43. Multifunctional Plant Surfaces and Smart Materials

The surfaces of plants represent multifunctional interfaces between the organisms and their biotic (living) and the nonbiotic solid, liquid, and gaseous environment. The diversity of plant surface structures has evolved over several hundred million years of evolution. Evolutionary processes have led to a large variety of functional plant surfaces which exhibit, for example, superhydrophobicity, self-cleaning, superhydrophilicity, and reduction of adhesion and light reflection. The primary surface of nearly all parts of land plants is the epidermis. The outer part of epidermal cells is an extracellular membrane called the cuticle. The cuticle, with its associated waxes, is a stabilization element, has a barrier function, and is responsible for various kinds of surface structuring by cuticular folding or deposition of three-dimensional wax crystals on the cuticle. Surface properties, such as superhydrophobicity, self-cleaning, reduction of adhesion and light reflection, and absorption of harmful ultraviolet (UV) radiation, are based on the existence of three-dimensional waxes. Waxes form different morphologies, such as tubules, platelets or rodlets, by self-assembly. The ability of plant waxes to self-assemble into three-dimensional nanostructures can be used to create hierarchical roughness of various kinds of surfaces. The structures and principles which nature uses to develop functional surfaces are of special interest in biomimetics. Hierarchical structures play a key role in surface wetting and are discussed in the context of superhydrophobic and self-cleaning plants and for the development of biomimetic surfaces. Superhydrophobic biomimetic surfaces are introduced and their use for self-cleaning or development of air-retaining surfaces, for, e.g., drag reduction at surfaces moving in water, are discussed. This chapter presents an overview of plant structures, combines the structural basis of plant surfaces with their functions, and introduces existing biomimetic superhydrophobic surfaces and their fabrication.