High frame-rate imaging of the shape oscillations and spreading dynamics of picolitre droplets impacting on a surface

The post-impact dynamics and spreading rates of picolitre liquid droplets on surfaces are critical to many practical and industrial applications, as well as respiratory disease transmission mechanisms. Here we use a high framerate imaging method to explore the shape oscillations and spreading of individual similar to 10-180 pl volume droplets (corresponding to similar to 30-70 mu m in initial droplet diameter) impacting on a surface under ambient conditions with 10 mu s temporal resolution. The method allows the surface tension to be accurately extracted from the measured frequency of post-impact shape oscillations for sessile droplets in this size range with surface tensions greater than 40 mN m(-1) and viscosities up to at least 3.2 mPa s. The Tanner's law model for predicting sessile droplet spreading rates provides an accurate account of the "steady-state" spreading rate for all droplets with surface tensions in the range 22-73 mN m(-1) studied here. However, Tanner's law does not account for the delay in the onset of spreading observed for droplets in this size range, caused by competition between shape oscillations and spreading governing the morphology at short time periods, particularly for surface tensions >22 mN m(-1).