propulsive forces. Our
measurements are supported by a theoretical model of dilute active rods
embedded within a periodic soft confinement potential.
