  Colloidal gels possess a memory of previous shear events, both steady and
oscillatory. This memory, embedded in the microstructure, affects the
mechanical response of the gel, and therefore enables precise tuning of the
material properties under careful preparation. Here we demonstrate how the
dynamics of a deformable inclusion, namely a bubble, can be used to locally
tune the microstructure of a colloidal gel. We examine two different phenomena
of bubble dynamics that apply a local strain to the surrounding material:
dissolution due to mass transfer, with a characteristic strain rate of
$10^{-3}$ s$^{-1}$; and volumetric oscillations driven by ultrasound, with a
characteristic frequency of $10^4$ s$^{-1}$.
  We characterise experimentally the microstructure of a model colloidal gel
around bubbles in a Hele-Shaw geometry using confocal microscopy and particle
tracking. In bubble dissolution experiments, we observe the formation of a
pocket of solvent next to the bubble surface, but marginal changes to the
microstructure. In experiments with ultrasound-induced bubble oscillations, we
observe a striking rearrangement of the gel particles into a microstructure
with increased local ordering. High-speed bright-field microscopy reveals the
occurrence of both high-frequency bubble oscillations and steady microstreaming
flow; both are expected to contribute to the emergence of the local order in
the microstructure. These observations open the way to local tuning of
colloidal gels based on deformable inclusions controlled by external pressure
fields.
