Extreme deformation of soft matter: tuning colloidal gel microstructure with ultrasound and microbubbles
Valeria Garbin, TU Delft
Controlling the microstructure of soft materials is essential to impart properties for advanced applications, for instance to obtain acoustic or photonic metamaterials, or by modifying their mechanical response. Imposing a steady or oscillatory shear during the manufacturing process is one of the preferred pathways to control the orientation, structure or mechanical properties of a wide range of microstructured materials. Colloidal gels are a broad class of industrially relevant and highly tunable soft materials, for instance in the paint and coatings industry. These microstructured materials are sensitive to shear due to their out-of-equilibrium kinetic arrest and undergo a transition from hexagonal to cubic ordering under simple, homogeneous, oscillatory shear. The impact of a heterogeneous shear or extensional deformation on these systems, despite being the norm in industrial processes, remains however largely unexplored. We use a gas inclusion (i.e., a bubble), activated by ultrasound at 10-100 kHz to locally and periodically strain a depletion colloidal gel at high frequency. The strain field applied by the bubble is in this case predominantly extensional and localised; the strain decreases as a power law away from the bubble. We monitor the microstructure of the gel under a confocal microscope close to the bubble before and after the acoustic excitation, and we are able to visualize some features of the flowing gel using high-speed, brightfield video microscopy. We find that the oscillations induce local rearrangements with striking patterns, which we quantify using structural indicators. The observed, irreversible rearrangements demonstrate the potential for controlling the microstructure of a colloidal-gel material locally and with a remote acoustic trigger.
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