Final Report Summary - USOFT (Ultrasound-based techniques for soft jammed materials)
Over the course of the USOFT project we developed new tools based on ultrasound for studying soft materials such as gels, suspensions, or emulsions. These materials generally show solidlike properties at rest and flow when submitted to moderate mechanical stress. This solid-to-fluid transition is of prime importance for practical applications where good control of the rheological properties is required, for instance food, cosmetic, pharmaceutical, or oil industries. In the first part of the USOFT project we have devised a custom-made ultrasonic scanner that allows ultrasonic imaging with frame rates up to 20,000 images per second. This new equipment has been used to follow gelation processes over time in agar and protein gels and to revisit the dynamics of various physico-chemical systems in soft matter under simple shear. We focused on surfactant wormlike micelles that show shear banding and elastic instabilities, on carbopol microgels where fluidization involves transient shear bands, on carbon black gels that present spatially heterogeneous yielding preceded by a creep regime reminiscent of solid systems, on protein gels where stress-induced failure is irreversible and proceeds through the growth of fractures, on cellulose microfibril suspensions that show three-dimensional features at low shear rates, on thermoresponsive particles whose jamming and slippage properties depend on temperature and on silica gel as well as shear-thickening suspensions. The possibility to explore fast transient regimes under constant shear and to image systems submitted to large amplitude oscillatory shear has provided crucial space- and time-resolved insights into the flow dynamics of these soft materials. In a second part of our project, ultrasound has been used at high acoustic powers to interact strongly with soft jammed materials. New experimental setups have been designed to submit gels and suspensions to a high-intensity ultrasonic field and study their response through optical imaging and mechanical characterization. A first study on granular suspensions has evidenced that high-intensity focused ultrasound can unjam and fluidize locally a dense packing of grains. A second study has shown that high-intensity ultrasound dramatically affects the elastic properties of colloidal gels eventually leading to their liquefaction at the highest achievable powers. These findings could have some applications in unclogging, filtration and acoustic control of material properties.