Superhydrophobic coatings are used to separate surfaces from the surrounding liquid by trapping an air layer called plastron in its microstructures. The durability and therefore the performance of these surfaces lie in the capacity to retain this plastron, which might be destabilized by excessive stresses or dissolution. Plastron monitoring is therefore essential to quantify the amount of gas shielding the surface from the liquid and to determine its temporal evolution. Such air layers, which may blend multiple length scales ranging from nm up to cm, are challenging to probe. In this paper, we develop a versatile technique based on the acoustic resonance of the plastron, which allows passive probing of the total volume of air trapped in the surface roughness. Validation of this technique is performed by comparing the mean acoustic thickness against direct optical observations and shows very good agreement. As this acoustic method uses portable transducers and is sensitive to the air layer's volume regardless of the superhydrophobic surface, we foresee that it can pave the way toward plastron monitoring under more realistic conditions such as complex geometries in different flow regimes. Additionally, this technique is shown to efficiently monitor plastron on timescales of a few milliseconds, enabling real-time characterization necessary to better assess destabilization mechanisms leading to the collapse of a plastron.

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