Investigations on the sound field between waveguide and counterpart induced by high-intensity focused ultrasound in thin polymer films

The use of high-intensity focused ultrasound (HIFU) to heat polymer films is getting more in focus of interest in the industry due to some advantages e.g. low damages of thin polymer films during film processing or cost reduction of the processing tool. The capability to heat thin polymer films using a solid waveguide transducer was already reported in the literature. However, some significant hurdles persist: the physical principles responsible for the heating of thin polymer films are not fully understood. This study aims to investigate the sound field occurring in thin polymer films between a solid waveguide transducer (having a resonance frequency of 1.1 MHz) and a solid counterpart and compared it to the sound field in mm-thick polymer sample. An analogy between the sound field measured in water and the sound field inside polymer films during the heating process is done. The sound field between counterpart and solid waveguide is analyzed by means of membrane hydrophone sound field measurements and finite element simulations. The impact of the transducer-counterpart gap size is studied. This investigation reveals that the behavior and characteristics of the sound field in water in a transducer-counterpart gap differ considerably from each other depending on the gap size. When the gap is smaller than a wavelength, the sound cannot be focused within the waveguide-counterpart gap and complex interferences occur inside the small gap. Furthermore, the investigation showed that similarities occur between the sound field measured in water and the sound field calculated in polymer. These findings are relevant for the understanding of HIFU-based processes involving thin polymer films.