FIG. 7.

(a) Experimental results depicting the relationship between the acoustic streaming velocity U_dc and the modulation index m at three different modulation frequencies f_m = 5 kHz (◇), f_m = 50 kHz (△), and f_m = 100 kHz (×) in a 5 μl droplet suspended with 6 μm particles. The power is fixed at W_e = 20 mW. The dotted lines represent least square fit—R² = 0.999 (◇), R² = 1.000 (△), and R² = 0.999 (×)—and suggests that $U_{\text{dc}}\sim {10}^m$, consistent with that shown in Fig. 6. (b) Experimental results showing the relationship between the acoustic streaming velocity $U_{\text{dc}}$ and the modulation frequency f_m at two different powers W_e = 92 mW (×) and W_e = 156 mW (○) in a 5 μl droplet suspended with 6 μm particles. The case f_m = 0 represents the condition in the absence of amplitude modulation. The piezoelectric substrate is excited by 30.5 kHz SAWs. Error bars indicate a ±2 standard deviation (95% confidence level) from the mean. Trendlines in (b) were added to aid visualization.