Bubbles stabilised by colloidal particles can find applications in advanced materials, catalysis and
drug delivery. For applications in controlled release, it is desirable to remove the particles from
the interface in a programmable fashion. We have previously shown that ultrasound waves excite
volumetric oscillations of particle-coated bubbles, resulting in precisely timed particle expulsion
due to interface compression on a ultrafast timescale [Poulichet et al., Proc. Natl. Acad. Sci.
USA, 2015, 112, 5932]. We also observed shape oscillations, which were found to drive directional
particle expulsion from the antinodes of the non-spherical deformation. In this paper we
investigate the mechanisms leading to directional particle expulsion during shape oscillations of
particle-coated bubbles driven by ultrasound at 40 kHz. We perform high-speed visualisation of
the interface shape and of the particle distribution during ultrafast deformation at a rate of up
to 105 s
−1
. The mode of shape oscillations is found to not depend on the bubble size, in contrast
with what has been reported for uncoated bubbles. A decomposition of the non-spherical
shape in spatial Fourier modes reveals that the interplay of different modes determines the locations
of particle expulsion. The n-fold symmetry of the dominant mode does not always lead
to desorption from all 2n antinodes, but only those where there is favourable alignment with the
sub-dominant modes. Desorption from the antinodes of the shape oscillations is due to different,
concurrent mechanisms. The radial acceleration of the interface at the antinodes can be up
to 105 − 106 ms−2
, hence there is a contribution from the inertia of the particles localised at the
antinodes. In addition, we found that particles migrate to the antinodes of the shape oscillation,
thereby enhancing the contribution from the surface pressure in the monolayer.