  Bacterial attachment to a fluid interface can lead to the formation of a film
with physicochemical properties that evolve with time. We study the time
evolution of interface (micro)mechanics for interfaces between oil and
bacterial suspensions by following the motion of colloidal probes trapped by
capillarity to determine the interface microrheology. Initially, active
bacteria at and near the interface drive superdiffusive motion of the colloidal
probes. Over timescales of minutes, the bacteria form a viscoelastic film which
we discuss as a quasi-two-dimensional, active, glassy system. To study late
stage mechanics of the film, we use pendant drop elastometry. The films, grown
over tens of hours on oil drops, are expanded and compressed by changing the
drop volume. For small strains, by modeling the films as 2D Hookean solids, we
estimate the film elastic moduli, finding values similar to those reported in
the literature for the bacteria themselves. For large strains, the films are
highly hysteretic. Finally, from wrinkles formed on highly compressed drops, we
estimate film bending energies. The dramatic restructuring of the interface by
such robust films has broad implications, e.g. in the study of active colloids,
in understanding the community dynamics of bacteria, and in applied settings
including bioremediation.
