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. 2020 May 22;10:8493. doi: 10.1038/s41598-020-65241-4

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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(A) The electrical impedance |Z | (red line) and phase angle φ (blue line) for the two transducers connected in parallel for the device before filling. The black lines show the corresponding impedance when water is added to the cell. The modulation of the impedance with a period of ~60 kHz, due to cavity resonance, is observed clearly. (B) The changes in the modulation seen in the impedance measurements as the volume of water is increased successively in 20 µL steps. The lowest trace corresponds to the underfilled device. The bold line corresponds to a level surface in the device as used in our measurements. For clarity the smooth unfilled cell impedance (red and blue lines in (A) has been subtracted and the measurements offset by +0.05Ω ( | Z | ) and +0.001 radians (φ) for each step. The large variations in the impedance close to the operating frequency of the device (6.77 MHz) can be observed.