  Colloidal gels, where nanoscale particles aggregate into an elastic yet
fragile network, are at the heart of materials that combine specific optical,
electrical and mechanical properties. Tailoring the viscoelastic features of
colloidal gels in real-time thanks to an external stimulus currently appears as
a major challenge in the design of "smart" soft materials. Here we introduce
"rheo-acoustic" gels, a class of materials that are sensitive to ultrasonic
vibrations. By using a combination of rheological and structural
characterization, we evidence and quantify a strong softening in three widely
different colloidal gels submitted to ultrasonic vibrations (with submicron
amplitude and frequency 20-500 kHz). This softening is attributed to
micron-sized cracks within the gel network that may or may not fully heal once
vibrations are turned off depending on the acoustic intensity. Ultrasonic
vibrations are further shown to dramatically decrease the gel yield stress and
accelerate shear-induced fluidization. Ultrasound-assisted fluidization
dynamics appear to be governed by an effective temperature that depends on the
acoustic intensity. Our work opens the way to a full control of elastic and
flow properties by ultrasonic vibrations as well as to future theoretical and
numerical modeling of such rheo-acoustic gels.
