Morphology–Controlled Fe/Silicone Composite Dielectric Layers via Ultrasonic Needle-Induced Acoustic Streaming for Flexible Capacitive Sensors
Achieving precise microstructure control in composite dielectric layers remains a key challenge for enhancing the sensitivity and reducing the power consumption of flexible capacitive sensors. In this work, an ultrasonic needle-induced acoustic streaming strategy is proposed to regulate the spatial distribution of Fe particles within a silicone matrix, enabling controllable particle migration and aggregation in liquid silicone. Multiphysics simulations reveal that, at an excitation frequency of 75.49 kHz, Fe particles are effectively driven toward the ultrasonic focal region, forming a tunable microstructure. Experimental results confirm that this method enables precise morphological control of the composite dielectric layer. The composite with 25 wt% Fe exhibits the highest measured relative permittivity of about 3.45, enabling a capacitive sensor sensitivity of 0.423 kPa−1 in the 0–1 kPa range. After acoustic-streaming optimization and integration into a four-unit capacitive array, the device achieved 0.509 kPa−1 sensitivity, retained 92.04% of its response after 5000 cycles at 3 kPa, and maintained 97.8% of its initial capacitance after 24 h. The proposed approach provides an effective route to improving sensor performance through microstructure engineering while maintaining low electrical loss. This work not only advances the design of high-performance functional composites but also expands the application of acoustic streaming techniques in low-power flexible electronics.