Lotus effect

Overview


The lotus effect in materials science is the observed superhydrophobic (also super hydrophobic or super-hydrophobic) and self-cleaning property found with lotus plants' leaves. In some Eastern cultures, the lotus plant is a symbol of purity. Although lotuses prefer to grow in muddy rivers and lakes, the leaves and flowers remain clean. Botanists who have studied lotus leaves have found that they have a natural cleaning mechanism. This cleaning mechanism was discovered by Wilhelm Barthlott in 1982. He also holds a patent.

The microscopic structure and surface chemistry of the leaves prevent them from being wetted by liquids having a contact angle of greater than 90° to an unstructured surface of the same material. With contact angles to water of up to 170°, droplets roll off a leaf's surface like mercury, taking mud, tiny insects, and contaminants with them. This is known as superhydrophobicity, or more commonly, the lotus effect. Water droplets on taro and nasturtium leaves exhibit similar behavior.

Some nanotechnologists have developed treatments, coatings, paints, roof tiles, fabrics and other surfaces that can stay dry and clean themselves in the same way as the lotus leaf. This can usually be achieved using special fluorochemical or silicone treatments on structured surfaces or with compositions containing micro-scale particulates. Super-hydrophobic coatings comprising Teflon™ microparticles have been used on medical diagnostic slides for over 30 years. It is possible to achieve such effects by using combinations of polyethylene glycol with glucose and sucrose (or any insoluble particulate) in conjunction with a hydrophobic substance.

In one method, an aluminium surface is made superhydrophobic by immersing it in sodium hydroxide for several hours (which roughens the surface) followed by spin coating a layer of perfluorononane to a thickness of 2 nanometers. This procedure increases the water contact angle from 67° to 168°, an effect that can be explained by Cassie's law. Electron microscopy shows that the aluminium surface resembles that of a lotus surface, with a porous microstructure containing trapped air. Any surface treatment that creates micro or nano scale roughness with features having a height to width ratio greater than one that is subsequently coated with a thin hydrohobic coating will exhibit the Lotus effect.

Commercial uses
Clothing that repels water has already been developed and marketed by brands such as Gap and Dockers; it uses a fabric named Nano-Care. Water repelling glass panels have also been brought onto the market for use on the roofs of conservatories. StoCoat Lotusan is an exterior coating (paint) that mimics the microstructure of the lotus leaf surface, gaining similar water-repellent and self-cleaning properties, termed the Lotus-Effect. Water does not adhere to the surface, but rolls off the paint, picking up and washing away debris in the process. By remaining dry, the coating also resists mold, mildew, and algae. Though hydrophobic, the coating is highly permeable to water vapor. Lotus effect superhydrophobic coatings applied to microwave antennas can significantly reduce rain fade and the buildup of ice and snow.