Off-centered droplet impact on single-ridge superhydrophobic surfaces

•Off-centered droplet impact on single-ridge superhydrophobic surfaces is studied.•Six typical types of droplet morphologic evolution are identified.•A theoretical classification model is proposed to predict different types.•Contact time increases linearly with off-centered distance within a certain range. Droplets impacting on superhydrophobic surfaces with submillimetric or millimetric macrotextures can rebound completely after a short contact time, which is of practical importance in applications like self-cleaning and anti-icing. Many researches have been done on the centered impact, but the off-centered impact is rarely studied, even though it occurs more often. In the present work, we investigate the off-centered droplet impact on single-ridge superhydrophobic surfaces and focus on the morphologic evolution and contact time. We identify six typical morphologic evolution types as off-centered distance changes, with the centered impact and flat impact being the two extreme types. A theoretical classification model is proposed to predict the ranges of off-centered distance for different types and the regime maps of morphologic evolution types are presented. The effects of Weber number, off-centered distance and structural parameters on the contact time are explored. Under the current experimental conditions, the contact time increases linearly with off-centered distance when the off-centered distance exceeds a certain value, and the linear relationship is independent of Weber number and structural parameters. This work extends the fundamental knowledge of droplet impact and could be useful for related engineering applications.