Nature represents an inexhaustible supply of solutions to many scientific and technical problems. Biological systems have evolved over billions of years to develop wetting strategies for favorable structure-performance-performance relationships that are critical to their survival. Wonderful functional behavior often arises from the interaction of building blocks on multiple length scales assembled into complex hierarchical architectures. For example, thanks to its micro/nano-textured surface and wax coating, the inner edge of the leg ruffle are waterproof and looks clean even in muddy environments.
Alfa Chemistry is committed to developing new strategies to enable the fabrication of bionic materials that mimic the multi-scale structure and function of naturally inspired sources. We focus on state-of-the-art nanofabrication technologies for creating bionic superwettability systems, with a focus on technologies with scalable production potential. Several fabrication techniques have been successfully designed for the design of exquisite heterogeneous architectures with unprecedented functionality, which in some cases equal or even exceed natural examples. The biomimetic super-wettable architecture has developed outstanding features such as self-cleaning, anti-fog, anti-fouling, anti-reflection and anti-icing properties. These superwettability-derived properties have been exploited in many applications, including photovoltaics, organic light-emitting diodes (OLEDs), cell trapping, and lab-on-a-chip devices.
Discovering Natural Surfaces
In nature, the surfaces of many plants and animals exhibit intelligent properties. These fascinating and diverse functions rely on the micro/nanoscale hierarchy of the surfaces of such organisms.
Superhydrophobic Surfaces: Lotus Leaves
The lotus leaf is probably the most intensively studied superhydrophobic surface with interesting self-cleaning properties. The lotus leaf is covered with randomly distributed micro-pillars. These micropillars are further decorated by nanoscale cilia-like structures that form a hierarchical structure. This micro/nanostructure is coated with a low surface energy wax layer. Thus the lotus leaf can suspend water droplets and form an air cushion between the droplets and the leaf surface, generating an apparent water contact angle greater than 150°; at the same time, the low surface energy wax coating reduces the adhesion of water to the lotus surface, thus ensuring a low contact angle hysteresis where the water droplets can roll off with an inclination angle of less than 5°.
Superhydrophobic surfaces with low contact angle hysteresis are also known as water-resistant surfaces. Waterproof surfaces offer great potential for the development of surfaces with anti-fog and anti-ice capabilities.
Fig 1. (a)The SEM image shows the unique micro/nanostructure of the lotus leaf with superhydrophobic surface; (b) SEM and TEM images of the surface structures of springtail skin at different magnifications. (Kong T, et al. 2019)
Omniphobic Surfaces: Springtail Skins
Superhydrophobic surfaces cannot be wetted by water, but are easily contaminated by low surface energy liquids, such as aqueous solutions containing surface-active ingredients. Superwettable surfaces with oil droplet contact angles exceeding 120° are called superhydrophobic surfaces. Since oil droplets with lower surface energy than water droplets are more difficult to suspend, superoleophobic surfaces are usually also superhydrophobic. Surfaces that cannot be wetted by any liquid, whether water-based or oil-based, are also called Omniphobic or super amphiphilic surfaces.
A particular example of an Omniphobic surface is the stratum corneum of the elasmobranch. The surprising non-wettability to all liquids is associated with interconnected concave microstructures on the skin of the elasmobranch. These re-entrant microstructures were found to be the most important feature for designing fully hydrophobic surfaces that repel all liquids. Interestingly, such re-entrant structures were also observed on the surface of protein-based nanoparticles, thus protecting against contamination.
Smooth Surfaces: Pigmented Grass
Smooth surfaces are textured surfaces covered by a thin layer of lubricant with the lowest surface energy. These surfaces should be made of fluorinated materials that can be used for lubricant injection. Unlike surfaces that use concave microstructures to repel liquids, these smooth surfaces use microscopic textures to lock in the lubricant and the lubricant layer repels the liquid. The smooth surfaces are inspired by the famous hogweed, into which insects slide and are then digested to obtain nutrients.
Fig 2. The slippery surface of the pitcher plant. (Chen H, et al. 2016)
Since the discovery of this ability, a new class of liquid-repelling surfaces with self-cleaning properties has been designed and manufactured. These surfaces, which are infused by a thin lubricating layer, are inherently smooth, defect-free, mechanically robust, and chemically inert. In addition, the lubrication layer can be recovered after physically damaging cycles. These water-resistant, smooth surfaces offer new opportunities for fluid handling instrumentation and antifouling coatings in extreme environments.
Anisotropic Wetted Surfaces: Spider Silk, Beetle Backs
Anisotropic wetting strategies have been widely used by plants and animals. Such natural surfaces often have anisotropic micro/nano textures that allow fluids to diffuse or move in one direction. For example, the mist-collecting ability of spider silk fibers is conferred by the periodic spindle knots and joints of the fibers, which exhibit varying degrees of surface micro/nanoscale roughness; the interwoven hydrophilic/hydrophobic pattern of the beetle dorsum facilitates the capture, nucleation, and transport of tiny water droplets. The mechanisms behind these micro/nanoscale structures in natural organisms provide a rich source of inspiration for the design and fabrication of anisotropic wetting materials for unidirectional fluid transport.
Dynamic Wetting Surfaces
With the development of biomimetic super wetting surfaces, stimulus-responsive functional polymers and nanoparticles have been used to create dynamic wetting surfaces that can respond to stimuli. These stimulus-responsive materials undergo significant changes in their properties when exposed to external stimuli such as temperature, pH, electrical potential, chemical potential, and light. A general strategy for creating dynamically wettable surfaces from these stimulus-responsive materials is that the wettability of the structure changes through changes in roughness in response to the stimulus.
Create Nature-Mimicking Surfaces
Alfa Chemistry focuses on micro/nanofabrication methods related to the structural features of bionanostructures to create simulated natural surfaces. These methods fall into the following three categories: random rough surfaces, explicit array-like surfaces, and explicit porous surfaces.
Fabrication of Architectures with Random Roughness
Fabrication of Micro-Array-Like Architectures
Fabrication of Porous Architectures
References
- Kong T, et al. (2019). "Bioinspired Superwettability Micro/Nanoarchitectures: Fabrications and Applications." Adv. Funct. Mater. 29: 1808012.
- Chen H, et al. (2016). "Continuous Directional Water Transport on The Peristome Surface of Nepenthes Alata." Nature. 532(7597): 85-89.
