Microbubble-based measurement of shear and loss moduli in polyacrylamide hydrogels at MHz frequencies

The rheology of soft materials is routinely measured at low strain rates to extract constitutive laws necessary for understanding and modeling their behavior. High-frequency rheology, however, remains difficult to access. Consequently, the mechanical properties of soft materials at MHz strain rates are largely unknown. Ultrasound-driven microbubbles, widely used in biomedical imaging, drug delivery, and therapy, act as efficient mechanical actuators at MHz frequencies. Their dynamics depend on nonlinear resonance behavior, the viscoelasticity of their stabilizing shells, and the viscoelastic properties of the surrounding medium. Here, we make use of (nonlinear) bubble dynamics to characterize the rheology of polyacrylamide (PAM) hydrogels at strain rates exceeding 10⁶ s^-1. Narrow resonance curves of single coated microbubbles embedded in PAM, obtained through high-speed imaging, were compared to a Rayleighâ€"Plesset-type model. The results show that the shear modulus is similar in both the Hz and MHz regimes, while the loss modulus behaves very differently, exhibiting an effective shear viscosity at MHz frequencies comparable to that of water. These findings demonstrate a new approach for probing the high-frequency rheology of viscoelastic media.