Impacting dynamics of nanodroplets on superhydrophobic surfaces decorated by a ridge

Reducing the contact time of impacting nanodroplets is of fundamental interest and importance due to its promising potential in various engineering applications, such as self-cleaning and anti-icing. In this work, nanodroplets impacting superhydrophobic surfaces decorated by a rectangular ridge are studied over a wide range of Weber number (We) and different sizes of the ridge via molecular dynamics. Six bouncing modes have been distinguished. The usage of the rectangular ridge significantly enhances the bouncing performance of nanodroplets, including enlarging the We range for bouncing and reducing the contact time. It is especially found that the rectangular ridge leads to an increased contact time compared with the flat surface in the moderate We region. The underlying mechanism of the differential effect of the rectangular ridge on the contact time is compressively elucidated by the retraction dynamics of distinct bouncing modes. Subsequently, the retraction models considering different retraction characteristics are developed for the unsplit case. To promote a universal understanding of the effect of the ridge on the retraction dynamics of nanodroplets, several typical ridge shapes (triangular, cylinder, and semi-circular) are further considered. The retraction models are extended and verified to consider the effect of the ridge shape by introducing the equivalent structure parameters. Based on such a universal understanding, a newly designed shoot-shaped ridge is proposed and can considerably reduce the contact time of unsplit nanodroplets in the moderate We region.