A method to characterize the cavitation pressure of soft matter under superimposed azimuthal shear

In many applications (e.g. contact sports, vehicular accidents, blunt force trauma, traumatic brain injury, drug delivery, surgeries), biological tissues and other soft matter are subject to complex, multi-axial stress states that can induce a variety of material deformation and failure modes. Cavitation is a particular soft matter failure mode that is insufficiently understood and poorly characterized. To the best of the authors’ knowledge, cavitation has only been investigated in initially stress-free samples. The lack of data for soft matter behavior under multi-axial stress states prevents the development and validation of generalized cavitation theories. This study introduces a superimposed shear cavitation (SSC) apparatus that enables an examination of the role torsional shear stress plays in cavity nucleation, expansion, and collapse in soft matter. Our proposed SSC test expands on the well-established needle-induced cavitation (NIC) experiment by housing the gel in a Taylor–Couette cell instead of a conventional beaker. This modification enables the application of azimuthal shear stresses to the gel sample prior to the insertion and pressurization of the syringe needle. To demonstrate its capability, our SSC apparatus was used to measure the critical pressure ($P_c$) of tri-block copolymer (PMMA-PnBA-PMMA) gel samples pre-loaded with various degrees of torsion. In a limited set of experiments, $P_c$ was found to generally increase with increasing amount of applied pre-shear stress, in qualitative agreement with a generalized cavitation theory first introduced by Lopez-Pamies and co-workers, thereby providing some degree of evidence that the SSC apparatus is functioning as intended. Motion tracker particles embedded in the top surface of the gel provide additional evidence that the SSC apparatus generates the intended azimuthal pre-deformation field.