FIG. 6.

Results from the numerical simulation showing the relationship between the average acoustic streaming velocity $U_{\text{dc}}={(u_{\text{dc}}^2+w_{\text{dc}}^2)}^{1/2}$ and the modulation index m at three different heights in the droplet: y ≈ 9.2λ_f (◇), 18.4λ_f (○), and 27.6λ_f (×) (see Fig. 2). The modulation frequency is fixed at f_m = 100 kHz and m = 0 represents the case where amplitude modulation is absent. The dotted lines represent least square fit—R² = 1.000 (◇), R² = 0.999 (○), and R² = 0.996 (×)—and suggests that U_dc ∼ 10 ^m.