This blog post focuses on explaining the principles, manufacturing methods, advantages and disadvantages, and everyday applications of anti-fingerprint films that utilize the “lotus leaf effect.”
The Lotus Leaf Effect and Its Principles
As society continues to evolve, daily life has become so inconvenient without a cell phone. With just a single cell phone, you can contact friends, surf the internet, play games, and use most computer functions. However, in the past, touching a cell phone screen left fingerprints, requiring periodic cleaning—a real hassle. Recently, “anti-fingerprint films” made from fingerprint-resistant materials have been developed to alleviate this inconvenience.
The core principle behind this fingerprint-resistant film is the “lotus leaf effect.” The phenomenon where water droplets form on a lotus leaf without wetting the surface is due to the countless nano-scale protrusions on its surface. These nano-protrusions cause water droplets to gather rather than spread out when surface tension acts upon them. Here, surface tension is the force that seeks to minimize surface area, serving to reduce the contact area with other substances. A related example is the phenomenon where a toothpick placed on water does not sink; this is because the surface tension between molecules on the liquid’s surface maintains their close packing, leaving no gap for the toothpick to enter the water.
Furthermore, nano-protrusions possess superhydrophobic properties and repel water. The angle between the edge of a water droplet and the underlying surface is called the contact angle. If the contact angle is less than 90 degrees and the water spreads out widely, the surface is hydrophilic; if it is greater than 90 degrees and the water forms a spherical shape, the surface is hydrophobic. On a superhydrophobic surface formed by nano-protrusions, the contact angle is very large, causing water to cluster into small droplets.
Manufacturing and Applications of Anti-Fingerprint Films
By utilizing this lotus leaf effect, special nanoparticles can be coated onto the surface of a smartphone film to create a nano-pillar structure. This significantly reduces the actual contact area between the surface and contaminants or fingerprints, preventing fingerprints from sticking. TiO2 particles are primarily used in anti-fingerprint films, and the “low-temperature spray coating method” is employed to coat these particles onto the film surface.
The low-temperature spray coating method is a technique that utilizes supersonic airflow generated by compression and expansion to adhere the powder to the substrate using the energy released when it collides with the surface. This method allows for coating at room temperature, preventing deformation or degradation of the material, and is widely used because it improves wear resistance, fatigue resistance, heat resistance, and corrosion resistance.
However, fingerprint-resistant films have one drawback because they involve coating the surface with nanoparticles. The nano-protrusions and uneven surface structure can cause light to scatter, potentially reducing screen visibility. To address this issue, technologies are being developed that allow for the application of nanoparticles while maintaining a clear screen view. For example, Toray Industries in Japan is attempting to solve the problem of reduced visibility by utilizing a nano-textured structure but increasing the density of the particles to create a nearly smooth surface, thereby enhancing light transmittance.
The lotus leaf effect is utilized in various fields beyond fingerprint-resistant films. Examples include waterproof clothing and paper that do not get wet, nano-foams sprayed on car windows during rain to easily shed water, and non-stick frying pans treated to prevent food from burning or sticking—all of which are applications of this effect.
In other words, materials engineering is a field dedicated to developing and widely utilizing new materials to resolve inconveniences in daily life. Just as a tree branches out from a single root, materials engineering gives rise to numerous applications from a single key principle, serving as the foundation for other scientific disciplines.
