Adaptive Wettability of a Programmable Metasurface

Abstract

Inspired by bionics and natural phenomena, the adaptable wettability of solid surfaces is receiving more and more interest. For example, self-cleaning of super-hydrophobic leaves, like the lotus leaf, is adapted to functional surfaces for new technologies and applications. [1-4] In general, there are two main approaches to adjust the wettability of solid surfaces: either by changing chemical composition, or by adapting the morphology of the surface. [5,6] In nature, lotus leaves achieve their unique wetting behavior using a combination of a microstructured surface as well as a functional layer of epicuticular wax. [7-9] As a result, the contact area between droplet and surface, and thus the resultant adhesive forces are reduced to achieve super-hydrophobicity and selfcleaning effects. With this, the lotus leaf is protected against fouling or contamination by microorganisms and particles, resulting in an effective increase in incident photosynthetically active radiation. [10,11] Several research groups have been working on different technical solutions to manufacture super-hydrophobic surfaces which can be used for applications like microdosing or filter systems. [1,12-14] Usually the focus is on static surface structures with certain chemical treatments or coatings. The topologies are stochastically ordered and hierarchically structured, whereas the maximum structure sizes are typically smaller than 100 μm to achieve superhydrophobicity. [7,11] Fabrication methods like two-photon polymerization allow the realization of nearly arbitrarily complex 3D geometries in this size regime. Within the metamaterial community, novel designs for artificial surfaces are getting more popular and many results of super-hydrophobic surfaces are published. [15-18] They show great utility of hierarchical structures for micro-patterning but are limited to a static surface morphology. Other groups work on functional surfaces with switchable wettability. They make use of molecular reactions at the surface, controlled by different stimuli like temperature-, [19] pH-value-, [20,21] UV-light exposure, [22,23] or electric potential change. They obtain great contact angle changes of water, from hydrophilic to super-hydrophobic, but only consider chemical effects with a static surface morphology. In addition, these chemical reactions have a certain time dependency and are rather slow (180 min, [23] 200 s [1]). Mechanical metamaterials, and especially programmable materials, represent a new possibility to change not only material properties, but also functionalities. In contrast to the common understanding in the literature, we transfer these principles to wetting phenomena and show the design and implementation of programmable adaptive metasurfaces.

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