Impacts of water droplets and oil-in-water emulsion droplets on superhydrophobic surfaces. (A) Schematic of experimental setup. (B) Snapshots of high-speed videos of emulsion and water droplets impacting on a surface. The emulsion is a hexadecane-in-water emulsion at a concentration of 20%. At We = 50, both droplets bounce. At We = 87, water bounces while the emulsion sticks. At We = 95, both droplets splash and bounce. (C) Schematic of water and emulsion impact dynamics showing the three phases: impregnation, oil ridge formation, and departure. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abl7160

Oil impregnation of surfaces during emulsion impacts. (A) Microscope images of surface after impacts of emulsion droplets with various concentrations on inclined superhydrophobic surfaces at We = 30. (B) Snapshots of high-speed bottom-view videos of the spreading phase of a 20% hexadecane-in-water emulsion droplet impact on a transparent superhydrophobic surface (We = 60). The focus of the lens is on the interfacial plane between the droplet and the surface. The black dots are oil droplets depositing on the surface. (C) Experimental measurements of normalized deposit diameter as a function of the Weber number for various concentrations of oil-in-water emulsions. (D) Oil coverage of the surface after impact. Symbols are experimental measurements (SD from six repeated experiments with varying We between 10 and 40), and the solid line is our model prediction. The shaded gray area shows model predictions for oil droplet radii ranging from 400 to 900 nm. The inset is a schematic of the change in shape of oil droplets when they impregnate the surface. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abl7160

Bouncing-sticking-bouncing transitions. (A) Schematic of a free-body diagram of a droplet retracting after impact, showing the pressure exerted by the atmosphere, the pressure exerted by the oil layer underneath, and the surface tension force along the contact line. (B) Snapshots of a water droplet with 10% hexadecane impacting with a Weber number of 24. The first row has photographs of the entire droplet in various stages, and the second row has zoomed-in photographs showing the oil ridge whenever visible. (C) Values of the calculated force ratio of the bouncing force to the sticking force in emulsion impacts experiments with various concentrations (left y axis) as a function of the experimental Weber number. Green symbols represent sticking droplets, while red symbols represent bouncing droplets. Line colors represent different oil concentrations. Shapes represent different instability patterns (squares for no instability, diamonds for splashing, and circles for rim instability and onset of splashing). The solid lines are model estimates of force ratios for three oil concentrations based on the derived equation for the force ratio. The dashed black line indicates a force ratio of 1, which is the theoretical transition from bouncing to sticking. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abl7160

Impregnation of surfaces by impacting emulsion droplets and effect on bouncing/sticking transition. (A) Snapshots of top-view high-speed video of a 10-cSt silicone oil in water emulsion impacting on a surface at We = 27. (B) Snapshots of top-view high-speed video of a 1000-cSt silicone oil in water emulsion impacting on a surface at We = 24. (C) Experimental impact outcomes of oil-in-water emulsions of various viscosities on superhydrophobic surfaces and of water droplets on liquid-impregnated surfaces (LIS) with lubricating oils of various viscosities. The lower solid line shows the limit below which maximal suction forces do not overcome the droplet inertia and where bouncing is expected to always occur for emulsions. The higher solid line shows the viscosity limit above which oil droplets in the emulsion do not have time to impregnate the surface during the contact time and the surface during the retraction phase diverges from an LIS-like surface. Inset: Surface coverage directly after rebound for different viscosity oils at a concentration of 10%. Other data points are silicone oils at various viscosities. The data were collected for We = 30 and We = 50, and there was no dependence of the Weber number. Error bars indicate the SD over 10 measurements for silicone oil and 6 measurements for hexadecane. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abl7160

Macroscopic spraying of emulsion on nonwetting surfaces. (A) Snapshots of high-speed video of water and emulsion (8% hexadecane in water) sprays on superhydrophobic surfaces. Spray droplets are on the order of 1 mm in diameter. Weber numbers were mostly in the 40-to-200 range. All water droplets bounce, while emulsion drops stick and accumulate on the surface (see movies S9 and S10). (B) Graphs of retained volume of sprayed liquid on superhydrophobic surface after repeatedly spraying fixed amounts of water and 20% hexadecane emulsions. Dashed lines are linear fits. The slope of the red dashed line corresponding to the emulsion case is 10 times larger than the slope of the water line. (C) Photograph of a hosta leaf after spraying the left side with water and the right side with a 20% hexadecane emulsion. The left side remains largely dry, while a film of liquid covers the right side. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abl7160