What are liquid crystals and why are they important?
As the name suggests, liquid crystals are materials which share some structural properties that are found in solid crystals (e.g., diamond), while at the same time displaying flow behaviour observed in classic liquids (e.g., water). They can therefore be thought of as an intermediate state of matter, existing between the solid and liquid phases (see Figure 1). In liquid crystal materials, the molecules lose the positional ordering found in solids, but not their orientational ordering (i.e., how the molecules are aligned in a sample). The orientational ordering and fluid flow in liquid crystal materials are coupled to each other, where molecular rotation can cause flow and vice versa. In classic liquids, the molecules have no positional or orientational ordering.
Liquid crystals were first discovered in 1888 by the chemist Friedrich Reinitzer, who observed that heating a sample of cholesteryl benzoate (a solid at room temperature) led to two melting points. The first of the melting points occurred at 145.5°C and led to the formation of a cloudy liquid. Upon further heating, a second melting point occurred at 178.5°C, at which the cholesteryl benzoate turned into a transparent liquid. The cloudy liquid reported by Reinitzer is now known to be a liquid crystal phase. Reinitzer sent a letter to the German physicist Otto Lehmann, along with two samples of cholesteryl benzoate, in which he requested an investigation to confirm his experimental observations. Examination of these samples led Lehmann to use the expression “flowing crystals” to describe these samples, before eventually settling on the term “liquid crystals” in 1900.
Since the invention of the first liquid crystal display in 1964 at RCA Laboratories in Princeton, New Jersey, liquid crystals have been the focus of an active and highly interdisciplinary research community, including chemists, engineers, mathematicians, and physicists. Each of these areas continue to make substantial contributions to the development of liquid crystal technologies in a wide range of applications which we make use of in our everyday lives (see Figure 2). Under current projections, the worldwide liquid crystal industry is expected to reach a market value of $1.8 billion by 2026, and currently is valued at $1.2 billion. Each year, an estimated 2 billion liquid crystal display units are sold each year across the world. This is an industry which continues to grow, with a high increase in demand for liquid crystals for the development of future generation technologies (e.g., 5G and 3D printing, and head up displays (HUDs) for car dashboards).