University of Minnesota
School of Physics & Astronomy


Liquid crystals

C. C. Huang
C. C. Huang
Alex Schumann

For the past ten years, Cheng-Cher “C.C” Huang and his collaborators have been exploring properties of liquid crystals with a technique called resonant x-ray scattering. This is the most reliable and effective method for getting to the specific liquid crystal states that Huang and his group are researching. Liquid crystals, a phase of matter between a liquid and a solid, have been used in technologies from high-resolution camera viewfinders to large-area “liquid crystal displays” used in monitors

Resonant x-ray scattering is the most reliable and effective method of studying liquid crystals, but it requires the use of synchrotron radiation at Brookhaven National Laboratory. Because of the group’s success with this method they have been given a prime slot at the lab which amounts to 10 days a year. “If we do not do preliminary studies,” Huang says, “we won’t get any physics out of that time.”

In order to optimize their results at Brookhaven, Huang and his group have come up with alternative methods of exploring liquid crystals in his laboratory in-house. Huang has developed a series of optical probes, which are elaborate laser/refractivity experiments set up on large stability tables in his labs. The light moves through various optical elements striking the sample to provide information. In one set up there is no light emanating through the sample. This is called null transmission ellipsometry and the goal is to find out the effect of the film the orientation of polarized light and how the film reacts to two simultaneous stresses. From this experiment Huang and his group can learn about the molecular packing of a free standing film.

In another set-up light moves through a polarizer, then through the sample of liquid crystal, and is collected and recorded by a CCD camera in a process is called depolarized reflected light microscopy. The sample of liquid crystal looks like a simple soap bubble on a slide. Huang uses the soap bubble analogy to explain liquid crystals since the structure is similar, but the components are different. In a soap bubble as in a liquid crystal there is movement similar to a liquid, but there is also a crystalline structure which makes the shape of the bubble. Liquid crystals of course are not soap and water, but are unique materials manufactured to exhibit certain properties. Huang is investigating the orientational order of liquid crystals because that is an important area with many unknown variables. As liquid crystal devices become more widespread, the need for understanding all phases and their reaction to electrical field, heat, light, etc. becomes more important.

Huang explains that the optical probes in their lab have been so accurate that they probably could publish their results based only on those techniques, but that the group still prefers to use the x-ray scattering technique at the Brookhaven synchrotron to verify their findings.

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