Water Droplet Hammer: Piezoelectric Sensing of Ultrafast Mechanical Transients During Droplet Impact

Erosion and fracture caused by repeated water-droplet impacts are often attributed to splash-mediated material removal, high-pressure impulsive loading, or cavitation. Yet, the mechanical response during the earliest instant of impact remains difficult to capture. Using a high-temporal-resolution polyvinylidene fluoride (PVDF) piezoelectric sensor combined with high-speed imaging, we directly measure substrate-coupled mechanical transients during a single water-droplet impact. A strong impulsive strain response appears within the first 2 ms after contact, followed by coherent 100-500 Hz oscillations at the water-solid interface during the first 20 ms, before any rebound occurs. The oscillation frequency is modulated by an effective interfacial stiffness, whereas the impulsive peak-to-peak response scales with droplet size and impact velocity. The experimentally captured signals provide direct experimental evidence of ultrafast substrate-coupled mechanical transients during droplet impact and establish a minimal inertial-capillary framework for understanding mechanical signatures that may contribute to low- to moderate-speed droplet-induced erosion and fatigue.