In this blog post, we’ll explore the principles behind producing single-crystal clear ice using radiative cooling, why regular ice becomes cloudy, and how this method differs from existing production technologies.
Ice has long been an essential item in modern daily life. It is used in various settings—including homes, coffee shops, cocktail bars, and ice sculpture art—and manufacturing technologies have also continued to advance. In particular, clear, transparent ice is highly valued in the premium beverage and food and beverage industries because it is visually more appealing and conveys a refreshing sensation compared to regular ice. Amid this trend, technology that uses radiative cooling to produce single-crystal transparent ice is gaining attention. In this article, before examining this method of producing transparent ice, we will first explore the causes of cloudiness in ordinary ice and the existing technologies developed to overcome this issue.
Most ordinary ice appears cloudy. The reason transparent water becomes opaque when it freezes lies in the ice formation process. Generally, water contains small amounts of dissolved air bubbles and impurities. When water begins to freeze, it cools from the edges first, and ice crystals form. During this process, air bubbles and impurities that cannot escape are pushed toward the center, where they become trapped and eventually concentrate in the core of the ice. These trapped air bubbles reflect and scatter visible light, causing the center of the ice to appear white and cloudy. Additionally, if ice forms rapidly, the crystal structure may not grow uniformly, resulting in a polycrystalline structure where crystals form in multiple directions; this structure also increases light scattering, contributing to the ice’s opacity.
As such, there are various reasons why ice becomes opaque, and manufacturing technologies to address this issue have been steadily researched. The most widely known method involves boiling water once to remove dissolved gases and then cooling it again. However, this method has the drawback of high energy consumption. Since then, various technologies for removing air bubbles have been developed, primarily by water purifier manufacturers. For example, methods such as using vibrations to remove dissolved gases during the cooling process, or slowly freezing water while spraying it—based on the principle that icicles freeze from the inside out—are currently in use. While these technologies are effective at producing ice that is more transparent than regular ice, they have had limitations in achieving the highest level of transparency through complete control of the crystal structure.
The core of the recently spotlighted single-crystal transparent ice production technology lies in slowly crystallizing the ice using radiative cooling—a method distinct from conventional cooling techniques. A single crystal refers to a state in which a single crystal structure extends continuously and uninterrupted throughout the entire sample; it is a structure of great importance in various high-tech industries, such as semiconductors and materials engineering. In other words, this technology is significant in that it goes beyond simply making ice clearer; it represents an attempt to achieve nearly perfect transparent ice by controlling even its microstructure.
Radiative cooling is a cooling principle that utilizes radiation, one of the methods of heat transfer. All objects emit infrared radiation at temperatures above absolute zero; specifically, when heat is emitted into outer space through the infrared wavelength range—which the atmosphere transmits relatively well—the object can cool to a temperature lower than that of the surrounding air. This principle is currently being actively researched in the fields of eco-friendly cooling and energy-saving technologies. Using radiative cooling, water crystallizes very slowly rather than freezing rapidly, allowing it to form large single crystals; this process significantly reduces air bubbles and crystal defects, resulting in highly transparent ice. Although research into the precise mechanisms behind the formation of transparent ice is still ongoing, the fact that radiative cooling has been applied to ice-making technology to produce single-crystal transparent ice is of great significance. The phenomenon of radiative cooling can also be observed in everyday life. Classic examples of radiative cooling include crops suffering frost damage on clear winter nights or the surface of a pond freezing even when the air temperature is high.
So far, we have examined the reasons why ordinary ice becomes cloudy, the challenges involved in producing transparent ice, and various technologies developed to overcome these challenges. We have also explored a more fundamental approach: the technology for producing single-crystal transparent ice using radiative cooling. At first glance, this might seem like a technology simply designed to make ice more transparent. However, this research demonstrates creativity by revealing new value even in water and ice—substances with which everyone is familiar. It also serves as an example of the importance of a research culture that fosters ideas that transcend conventional wisdom and integrates science and technology across various fields. It will be interesting to see what new applications emerge in the future based on radiative cooling and single-crystal control technologies.
