Droplet impaction with a substrate—The critical condition for detachment

Abstract

Abstract A new experimental device was developed for the purpose of determining the critical condition associated with the release of an attached droplet from a decelerating substrate. A droplet of known size was located 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 volume was varied until the critical size for detachment was obtained. A system consisting of a water droplet in contact with a PTFE substrate was examined using substrate decelerations ranging from about 200 to 4000 m s −2 . The critical condition for detachment of “small” droplets followed the simple scaling law, α ∝ D −2 , where α is the critical deceleration and D is the droplet diameter. This result is consistent with that expected from an analysis of the drop-weight model, as used for measuring surface tension. The results are relevant to the more general problem of a free droplet impacting with a substrate.