Focus: Liquid Crystal Fingers
Liquid crystals make the numbers on your digital watch, but they are also a fascination to physicists who study phase transitions. This unique class of molecules has several electrical and optical properties that vary with temperature and make it easy to monitor the various phases of liquid crystals in the lab. Most phase transitions, such as salt crystallizing out of solution, propagate with dendritic, finger-like patterns in two or three dimensions, but theorists are hard-pressed to completely describe such processes. In the 18 May PRL, a team describes a liquid crystal phase transition that propagates in one dimension at uniform temperature–the first of its kind in any system, and one that may allow deeper understanding of such growth phenomena in higher dimensions.
Most liquid crystal molecules are roughly cigar-shaped, typically five times longer than their diameter. At high temperatures the cigars are oriented randomly, but cooling down causes a series of phase transitions, as the cigars become more ordered. In the smectic C phase, the molecules are in layers in which they flow freely, liquid-like, but cannot easily move from one layer to the next. At the same time, they remain oriented at a specific tilt angle, not parallel to the layers, nor perpendicular to them. At lower temperatures some liquid crystals can enter the smectic phase, where molecules in alternate layers point in directions that differ by a 180 degree rotation about the layer normal.
Xin-Yi Wang, of the University of Akron, Charles Rosenblatt, of Case Western Reserve University, and their colleagues, studied this smectic C to CA phase transition. They started with a sample in the smectic C phase, just above the transition temperature (about 100 degrees Celsius), and suddenly brought it to a temperature below the transition. The phase change propagated as narrow, parallel “fingers,” a few µm wide, easily viewed in a microscope because of the differing optical properties of the two phases. The fingers grew at a constant speed that was essentially independent of the size of the temperature change.
Rosenblatt says the process is “different from any other [one-dimensional] phase transition in nature that we know of,” because it does not require a temperature gradient or any other special methods. The simplicity of the system allowed the authors to describe the basic properties of the finger growth with a relatively simple equation and derive the speed of propagation. They hope to further refine their model of the non-linear, non-equilibrium growth phenomenon in a way that is not possible with the theoretical complications of two or three dimensions.