  We study the motion of a sphere of diameter 330 $\mu$m embedded in a Carbopol
microgel under the effect of the acoustic radiation pressure exerted by a
focused ultrasonic field. The sphere motion within the microgel is tracked
using videomicroscopy and compared to conventional creep and recovery
measurements performed with a rheometer. We find that under moderate ultrasonic
intensities, the sphere creeps as a power law of time with an exponent $\alpha
\simeq 0.2$ that is significantly smaller than the one inferred from global
creep measurements below the yield stress of the microgel ($\alpha \simeq
0.4$). Moreover, the sphere relaxation motion after creep and the global
recovery are respectively consistent with these two different exponents. By
allowing a rheological characterization at the scale of the sphere with forces
of the order of micronewtons, the present experiments pave the way for acoustic
"mesorheology" which probes volumes and forces intermediate between standard
macroscopic rheology and classical microrheology. They also open new questions
about the effects of the geometry of the deformation field and of the sphere
size and surface properties on the creep behaviour of soft materials.
