Surface Microstructures that Create New Functionalities: Nanometer-Scale Precision Machining

Jiwang Yan (Professor, Department of Mechanical Engineering)

In our daily lives, we can find natural surfaces that exhibit various fascinating phenomena. Examples include lotus leaves that repel water and the vividly colored wings of Morpho butterflies. These surfaces appear smooth at first glance, but when magnified tens of thousands of times with an electron microscope, extremely small protrusions and depressions on the micron scale or smaller become visible. In fact, it is these microstructures that are responsible for the surface's water repellency and color.

If we could artificially create such microstructures found on natural surfaces using machining technology, it could contribute to enhancing the functionality of industrial products. In particular, creating these microstructures on artificial materials like metals, ceramics, glass, and polymers, instead of natural ones, could lead to the development of many groundbreaking products. For example, we could expect to see rust-proof iron with water-repellent properties, stain-resistant glass, decorative items that never fade, and pitch-black solar cells capable of total light absorption. Furthermore, it is believed that machining microstructures onto the surfaces of ships and airplanes could significantly reduce the fluid resistance they experience. In other words, it is no exaggeration to say that surface microstructures hold infinite possibilities.

On the other hand, machining precision is crucial when creating surface microstructures. This is because even slight errors in the shape or dimensions of the microstructure can significantly alter the surface's function. For instance, when creating an artificial colored surface, a small change in the spacing of microgrooves can cause a large shift in color. To form a uniform color, nanometer-level precision machining is required.

Among microstructures, three-dimensional structures with curved surfaces are considered particularly difficult to machine. Let's take the compound eye structure of a fly or dragonfly as an example. On the surface of a dome-shaped compound eye, about 1 millimeter in diameter, countless ommatidia (single eyes) of about 10 microns in diameter are arranged. Furthermore, some of these have a large number of hemispherical protrusions, hundreds of nanometers in diameter, arranged on the surface of each ommatidium. This complex structure not only gives the compound eye a multidirectional field of view but is also thought to prevent the eye from fogging up in humid conditions, as it possesses water-repellent properties similar to the surface of a lotus leaf. However, fabricating complex microstructures with curved surfaces, like a compound eye, requires extremely advanced machining technology with nanometer-level control accuracy.

The Yan Laboratory in the Department of Mechanical Engineering is taking on the challenge of developing technologies to efficiently machine various microstructures onto the surfaces of different materials.

Figure 1 shows the surface measurement results of a microlens array fabricated by nano-cutting as a simulated compound eye structure. Figure 2 shows an example where hydrophilic and hydrophobic properties are imparted to a copper surface by cutting microgrooves a few microns deep. In this way, we can freely control the surface wettability by changing the groove shape and depth.