Experimental study on the evolution of microdroplets in acoustic traveling wavefields

We present an experimental study on the evolution of microdroplets in acoustic traveling wavefields. The experimental setup comprises a 4 m-long tunnel with loudspeakers installed at both ends to generate plane traveling acoustic wavefields. A humidifier produces microdroplets that enter the tunnel and are propelled by a velocity regulator. A laser and a high-speed camera were employed to observe the motion of the microdroplets. Using Gaussian filtering and the Hough transform, we identified the microdroplets and their trajectories, and derived their diameters for further statistical analysis. Eight experimental sets were conducted at two acoustic frequencies, 200 and 500 Hz, under varying acoustic pressure conditions, and we analyzed microdroplets' evolution of those different sets from number density, probability density function (PDF), and cumulative mass distribution (CMD). The results revealed the effects of the acoustic wavefield on the condensation and coalescence of microdroplets. The condensation is primarily reflected in a slight rightward shift of the PDF within the first 30 s of the acoustic action, while the coalescence is evidenced by a decrease in the number density and an increased proportion of large microdroplets in both the PDF and the CMD. Specifically, acoustic pressure influences the rate of microdroplet evolution: The higher the acoustic pressure, the faster the evolution. Frequency influences the preferred coalescence pairs. The 200 Hz acoustic wavefields promote coalescence between large (d20 μm) and small (d≤20 μm) microdroplets more easily. In comparison, the 500 Hz acoustic wavefields are more likely to facilitate coalescence between small microdroplets.