Interactions H.I.F.U. / polymer films

In order to obtain the modification of a defined volume of a polymer layer, preliminary tests of sonication (HIFU 0.75 and 3 MHz) were carried out to obtain an accurate characterization of the acoustic fields as well as convective flow velocities measured close to the surface by Particle Image Velocimetry and luminol mapping. Then, sonication of a delimited zone of an acrylic resin deposited upon a stainless steel thin substrate highlighted a strong dependence of the polymerization or reticulation degree, with different exposure times and different power thresholds for the same final modification. The point was to examine the ability of the thin polymer film to absorb ultrasonic energy or to act as a perfect reflector which induces asymmetric cavitation and therefore violent collapses to take place close to the surface. In the first case, the wave absorption by the film is converted into heat and then acts for the polymer alteration under the high convective flow velocities. For the highest acoustic intensities and for the hardest films, cavitation occurs and the produced microbubbles oscillated and grow close to the surface. Then, the shocks resulting of the bubble collapse eroded mechanically the film.