Creation of localized heating in complex solids at a specific location and with a controlled intensity is of great importance in pyrotechnics and explosives’ handling, as well as material processing and fabrication in general. Our predictive models of such events (e.g., impact initiation of explosions), however, are poor and our control of such events on a microscale with typical energy sources (such as laser, electric current or shockwaves) are either non-selectively destructive or limited to specific material compositions. On the other hand, ultrasound is a relatively weak delocalized energy, but one that can induce dramatic thermo-mechanical effect if localized in a small region. Examples include acoustic cavitation in liquids, ultrasonic welding with the help of energy guides in solids, and clinical hyperthermia by a focused ultrasonic beam. How ultrasound can induce intense localized heating at the interior of bulk solids, however, remains barely explored.
In this work, we have generated well-controlled hot spots in polymeric composites using ultrasound and study their dynamics with a thermal imaging microscope. We observe that ultrasonic energy spontaneously and exclusively concentrates on delaminating interfaces in such composites, with local heating rate up to 30,000 K/s on sub-mm scale, while the rest of the material remains cool and undisturbed. Hot spot locations can be precisely controlled by manipulations of interfacial adhesion, and hot spot intensity can be controlled by ultrasonic energy input. Insights from this work provide interesting possibilities for a new degree of control over the thermal events as well as chemical reactions inside composite solid structures.
University of Illinois at Urbana-Champaign
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