P. Lidon – 9 avril

Soft materials are known to exhibit complex and heterogeneous response to mechanical excitations. However, conventional rotational rheometry which is the most common tool to study mechanical response only provides us with information averaged on a macroscopic scale. In order to access local dynamics of complex fluids, a strong research effort has been done to study the motion of small tracers, either due to brownian motion (passive microrheology) or under an external force generated by optical or magnetic tweezers (active microrheology). These methods bring significant information on the rheological properties at a micrometric scale, but the range of accessible deformations is small and thus essentially restricted to the linear regime.

In this presentation, I will describe how to use the acoustic radiation force to perform microrheology in order to probe larger force and length scales than conventional microrheology. By focusing high intensity ultrasound on a small sphere embedded in a soft microgel, we can trigger creep deformations of the gel matrix up to yielding at high enough forcing as well as relaxation after the end of excitation. I will compare our results with rheological measurements under similar excitation and show that the observed local dynamics is qualitatively mostly similar to macroscopic observations, despite some quantitative discrepancies.