Asymmetries in the spread of drops impacting on hydrophobic micropillar arrays

Abstract

Studies of water drop impacts on microstructured surfaces are important for understanding dynamic wetting on rough surfaces, and for developing related design principles. Here, high-speed imaging has been used to study asymmetries within the spreading phase following vertical water drop impacts at Weber numbers between 34 and 167. The eleven polydimethylsiloxane surfaces studied had micropillars arranged in square and rectangular arrays, with feature sizes ranging from ∼5 μm to ∼240 μm and various pillar cross-sections, in most cases supporting a static Cassie state. Two contrasting and apparently independent asymmetries were identified. Firstly, partial (rather than full) microstructure penetration occurred on five of the surfaces, with the edges of the penetrated profiles tending to lie parallel to the array rows and columns. These observations are best explained by considering surface energies. Secondly, the perimeter of a spreading drop tends to lie at 45° to the rows and columns. This shape is caused by movement of air from underneath the impacting drop, which generates jets and subsequent fingers in preferred directions at the edge of the drop. The area of the 'corridor' through which the air escapes is an important quantitative parameter. Experiments also demonstrate the effects of microstructures on the maximum spreading diameter, and formation of off-centre microbubble patterns.

Local dissipation limits the dynamics of impacting droplets on smooth and rough substrates

“…Another parameter that can influence droplet spreading upon impact is the substrate topography 34,38 . For spontaneous spreading of droplets (V i = 0) the friction factor µ f has already been shown to rationalize spreading dynamics on rough substrates where the magnitude of µ f depends on the substrate roughness factor (S).…”

“…Droplet impact on regular micro-textured substrates 33,34,38 show that β max is influenced by the substrate topography. Even a substrate with small aspect ratio roughness hinders droplet spreading 35 , although the effect becomes less pronounced.…”

How a raindrop gets shattered on biological surfaces

“…Recently, drop impact on engineered microscale structures has been known to exhibit asymmetric spreading (11) and retraction (4) and a pancake-shaped rebouncing (12), finally leading to the rapid drop detachment with a significant decrease in the contact time (4,12). However, the role of microscale structures during drop impact was underestimated since most studies have focused on drop impacts at low speeds (4,5,13,14), much lower than real raindrop impact speeds in natural events.…”

High‐speed photography of water drop impacts on sand and soil

“…The major difference in those experiments was that the granular targets had relatively narrow size probability distribution functions, and were typically ∼ 100 μ m in size or smaller. More broadly, Equation (2) does not account for the nature of the solid surface, and has been most successful elsewhere for solid hydrophobic and partially wet surfaces (Marengo et al ., ; Josserand & Thoroddsen, ), including superhydrophobic leaves (Fritsch et al ., ) and hydrophobic polymer pillar arrays (Robson & Willmott, ). Likewise, the other capillary‐driven models tested predicted greater spreading than in our experiments, and we note that these scaling relations have usually been developed under specific experimental conditions, limiting their universality (Marengo et al ., ; Josserand & Thoroddsen, ; Wildeman et al ., ).…”