Researchers from Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences developed a compact liquid sensor based on a liquid-filled glass cylindrical shell whose intrinsic circumferential modes were excited acoustically and subsequently detected, the circumferential resonances of this cylindrical shell could be used to sense the properties of liquid. 

According to the study published in Sensors and Actuators A: Physical, the corresponding resonant field was confined at the shell surface and enhanced the interaction between the acoustic wave and the liquid sample in the shell, thereby improves the sensitivity. 

There is a universal and desirable need for analyzing the properties of a liquid rapidly and precisely nowadays, such as food quality control, petrochemical composition analyzing and environmental monitoring. 

Phononic crystals, which are efficient at modulating the propagation and distribution of acoustic waves, have been designed as liquid sensors based on localized modes. However, the complex structure of this kind of sensors limits their portability and integration capabilities, and most sensors based on phononic crystals remain as early-stage laboratory prototypes. 

Based on previous analytical optimization, Dr. LIN Qin developed the fabricated system, which was shown in the Figure, comprised of a glass cylindrical shell with an outer radius=150.01μm, an inner radius=119.98μm, and a length=10 mm. The shell was filled with the liquid to be sensed, and the sensor sample volume approximately 0.45 μL. 

To evaluate the performance of the cylindrical-shell system, they also theoretically and experimentally investigated the transmission coefficients of mixtures of water and sodium iodide (NaI) of varying concentrations inside the shell but always pure water around the shell.  

When a plane acoustic wave of proper resonance frequency traveled through the shell filled with the liquid medium excited the circumferential resonance of the shell, the acoustic field localized around the shell surface could interacted intensively with the liquid sample, with the resonant transmission dip being strongly dependent on the acoustic properties of the liquid. Therefore, the position of the resonant transmission dip could be used to measure the acoustic properties of the liquid. 

For the future research, the cylindrical shell is disposable and compatible with other microfluidic components, could to be integrated with lab-on-a-chip devices for various microfluidic sensing applications. 

Figure. (a) Schematic illustration of the experimental set up. The glass cylindrical-shell sensor sample (White) is placed between two flat piezoelectric immersion transducers (grey). The liquid (Yellow) mixture to be sensed is injected into the shell by the injector. The environment liquid is pure distilled water (Blue). Short-pulse incident signals are generated by a pulser and receiver (5800PR), and the received signals are processed on a computer. (b) Schematic illustration of the cylindrical shell sensor in the x–y plane. (Image by Dr. LIN Qin)