Magnetically driven superhydrophobic/slippery surfaces – MADNESS

Bio-inspired superhydrophobic surfaces have been known for more than two decades, following the work of the botanists Barthlott and Neinhuis. Superhydrophoby corresponds to hydrophobic rough surfaces that can partially encapsulate air at the interface between the liquid and the solid. This discovery has initiated numerous publications dealing with surface synthesis, characterization, and modelling. Nevertheless, there is still some controversy in our understanding of their wetting limits. Non-wetting surfaces can be classified into three main categories. The first one deals with unchanging surfaces, with rigid roughness. This corresponds to the large majority of published articles, due to the large variety of roughnesses that can induce non-wetting behavior. However, the applications are still limited, mainly because of the fragility of the nano/microstructured surfaces needed to obtain superhydrophobicity.

The second class is represented by networks of soft hairs that can additionally deform under stress. The comprehension of their wetting properties is still in its infancy. Indeed, even hydrophilic hairs can induce superhydrophobicity. The third one has been observed in Nepenthes pitcher plant and corresponds to slippery surfaces. In this case, the roughness is imbibed with oil, which considerably modifies the adhesion of water droplets or even insects. Indeed, the liquid-liquid interface reduces adhesion by drastically reducing the anchoring of the contact line. These surfaces are considered for application as anti-icing materials.

These two last categories have not been as intensively explored as the rigid superhydrophobic surfaces. Indeed, they have been observed more recently and the elastic and the liquid-liquid interfaces are more difficult to model. Thus, there are neither a global image of the non-wetting particularities of those surfaces, nor attempts to combine these two kinds of characteristics. In the MADNESS project, we aim at focusing on those interfaces by analyzing the wettability of soft, magnetic, model pillar arrays. Those surfaces can be tuned in term of Young's modulus, surface pillar densities to contribute to a fundamental understanding of wetting transitions (Cassie to Wenzel, impact, vibration) in new experiments developed for this project. Moreover, we want to introduce a magnetic stimulus to exploit the possibility to reconfigure these soft matter surfaces. In a previous study, we have evidenced the role of elasticity on wettability. Introducing magnetic reconfiguration of the surface (either by magnetic dipolar interaction between pillars or between the pillars and the magnetic oil for slippery surfaces), we want to induce transitions between isotropic to anisotropic anchoring of the contact line and analyze the wetting transitions between classical superhydrophobicity to slippery ones. These magnetic slippery surfaces will be ultimately transposed into innovative materials for which the hydrophobic oil will be confined at the pillar extremities. Doing so, we will be able to study the transition between Cassie model to slippery one on a single surface. Combining innovative wetting experiments with modelling will allow us to understand and potentially adapt these surfaces toward unexplored application.