Droplet shape oscillations: Effects on spray impaction and retention and Fourier analysis of their natural frequencies

In-flight shape oscillations for 111 to 383-mu m diameter spray droplets we re observed under moderate dynamic conditions using high-speed videography. Fourier analysis of oscillations determined droplet natural frequencies of 1.5 to 9 kHz, results which agreed with a capillary-oscillation model. In addition, an ellipsoid model was employed to estimate accompanying changes in droplet surface areas during flight Results suggest droplet oscillations cart limit surfactant adsorption at liquid-air and liquid solid interfaces such that droplets will arrive at a plant surface without the desired lowe ring of surface tension being achieved. Consequently, full benefits of a su rfactant in reducing droplet reflection and promoting retention will not be realized. However droplet shape oscillations can strengthen fluid motions within droplets, with some increase in rates of surfactant transfer to the interface. Results also showed the high natural frequencies characteristic of droplet sizes below 200 mu m will impose limits on cinematography or vid eography when attempting to observe droplet distortions or identify energy dissipation pathways at droplet impact.