Compressional acoustic wave generation in microdroplets of water in contact with quartz crystal resonators

Resonating quartz crystals can be used for sensing liquid properties by completely immersing one side of the crystal in a bulk liquid. The in-plane shearing motion of the crystal generates shear waves which are damped by a viscous liquid. Thus only a thin layer of fluid characterized by the penetration depth of the acoustic wave is sensed by a thickness shear mode resonator. Previous studies have shown that the finite lateral extent of the crystal results in the generation of compressional waves, which may cause deviations from the theoretical behavior predicted by a one-dimensional model. In this work, we report on a simultaneous optical and acoustic wave investigation of the quartz crystal resonator response to sessile microdroplets of water, which only wet a localized portion of the surface. The relationship between initial change in frequency and distance from the center of the crystal has been measured for the compressional wave generation regions of the crystal using 2 and 5 μl droplets. For these volumes the initial heights do not represent integer multiples of a half of the acoustic wavelength and so are not expected to initially produce compressional wave resonance. A systematic study of the acoustic response to evaporating microdroplets of water has then been recorded for droplets deposited in the compressional wave generation regions of the crystals whilst simultaneously recording the top and side views by videomicroscopy. The data are compared to theoretically expected values of droplet height for constructive acoustic interference. Results are highly reproducible and there is good correlation between theory and experiment.