Internal rupture and rapid bouncing of impacting drops induced by submillimeter-scale textures

We demonstrate an internal breakup mechanism for high Weber number drop impact on superhydrophobic surfaces uniformly patterned with submillimeter-scale textures, in which the liquid film ruptures from both interior and rim. The employment of submillimeter-scale posts could help decrease the critical Weber number of internal rupture, due to the small solid fraction and the large dimension ratio between primary structures and droplets. The internal rupture is found to promote more rapid drop bouncing than conventional rebound and rim breakup on superhydrophobic surfaces with small roughness, with a 10%–50% reduction of contact time. The internal rupture results from the film instability inside and the jet instability outside.

Figure 1

(a) Scanning Electron Microscope (SEM) image of the submillimeter-scale pillared surfaces before nanotreatment and the schematic diagram of the geometric parameters (edge length $P$, post spacing $S$, and post height $h$). (b) Schematic diagram of the experimental equipment.

Figure 2

Impact dynamics of droplets on various superhydrophobic surfaces. (a,b,c) Droplets ($\mathrm{We}=430,D_0=2\mathrm{mm},U_0=4\mathrm{m}/\mathrm{s}$) on superhydrophobic silicon substrate decorated with nanoparticles and superhydrophobic substrates patterned with submillimeter-scale posts P100S50 and P100S500. (d) Droplets ($\mathrm{We}=108,D_0=2\mathrm{mm},U_0=2\mathrm{m}/\mathrm{s}$) on superhydrophobic substrate patterned with submillimeter-scale posts P200S100. The first row in each figure indicates the selected cross-sectional high-speed images and the second row in each figure demonstrates the selected 45° top-view snapshots captured by a high-speed camera. The last snapshot in each row indicates the moment when the whole droplet bounces off the substrate. The right insets demonstrate the different breakup and bouncing mechanisms on different substrates, i.e., rim breakup, internal rupture, and pancake bouncing. The left insets show the scanning electron micrograph of various experimental surfaces. Scale bars: 2 mm.

Figure 3

Schematic diagram of the internal rupture at $\mathrm{We}=430$ on superhydrophobic substrate with submillimeter-scale posts P100S500. The insets are selected snapshots of the impact dynamics captured by a high-speed camera. The schematic diagrams demonstrate the typical rupture behaviors of the central liquid film [as marked with a blue circle in the inset in (a)] and the most powerful fourfold liquid jets [as marked with red circles in the inset in (a)]. The colliding drop evolves into a central liquid lamella with several short radial liquid jets at 1 ms. The radial liquid jets elongate and holes arise in the central liquid lamella at 2 ms. The central liquid film is completely broken into small pieces at 3 ms, which depart from the liquid jets. The liquid jets decelerate and break up eventually at 4 ms. Scale bars: 2 mm.

Figure 4

Variation of the dimensionless capillary empty time $k$ with $\sqrt{\mathrm{We}}$ corresponding to varied superhydrophobic surfaces patterned with submillimeter-scale posts with different geometric parameters. The green region corresponds to the internal rupture occurring on varied superhydrophobic surfaces with submillimeter-scale posts. The solid dots in various colors and shapes indicate the experimental results with internal rupture. The occurrence of internal rupture requires the dimensionless capillary empty time scale $k$ larger than 1.5. The open dots magnify the experimental data of pancake bouncing, the occurrence of which requires a limited value of $k$ ranging from 0.5 to 1.5. The dots with $+$ at the center reveal experimental data of conventional rebound, with rim breakup at high Weber number, locating at the range of $k$ below 0.5. The inset demonstrates the enlarged view of the distinct boundaries between each two types of bouncing patterns.

Figure 5

Comparison of drop impact dynamics on superhydrophobic surfaces with submillimeter-scale posts and cavities. (a) Selected snapshots of the collision between drops ($\mathrm{We}=118,D_0=2.2\mathrm{mm},U_0=2\mathrm{m}/\mathrm{s}$) and submillimeter-scale post substrate P200S200 captured from 45° top view. The yellow circles indicate the edges of the penetrated liquid, which shrinks with time before the retracting liquid rim. (b) Selected 45° top-view images of the collision between drops ($\mathrm{We}=118,D_0=2.2\mathrm{mm},U_0=2\mathrm{m}/\mathrm{s}$) and submillimeter-scale cavity surface C50S400. The green circles indicate the boundaries between the wetted and unwetted liquid zones, which do not change until they overlap the rim of the retracting lamella. Scale bars: 2 mm. The insets show the microscopic morphology of the superhydrophobic surfaces with submillimeter-scale posts and cavities, with scale bars indicating 200 μm. The time with the up arrow in each figure indicates the contact time when the drop departs from the surface completely.

Figure 6

Evolution of the dimensionless contact time on different kinds of superhydrophobic surfaces with one-tier or two-tier roughness (with or without submillimeter-scale posts) as a function of Weber number. The solid dots in different colors indicate the results on substrates patterned with submillimeter-scale posts with various geometric parameters. The open dots indicate the data on superhydrophobic silicon surfaces with one-tier roughness. Pink, yellow, and green indicate the partition among rebound, internal rupture, and pancake bouncing. The insets show the cross-sectional snapshots of three kinds of drop bouncing mechanism: rebound, internal rupture, and pancake bouncing, captured by a high-speed camera. Scale bars: 2 mm.