The Dynamics of SAW Streaming and its Application to Fluid Devices
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oscillation. When the SAW amplitude at the 50 MHz with a single pulse frequency is increased, the transformation of the droplet vibration with the single frequency spectrum to 1/2 and its harmonics are observed. 2 EXPERIMENTAL The typical experimental set up for SAW streaming excited by an interdigital transducer (IDT) is shown in Fig. 1. 2.1 Observation of SAW streaming A photograph of SAW streaming is shown in Fig. 2. Here, the IDT with 10 finger pairs and an aperture width of 2 mm was fabricated on 128 rot. Y-X LiNb0 3 substrate and excited continuously at the center-frequency of 50 MHz. For the purpose of visualizing the liquid streaming, the propagation surface was coated with Al to increase the optical reflectivity. In this experiment, a little detergent was mixed in the water in order to suppress the surface tension. A water droplet with volume of about 5 g I is first put on the propagation surface by a syringe. If SAW is excited with a sufficient input power, the water begins to move in the direction of acoustic power flow. When the input power was 25 VV_,the SAW displacement without liquid loading on the surface was estimated by optical probing method to be about 5 A. The streaming velocity was about 0.2 cm/s. In the photograph one can see clearly the two parts: a thin layer of liquid where the standing waves are observed and a thick layer with a liquid height of about 0.3 mm. In the thin-layer part, the leaky SAW radiates a longitudinal wave into the liquid and decays exponentially along the propagation path, so that in the thick layer part the leaky SAW power will sufficiently decay.
It is found experimentally that the SAW streaming velocity depends on the SAW power, frequency, volume of liquid and surface-chemical condition of the substrate. The surface-chemical condition means whether the surface is hydrophobic or hydrophilic. Since an AI-metallized surface is highly hydrophilic, streaming velocity is suppressed due to the friction between the wa-
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Droplet formation
rtreaming
Fig. 1. Water streaming or droplet formation on the SAW propagation surface. 54
Fig. 2. Observed SAW streaming.
ter and the substrate surface. For hydrophobic surface condition, we used steariltrichlorosilane (CH (CH2) SiCI 3 17 3) to form a chemically bound monolayer on the surface. On the silanized surface, a stream of droplets was expelled from the liquid upward at an oblique angle. Figure 3 shows the droplet formation time dependence after exciting the input IDT at a frequency of 50 MHz with a pulse frequency of 100 Hz and SAW input power of 2 W. Observations were made with a high speed video-camera with a microscope and the shutter speed of 1/500 sec.. The water volume put on the surface was about 6 pl. Following the pulsed input acoustic energy, a mound rose up from the liquid surface shown in Fig. 3(a), and a droplet was formed progressively as shown in Fig. 3 (b), (c), and (d). In this photograph the observed droplet diameter was 0.5 mm and ejection velocity was 1.5 m/s. The droplet size, the number of droplets p
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