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.

Figure 1. Functional surface texture in nature. (a) Fog harvesting Darkling beetle [4] (b) Water harvesting in Cacti [5] (c) superhydrophobic lotus leaf [6].

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.

Figure 2. The basic elements of texture. Image adapted from [7].

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.

Figure 3. Types of nanotopology.

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.

Figure 4. Nanodimensional textures for fish-scale-like properties. (a) Top view SEM image (b) Tilted view SEM image (c) Hydrophilicity in air (d) Hydrophobicity in water. Images adapted from [12]

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 techniques, inspired by the desert beetle's surface texturing to collect water. Engineers have created biphilic nanoscale surface textures, with hydrophilic nanobumps on a superhydrophobic substrate, that can coalesce water droplets condensed from the air. These fog harvesters have shown impressive results, with a water collection rate of 349% and a heat transfer coefficient of 184%, demonstrating the potential of nanostructured biphilic topology in real-life water harvesting applications Figure 5 [13].

Figure 5. Biphilic nanostructured surface and water collector design. Images reproduced without modification from [13].

Submicron and nanodimensional topography have a significant impact on biological processes such as protein adsorption and conformation, cell behaviour, bacterial adhesion, and blood-contacting properties, with implications for materials used in microfluidic devices, biosensors, and implants [14] Figure 6.

Figure 6. Interactions between the bone and the implant surface at different topographical scales. Image reproduced without modification from [15].

Surface topography has been explored as a means of creating surfaces that are both resistant to fouling (e.g., protein, blood, or bacteria) and have improved protein uptake properties, without the need to modify the material's bulk properties or surface chemistry. The surface shape modification of Sharklet micropatterned catheter surfaces has been demonstrated to impede the colonisation and migration of uropathogenic E. coli under in vitro growth conditions, without requiring the use of chemical agents Figure 7[16].

Figure 7. Fewer E-coli growth on sharklet patterend catheters (a) than untextured catheters (n) Images reproduced without modification from [16].

Another practical application of submicron surface texture is degassing. While bubbles play a beneficial role in certain processes such as heat transfer and liquid pumping in microchannels, they can also obstruct flow and circulation, which makes them detrimental in such applications as direct methanol fuel cells and biological systems. Bubbles can also cause cavitation damage to structures. Textured surfaces can facilitate bubble nucleation and capture Figure 8. Furthermore, the increased surface area during bubble capturing leads to energy reduction and a larger interface compared to a flat surface.

Figure 8. Interactions between bubbles and various textures. Image adapted from [17].

In the field of photovoltaic energy conversion, the light-trapping capabilities of textured optical sheets have recently gained attention due to their potential to significantly reduce the thickness of active solar cell material. Various texture schemes have been proposed for the front of c-Si silicon wafer solar cells Figure 9, including the pyramidal, reverse pyramid, bipyramid, and “patch” textures. Such textures trap light effectively and offer superior optical performance, especially in narrow-band applications.

Figure 9. Nanotexturing of Si for solar cells (a) Pyramidal [18] (b) reverse pyramid (c) multicrystalline [19].

Texture plays an important role in tribology [20]. As devices get smaller, surface forces become more significant and result in permanent adhesion and high friction in micro and nanoelectromechanical systems. By altering surface roughness, including nanotexturing, adhesive and friction forces can be adjusted to meet specific requirements [21]. Nanorod and nanocomposite textures can reduce the contact area between surfaces, resulting in lower adhesive and friction forces [22]. Lubricants residual in these surfaces can assist in improving the overall friction and wear behaviour thus controlling functionality dependent on the application.

Surface texture is gaining importance in a variety of commercial applications. One such area is gloves, where textured surfaces can be designed to provide a better grip or protect against hazardous materials [23]. In the medical field, surgical tools with textured surfaces can provide greater precision and control, ultimately leading to better patient outcomes [24]. These textured surfaces offer significant commercial value as they improve the efficiency and effectiveness of the tools and products into which they are integrated.

The effects of surface texture on various properties such as mechanical, electrical, corrosion, fatigue, or adhesion have not been consolidated in review articles as yet. To better understand these effects, modelling studies are necessary to simulate the numerous combinations of structures that can be templated or created. Additionally, long-term evaluations of surface activity can change chemical processes and the numerous end products that can be derived, making it important to evaluate them in greater detail.

The opportunities are left to the imagination and emulating nature can be beneficial in many new possibilities. In addition, we can envision expanding the value of each functional surface through painstaking research evaluations and characterisation with the new tools that are available which therefore enhance the various possibilities and engineering creations.