Valeria Garbin (Applied Sciences, Delft University of Technology, The Netherlands). Acoustic manipulation of armored bubbles: shape oscillations, particle expulsion, and pattern formation.

Armored bubbles, covered by a monolayer of colloidal particles, exhibit outstanding stability against dissolution and remarkable mechanical properties. In this seminar I will give an overview of the work in our group on acoustic manipulation of armored bubbles, and the intriguing phenomena we have observed: shape oscillations, particle expulsion, and pattern formation. Armored bubbles respond to ultrasound by undergoing volumetric oscillations and, under the right conditions, they exhibit shape oscillations. We found that shape oscillations of armored bubbles exhibit period doubling, but the mode of does not depend on the bubble size, in contrast with bare bubbles. For large-amplitude ultrasonic forcing, we observed expulsion of colloids from the interface of armored bubbles. We uncovered different particle expulsion scenarios depending on the mode of bubble deformation, including highly directional patterns of particles release during shape oscillations. Monolayers of colloidal particles are also a convenient two-dimensional model system to visualize emergent behaviors in soft matter, but previous studies have been limited to slow deformations. We have used ultrasonic driving of armored bubbles to probe and visualize the evolution of a monolayer of colloids during high-rate deformation at 10,000 Hz. We observed the emergence of a transient network of strings, which points to directional interparticle interactions during forcing. We used discrete particle simulations to show that it is caused by a delicate interplay of dynamic capillarity and hydrodynamic interactions between particles oscillating at high frequency. These results highlight the importance of inertial effects, which are normally negligible in a colloidal system, caused by accelerations approaching 10,000g. These phenomena have no counterpart at lower deformation rates, making armored bubbles driven by ultrasound a promising platform to study extreme deformation of soft matter.