When a liquid drops impinges a hydrophobic rough surface it can either bounce off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel droplets). The analysis of drop impact and quasi static "loading" experiments on model microfabricated surfaces allows to clearly identify the forces hindering the impalement transitions. A simple semi-quantitative model is proposed to account for the observed relation between the surface topography and the robustness of fakir non-wetting states. Motivated by potential applications in microfluidics and in the fabrication of self cleaning surfaces, we finally propose some guidelines to design robust superhydrophobic surfaces.Some plants leaves and insects shells exhibit extreme hydrophobicity, making the deposition of water drops on their surface almost impossible [1]. All these superhydrophobic biosurfaces share two common features: they are made of (or covered by) hydrophobic materials, and are structured at the micron and sub-micron scales.During the last decade much effort has been devoted to design artificial solid surfaces with comparable water-repellent properties. Their potential applications range from lab on a chip devices to self cleaning coating for clothes, glasses,... The actual strategy consist in mimicking superhydrophobic biosurfaces designing rough substrates out of hydrophobic materials. To achieve this goal both top-down and bottom-up approaches have been successfully developed: chemical synthesis of fractal surfaces [2], growth of carbon nanotube forests [3], deep silicon dry etching [4], see also [5] and references therein. We briefly recall the paradigm to account for superhydrophobicity. Two different wetting states can be observed on microstructured hydrophobic surfaces: (i) Wenzel state: the liquid follows the topography of the solid surface. Defining the surface roughness ζ as the ratio between the total surface area over the apparent surface area, the equilibrium contact angle of a liquid drop is given by cos(θ) = ζ cos(θ flat ),
The rheology near jamming of a suspension of soft colloidal spheres is studied using a custom microfluidic rheometer that provides stress versus strain rate over many decades. We find nonNewtonian behavior below the jamming concentration and yield stress behavior above it. The data may be collapsed onto two branches with critical scaling exponents that agree with expectations based on Hertzian contacts and viscous drag. These results support the conclusion that jamming is similar to a critical phase transition, but with interaction-dependent exponents.
Adhesion at polydimethylsiloxane (PDMS)-acrylic adhesive interfaces is shown to be enhanced through micropatterning of the PDMS substrate. By varying the geometry of the patterns (groves and hexagonal arrays of pillars of micrometer sizes, obtained through soft lithography techniques) and comparing rigid and deformable substrates, the respective roles of the geometry and the size and flexibility of the pattern features on the level of adhesion have been analyzed. For cylindrical pillars, two regimes are clearly identified: for a relatively low aspect ratio (h/r < 3, with h and r, respectively, the height and the radius of the pillars), soft patterned substrates are more efficient than rigid ones at increasing adhesion, pointing out the role of the elastic energy associated with the deformation of the pattern that is lost when the adhesive detaches from the substrate. Using scaling laws, the predominant contribution to that elastic energy can be further identified: deformation of the substrate underlying the pillars for h/r < 1.6 or bending of the pillars for h/r > 1.6.; for a high aspect ratio (h/r > 3), only rigid patterned substrates enhance adhesion, then the only possible contribution to energy dissipation comes from the enhanced viscoelastic losses associated with the pattern that induce modifications of the strain field within the adhesive layer. Soft, high aspect ratio patterns lose their efficiency even if still bent under the effect of the peel forces. This is because when bent, some of the pillars touch each other and remain stuck together, lying flat on the surface after the passage of the peel front. The bending elastic energy of the pillars (which is still lost) is then balanced by the corresponding gain in surface energy of the substrate in the peeled region. These systematic experiments demonstrate that the ability of the patterned surface to be deformed plays a crucial role in enhancing adhesion and allow us to propose a way to fine tune the level of adhesion at PDMS-acrylic adhesive interfaces, independently of the chemistry of the adhesive.
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