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 com pletely immersing one side of the crystal in a bulk liquid. The in-plane sh earing motion of the crystal generates shear waves which are damped by a vi scous liquid. Thus only a thin layer of fluid characterized by the penetrat ion depth of the acoustic wave is sensed by a thickness shear mode resonato r. Previous studies have shown that the finite lateral extent of the crysta l results in the generation of compressional waves, which may cause deviati ons from the theoretical behavior predicted by a one-dimensional model. In this work, we report on a simultaneous optical and acoustic wave investigat ion of the quartz crystal resonator response to sessile microdroplets of wa ter, which only wet a localized portion of the surface. The relationship be tween initial change in frequency and distance from the center of the cryst al has been measured for the compressional wave generation regions of the c rystal using 2 and 5 mul droplets. For these volumes the initial heights do not represent integer multiples of a half of the acoustic wavelength and s o are not expected to initially produce compressional wave resonance. A sys tematic study of the acoustic response to evaporating microdroplets of wate r has then been recorded for droplets deposited in the compressional wave g eneration regions of the crystals whilst simultaneously recording the top a nd side views by videomicroscopy. The data are compared to theoretically ex pected values of droplet height for constructive acoustic interference. Res ults are highly reproducible and there is good correlation between theory a nd experiment. (C) 2001 American Institute of Physics.