UC Riverside

The physical behavior of a single droplet impacting a surface is one of the most fascinating facets of spray research. Under some conditions, a droplet will strike and spread across a solid surface without splashing or rebounding. That droplet will spread and recoil for some time, oscillating between a disk and a hemisphere until these fluctuations diminish due to viscous damping. These oscillations affect the liquid coverage area and are essential in droplet solidification applications; yet little is known about them; Knowing more will, for example, enable higher-precision three-dimensional printing or enhanced droplet and spray cooling. Using mixtures of water and glycerol, oscillations of droplets with kinematic viscosities between 1.0 × 10-6and 1.1 × 10-4m2=s are explored, focusing on the damping behavior. Several impact substrates were used. Droplets freefall onto the target with velocities of 0.5-1.5 m/s. TheWeber number of the droplets ranged from 10 to 100 and the Reynolds number from 15 to 4000. The impact velocity, spreading lamella diameter, and thickness at the center of each droplet were measured. Droplet kinematic viscosity, impact velocity, and surface tension effects are found to play a role in oscillations, which occur at approximately 75-90 Hz. For the liquids tested, a hydrophilic surface thins the droplet, arresting oscillations quickly, whereas a hydrophobic surface sustains oscillations. Correspondingly, a highly viscous droplet tends to stop oscillations sooner than a less viscous droplet. Increasing the velocity of impact restricts oscillations by spreading liquid across a larger area. For the range of conditions studied, viscosity dominates droplet oscillations when compared to the surface effects. We explore the interplay between viscous and surface tension effects in the oscillations. The spring constant and damping coefficient of an analogous harmonic system are calculated for the observed droplet oscillations. The tested liquid droplets generally exhibit underdamped behavior; higher damping coefficients are associated with more wetting and more viscous droplet liquids - a 103increase in viscosity corresponds with an approximately 101increase in damping. The spring constant appears to be influenced by the droplet composition and the surface wettability in a less trivial manner, with similar magnitudes and no discernable pattern in the spring constant (1.5-3.0 N/m) for all droplets and surface conditions examined.