The effect of the contact angle and radius of a microsize droplet on the surface acoustic wave (SAW) response for microfluidic applications is reported. It is studied through the dynamic change of the droplet shape during the evaporation process. An aluminium nitride SAW device, operating at 125.7 MHz, is utilized to investigate the deformation of the droplet shape (contact angle and contact radius) caused by shrinking. The large cavity placed on the propagation path distorts the in-band SAW response one time at the centre frequency. The fractional coefficient of the SAW insertion loss, before and after dropping the liquid on the propagation path, is continuously recorded. The change in the fractional coefficient shows that the radiated acoustic kinetic energy depends on the contact area between the sessile micro-size droplet and the SAW device more than the contact angle of the droplet. Three droplet volumes have been considered, namely 0.05, 0.1 and 0.13 μl, and the electrical results show a better agreement with the theoretical data than the optical image data. The average duration of the fractional coefficient change for these cases is 420, 573 and 760 s, respectively. The effect of the hydrophobicity versus hydrophilicity of the contact surface on the duration of the fractional coefficient change is studied by coating the SAW with a silicon oxide or hexamethyldisilazane (HMDS) thin layer. For the same 0.05 μl sessile droplet on the hydrophobic surface, this duration is on average 110 s longer than that on the hydrophilic surface.

A surface-acoustic-mode aluminum nitride (AlN) transducer is utilized to determine the type of liquid dropped on the propagation path. It is based on tracking the shrinking droplet radius and observing stagnant liquid molecules during and after the liquid evaporation process. The device configuration is suitable to test small amounts of liquids, in the microliter range. According to both mass loading and physical property mechanisms, eight samples of liquids, isopropanol (IPA), ethanol (ETH), deionized-water (DW), tap water (TW), heptane (HEP), propylene glycol monomethyl ether acetate (PGMEA), hexamethyldisilazane (HMDS) and acetone (ACE), which have different equilibrium vapor pressures, molecular weights and boiling points, are accurately detected. The experimental results show that the rate of the change in the energy loss including a slow and fast attenuation region depends on the change of physical properties, such as density, sound speed in liquids and evaporation rate, during the evaporation process. As the evaporation rate of the DW is rather slow, the slow attenuation region occurs for a longer time than the fast one. Consequently, the whole oscillation duration of the attenuation occurs for a longer time, whereas that of the other liquids studied, like ACE, ETH, and IPA, having a faster evaporation rate is shorter. Sensitivities of the surface-acoustic-mode transducer to the evaporation process of liquids such as DW, TW, PGMEA, HMDS, HEP, IPA, ETH and ACE are −29.39, −29.53, −31.79, −34.12, −33.62, −32.87, −32.67, and −32.82 dB μm⁻², respectively. The concentration of stagnant liquid molecules causes a change in the surface mass of the micro-electro-mechanical transducer, which causes a frequency shift and increases the signal noise at the receiver after the liquid evaporation process. The average frequency shifts of ACE, HEP, HMDS, ETH, IPA, PGMEA, TW and DW are 241, 206, 172, 117, 76, 27.3, 11.6 and 0 kHz, respectively, coherent with the type of formed liquid pattern on the device surface, thus allowing to detect liquid samples effectively.

This paper presents the effect of an arbitrary interruption of the propagation path in Surface Acoustic Wave (SAW) microdevices on the intensity of the scattered surface waves. Using finite element modeling, simulations have been carried out to validate a new equivalent circuit based on the conventional Mason and Smith model. In addition, experimental results obtained with 30, 50 and 100 μm diameter microholes are reported. The comparison of theory, simulation and experiment proves that it is possible to fabricate an interruption like deep microcavities or microholes in the propagation path which results in an acceptable signal magnitude attenuation, but without shift in the operating frequency.