DROPLET IMPACTION WITH A SUBSTRATE - THE CRITICAL CONDITION FOR DETACHMENT

A new experimental device was developed for the purpose of determining the critical condition associated with the release of an attached dro plet from a decelerating substrate. A droplet of known size was locate d on the test substrate which, in turn, was located on a spring-loaded platform. Upon release of the spring, the substrate impacted into the droplet causing an advancement followed by a receding of the triphase line. Any droplet detachment, as revealed by a high-speed video, was noted. Keeping the deceleration conditions constant, the droplet volum e was varied until the critical size for detachment was obtained. A sy stem consisting of a water droplet in contact with a PTFE substrate wa s examined using substrate decelerations ranging from about 200 to 400 0 m s(-2). The critical condition for detachment of ''small'' droplets followed the simple scaling law, alpha proportional to D-2, where alp ha is the critical deceleration and D is the droplet diameter. This re sult is consistent with that expected from an analysis of the drop-wei ght model, as used for measuring surface tension. The results are rele vant to the more general problem of a free droplet impacting with a su bstrate.