  Colloidal gels made of carbon black particles dispersed in light mineral oil
are "rheo-acoustic" materials, i.e., their mechanical and structural properties
can be tuned using high-power ultrasound, sound waves with submicron amplitude
and frequency larger than 20~kHz . The effects of high-power ultrasound on the
carbon black gel are demonstrated using two experiments: rheology coupled to
ultrasound to test for the gel mechanical response and a timeresolved ultra
small-angle X-ray scattering experiment (TRUSAXS) coupled to ultrasound to test
for structural changes within the gel. We show that high-power ultrasound above
a critical amplitude leads to a complex viscoelastic transient response of the
gels within a few seconds: a softening of its storage modulus accompanied by a
strong overshoot in its loss modulus. Under high-power ultrasound, the gel
displays a viscoelastic spectrum with glass-like features and a significant
decrease in its yield strain. Those effects are attributed to the formation of
intermittent micro-cracks in the bulk of the gel as evidenced by TRUSAXS.
Provided that the shear rate is not large enough to fully fluidize the sample,
high-power ultrasound also facilitates the flow of the gel, reducing its yield
stress as well as increasing the shear-thinning index, thanks again to the
formation of micro-cracks.
