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

Abstract

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 characterised 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 0 crystal using 2μl and 5μl 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 is 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. PACS 43.35 Bf

Evaporation of binary mixtures and precision measurement by crystal resonator

“…The reflective wave interacts with the emitting wave constructively and destructively, resulting in resonant spikes. Each spike appears at an incremental film thickness that is multiples of the half wave length of the acoustic wave in liquid [17,35,8,7,9]. The longitudinal wave creates the spikes on the resonant frequency shift (Fig.…”

“…[2,3,5,4,6]. A few investigations have shown that evaporation kinetics of pure [7][8][9] and complex fluids including suspensions [11,10] and polymer solutions [12] can also be measured by crystal resonators. However, theoretical modeling and data interpretation on multi-component systems are only at the beginning stage.…”

Nonspecific binding removal from protein microarrays using thickness shear mode resonators

“…Quartz crystal resonators (QCRs) are commercially available and commonly used in the microelectronics industry. Routinely, resonators have been used as ultra-sensitive mass detectors, and are typically referred to as quartz crystal microbalances [4]. We demonstrate the ability of compact, reliable quartz crystal resonators to remove nonspecific binding, and improve fluorescent biosensor signal accuracy.…”

Adjustment and Measurement of Contact Angle with Electrowetting on a Quartz-Crystal Microbalance

“…in which r e is the radius of the electrode, K is the maximum sensitivity at the center of the resonator that can be deduced from the experiment on covering the electrode completely with a liquid of known density and viscosity, and β defines the sensitivity dependence on r (Figure 1b), which was empirically decided as 2-2.15 [30,31]. Different from a QCM entirely covered with one liquid (Equations (5) and (6)), sensitivity S is important when measuring the position of the moving boundary of the droplet and a medium driven by EWOD.…”