Visualization of sonication in high-viscosity polymer melts

The dispersion of nanoparticles in polymer melts is a major challenge in composite processing, especially in highly viscous systems where conventional extrusion provides poor mixing. Concomitantly, ultrasound has proven effective to disperse nanoparticles in low-viscosity fluids through cavitation and acoustic streaming, but its role in polymer melts remains unclear. This study examines ultrasound-induced flows in polydimethylsiloxane (PDMS) of three viscosities (5, 30, and 300 Pa.s) under partially degassed and non-degassed conditions. Flow visualization and Particle Image Velocimetry (PIV) were used to characterize velocity fields generated by a sonotrode. In non-degassed samples, dense bubble clouds strongly enhanced acoustic streaming, producing velocities up to 20 mm/s at moderate input power. Degassing reduced bubble formation, leading to lower velocities (3---6 mm/s) and more stable flow patterns. At very high viscosity (300 Pa.s), velocities decreased by approximately an order of magnitude, indicating that limited bubble formation and bulk viscosity-dependent attenuation strongly influences ultrasound-induced flow. Temperature measurements and bubble-cloud evolution were consistent with a thermally influenced mechanism: however because temperature and velocity could not be measured simultaneously, no direct causal relationship was extablished. These results underline the importance of dissolved gases, viscosity and thermal evolution in acoustic streaming in viscous polymers-like media and identify key factors to consider in the development of ultrasound-assisted extrusion.