Local assessment of visco-elastic properties with acoustical tweezers

Assessing the mechanical properties of soft matter is a subject of major interest, specifically if it can be performed in a local and non-invasive way. Here, we explore the use of single-beam acoustical tweezers as a novel active rheology approach. Individual microbubbles (∼50–100 μm in size) are generated in soft hydrogels (Carbopol) and pulled by the radiation force of a focused vortex beam. Due to the good acoustic contrast of a microbubble, forces in the micronewton range were applied at low acoustic intensities (<10 W/cm2). The microbubble displacement is tracked using optical microscopy, and we are able to detect the small displacements, u, of a few microns induced in the soft hydrogel. Combined with radiation force calculations and a simple elastic model for the displacement, local values of the shear elastic modulus, G, are obtained. Our local measurements are in very good agreement with those obtained using standard bulk shear rheometry. We then further focus on the full visco-elastic behavior of the hydrogel by exploring the microbubble dynamics in response to a time-varying radiation force. Overall this new approach offers real advantages over traditional bulk rheologymethods, as it can be considered minimally intrusive, local, and well adapted to probe inhomogeneous, anisotropic, and opaque-to-light materials.