Non-linear response of colloid monolayers at high-frequency probed by ultrasound-driven microbubble dynamics

Hypothesis: High-frequency interfacial rheology of complex interfaces remains challenging yet it is central to the performance of multiphase soft matter products. We propose to use ultrasound-driven bubble dynamics to probe the high-frequency rheology of a colloid monolayer used as model system with controlled interactions and simultaneous monitoring of the microstructure. We hypothesize that by comparing the response of colloid-coated bubbles with that of a bare bubble under identical experimental conditions, it is possible to detect the non-linear response of the monolayer and use it to extract interfacial rheological properties at 10⁴s⁻¹. Experiments: Using high-speed video-microscopy, the dynamics of colloid-coated bubbles were probed to study the micromechanical response of the monolayer to high-frequency deformation. Protocols analogous to stress-sweep and frequency-sweep were developed to examine the stress–strain relationships. A simple model, motivated by the observed non-linear responses, was developed to estimate the interfacial viscoelastic parameters. Findings: The estimated elastic moduli of colloid monolayers at 10⁴s⁻¹ are about an order of magnitude larger than those measured at 1 s⁻¹. The monolayers exhibit non-linear viscoelasticity for strain amplitudes as small as 1%, and strain-softening behaviour. These findings highlight the applicability of acoustic bubbles as high-frequency interfacial probes.