Self-cleaning surfaces
The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few.
The ultimate goal in developing superhydrophobic surfaces is to recreate the self-cleaning properties of the Lotus Leaf that has the inherent ability to repel all water in nature.
Heating of the surfaces via passing current through a conductive transparent film has been shown to repel and remove contamination.
[9] The lotus leaves (Nelumbo nucifera) are water-repellent and poorly adhesive which keep them free from contamination or pollution even being immersed in dirty water.
This ability, called self-cleaning, shields the plant from dirt and pathogens and plays a vital role in providing resistance towards invading microbes.
[10] It had been a curiosity how lotus flower could remain clean even in muddy water, until German botanists, Barthlott and Neinhuis, introduced the unique dual structure of leaves with the help of a scanning electron microscopy (SEM).
On top of microscale roughness, the papillae surface is superimposed with nanoscale asperities consisting of three-dimensional (3-D) hydrophobic hydrocarbons: epicuticular waxes.
Basically, the plant cuticle is a composite material composed of a network of cutin and low surface energy waxes, designed at different hierarchical levels.
[13][14][15] The various leveled surface of lotus leaves is made out of convex cells (looks like bumps) and a much smaller layer of waxy tubules.
[17] With small tilting angles, water droplets on the leaf roll off and take any dirt or contaminant along, leading to self-cleaning.
[20] The Nepenthes carnivorous pitcher, widespread in a lot of countries such as India, Indonesia, Malaysia and Australia, possesses a superhydrophilic surface, on which wetting angle approaches to zero to create uniform water film.
The absence of wax crystals and microscopic roughness enhance the hydrophilicity and capillary forces, in doing so, water can swiftly wet the surface of rim.
If the water bead is along the radial outward (RO) direction from the body’s central axis, it rolls off and cleans the dirt away, leading to self-cleaning.
On the other hand, if droplets stand against the opposite direction, they are pinned at the surface, leading adhesion and securing the flight stability of the butterfly by preventing deposit of dirt on the wings near the center of the body.
SEM micrographs of wings exhibit hierarchy along the RO direction, arising from aligned microgrooves, covered by fine lamella-stacking nanostripes.
In 2000, Autumn et al. revealed the origin of gecko’s strong adhesion by investigating the surface features of the toes under electron microscope.
In addition to strong adhesion, the gecko foot has a unique self-cleaning property which does not require water as the lotus leaf.
Increasing the aspect ratio of the nanofibers disrupted the uniform hexagonal pattern and caused the fibers to form bundles.
However, instead of utilizing high pressure, when the temperature is raised above the Tg, capillary forces enable the polymer to fill the voids within the mold.
One study used capillarity to fill PDMS molds with PUA, first partially curing the polymer resin with UV light.
[29] For example, brucite-type cobalt hydroxide crystalline surfaces were produced by chemical bath deposition and coated with lauric acid.
These surfaces had individual nanofiber tips with diameters of 6.5 nm, ultimately resulting in a contact angle as high as 178 degrees.
[9] Refer to section on Wenzel and Cassie-Baxter models for information on the different behaviors of droplets on topographical surfaces.
Lau et al. created vertical CNT forests with a polytetrafluroethylene (PTFE) coating that was both stable and superhydrophobic with an advancing and receding contact angle of 170° and 160°.
[43] Small and large spacing shows increased drop spreading, while horizontal orientation may even display hydrophilic properties.
Glass silica beads in an epoxy resin,[44] and the electrochemical deposition of gold into dendritic structures[43] has also created synthetic biomimetic surfaces similar to lotus leaves.
[40] Biomimetic materials based on the cicadia wing have also been made from polytetrafluoroethylene films with carbon/epoxy supports treated with argon and oxygen ion beams.
[52] However, a hydrophobic surface alone does not explain the perpetually clean toe pad of the gecko, even in dry environments, where water is not available for self-cleaning.
[40] Fish scale properties have been mimicked by polyacrylamide hydrogels, which are both hydrophilic and mimic the mucus’ retention of water.
[55] These replicas have also shown that the structure of shark skin reduces the fluid drag caused by turbulent flow.
