Brookhaven Soft Condensed Matter Physics

A subset of condensed matter, soft matter encompasses polymers, certain types of organic and biological matter, liquid crystals, and other self-assembled organic materials. These systems often have structural order that is intermediate between normal solids and liquids. The underlying physics and behavior of these materials is relevant to developing improved electronic displays, molecular electronics (tiny circuits based on single molecules), and novel biomaterials.

An example of this research is the study of “surface freezing,” the phenomenon by which the top molecular layer of a material—as thin as a single molecular layer—freezes at a temperature at which the underlying bulk remains liquid. This is a surprising result because in most materials, the opposite phenomenon, surface melting, is observed. Surface freezing occurs in wax-like materials called alkanes, which are composed of hydrocarbon molecules in simple linear chains. Alkanes are major components in oil, fuels, polymers, and lubricants. By shining x-rays at grazing angles on a puddle of alkanes, the crystalline structure of the surface-frozen, molecule-thick layer is readily distinguished from the underlying liquid. To date, the theoretical physical laws that underlie surface freezing are still unresolved.

One objective of the soft matter program at Brookhaven is to understand the behavior of ultra-thin organic films on solid and liquid surfaces. Liquid surfaces have properties that make them ideal substrates. Using x-rays to study films on a liquid mercury surface, for example, reveals that the molecules in the film typically progress with increasing coverage from a lying-down phase to a standing-up phase, often with an intermediate tilted phase. The specific type of organic molecule in the film affects the phase sequence and also how the film will crystallize. These specific structural and chemical details are important for understanding the electronic properties of thin films, an important step for developing applications in molecular electronics.

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Last Modified: January 4, 2006