Ionogel Microphones Detect Underwater Sound with Directivity and Exceptional Stability

Engineering the electric double layer (EDL) next to the electrode surface as an impedance-dominating location inside an ionogel provides us an opportunity to detect sound underwater, especially sound from different directions. In response to these vector stimuli, subtle changes in the interface/EDL were easily captured by high-frequency alternating current (AC) modulations. In contrast to capacitive mechanisms under direct current (DC) operations, this AC mode generates an electric field at the interface which is orders of magnitude weaker than its DC counterpart. This removes any electrochemical reaction in the electrolytic environment, resulting in an exceptional signal-to-noise ratio (SNR) over 3000 min of continuous operations. Moreover, this ionogel-based microphone is found to be responsive to the whole range of low-frequency sounds, producing 60 dB (1000 times) stronger signals than the commercial hydrophone. Another unique feature of this microphone is its directivity even when the wavelength of the incoming sound far exceeds the size of the device, filling a property gap that affects the latest piezoelectric ceramic-based sound navigation ranging (SONARs).