Functional surfaces through texture management

Surfaces define the outer boundaries of an object and interact with the surrounding medium in a multitude of ways. Surface texture, defined as “the local deviation of a surface from a perfectly flat plane “[1], is a crucial determinant of the functionalities of various materials, be they natural or man-made. In nature, surface texture has evolved to meet the diverse survival needs of living organisms. For instance, Darkling beetles Figure 1(a) and some types of cacti Figure 1(b) that inhabit desert environments possess specialised bumps, grooves, or 3D hierarchical structures on their body surfaces, which condense water from the air [2,3]. The surface texture of the lotus leaf Figure 1(c) is the most cited example of hydrophobic surfaces, bordering on being a cliché. Manmade surface textures can be perceived as nominal or actual. The nominal surface refers to the intended contour of the surface, while the actual surface is determined by the manufacturing processes used to create it [5]. Surface texture is typically categorised into roughness, waviness, lay, and flaws Figure 2. Roughness is determined by the characteristics of the materials and processes used to form the surface and manifests as small, finely-spaced deviations from the nominal surface. Waviness, on the other hand, consists of much larger deviations caused by factors such as work deflexion, vibration, and heat treatment. Roughness is typically superimposed on waviness. The lay of the surface texture refers to the predominant direction or pattern of the surface, while flaws are irregularities that occur occasionally on the surface, such as cracks, scratches, and inclusions. Although flaws are related to surface texture, they also affect surface integrity. The texture of a surface can contribute to aesthetics, safety, assembly, and functionality. For example, the shininess or dullness of a surface can impact its perceived aesthetic value, while the surface’s mechanical properties, absorption, friction and wear, corrosion and wear behaviour, adhesion, and electrical and thermal conductivity, can affect its overall functionality. Smooth surfaces are better suited for electrical contacts, while rough surfaces are better suited for water repellency and friction like in brakes. The texture is the single driving cause for the presence or absence of friction between mating surfaces. As early as the 18th century, Bernard Forest de Bélidor recognised that friction arises from the numerous hemispherical peaks and valleys on the mating surfaces, a concept furthered by Coulomb in his exposition of lubrication [8]. There are various categories of texturing methods, including addition, removal, displacement of material, and self-forming methods [9]. The most common industrial texturing processes such as shot blasting, milling, grinding, etching, lithography, laser methods, and manual polishing fall under the removal category. Replica methods such as master printing and microcontact printing, and 3D additive manufacturing are addition-based methods. Each of these techniques has its unique advantages and disadvantages, and the selection of the appropriate method depends on the type of surface, the desired texture, and the required accuracy. For example, shot blasting is ideal for creating a rough texture on a metal surface, while etching is useful for producing precise patterns on glass. All these methods are industrially available and the economies of scale have been achieved for specific functionalities and applications Additive manufacturing methods have enabled the production of complex and multi-scale materials with intricate submicron and nano-dimensional architectures, mimicking nature more closely than ever before [10]. Two-photon lithography (TPL), for example, can produce three-dimensional (3D) structures with submicrometer resolution of any complexity [10]. By controlling the shape, size, and distribution of surface features at the submicron and nanoscale Figure 3, it is now possible to create surfaces with unprecedented capabilities, such as tailorable superhydrophobicity [11], enhanced tribological performance, and increased biocompatibility. Femtosecond laser microfabrication has recently allowed for the nanotexturing of solid surfaces to exhibit superhydrophilicity in air and superoleophobicity underwater, mimicking fish scales Figure 4. Some of the applications of biomimetic submicron topology engineering have gone beyond academic interest and offer real-world solutions. One such example is the development of fog harvesting