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The coupling acoustic matching is investigated to realize acoustic sensors with a microfluidic channel, in which two overlap layers couple three local quarter-wavelength transmission elements. Provided that the reflecting waves from both sides of each overlap layer satisfy destructive interferences, most acoustic energy can be transmitted. Especially, the microfluidic channel is sensitive to realize a function of the acoustic matching, which could induce effects of an approximate half-wavelength transmission and much acoustic dissipation. With an optimized design, the detections of liquid properties with a high quality factor 300 in liquid environment, and a mass sensitivity of 95 kHz mm2/ng are obtained.

An acoustic sensor with composited transmission and reflecting structure is investigated for detection in liquid medium, in which a temperature compensational layer is deposited on the Si wafer to create a low temperature coefficient of frequency. This type of structure regulates most acoustic energy into the substrate, i.e., Si wafer and the compensational layer. Consequently, the standing resonant vibrations in the substrate are induced, as its thickness is integer multiples of a half wavelength. The acoustic sensor shows a low frequency shift with temperature, and the temperature coefficient of frequency can be controlled to about zero at the room temperature. Additionally, the acoustic sensor for the liquid detection on the viscosity and mass can be estimated 35 kHz/Pa · s and 23.25 kHz cm2/ng, respectively.

We report acoustic sensors for liquid detection by designing a composite structure. By controlling the substrate thickness and the wavelengths are at the same order, the longitudinal wave energy is regulated and the peak values of the quality factor Q up to 1270 are realized. Meanwhile, part of the energy is regulated by two pairs of optimized Bragg reflecting structures in which the functions of the liquid layer are enhanced and Q becomes more stable relative to the thickness of the micro-channel. These features present conveniences for fabrications and give high sensitivities in liquid environments.