Showing papers by "John Bechhoefer published in 1991"

Pattern Formation During the Growth of Liquid Crystal Phases

Abstract: Liquid crystals, discovered just a century ago, have wide application to electrooptic displays and thermography. Their physical properties have also made them fascinating materials for more fundamental research.The name “liquid crystals” is actually a misnomer for what are more properly termed “mesophases,” that is, phases having symmetries intermediate between ordinary solids and liquids. There are three major classes of liquid crystals: nematics, smectics, and columnar mesophases. In nematics, although there is no correlation between positions of the rodlike molecules, the molecules tend to lie parallel along a common axis, labeled by a unit vector (or director) n. Smectics are more ordered. The molecules are also rodlike and are in layers. Different subtypes of smectics (labeled, for historical reasons, smectic A, smectic B,…) have layers that are more or less organized. In the smectic A phase, the layers are fluid and can glide easily over each other. In the smectic B phase, the layers have hexagonal ordering and strong interlayer correlations. Indeed, the smectic B phase is more a highly anisotropic plastic crystal than it is a liquid crystal. Finally, columnar mesophases are obtained with disklike molecules. These molecules can stack up in columns which are themselves organized in a two-dimensional array. There is no positional correlation between molecules in one column and molecules in the other columns.

Critical Behavior of Crystal Growth Velocity

Abstract: Within a phase-field model that couples a nonconserved order parameter to the temperature field, we study the crystal growth velocity υ in a supercooled liquid as a function of the undercooling Δ. The velocity shows critical behavior in Δ depending on a the ratio p of effective diffusion constants and on δ, which measures the coupling between the order parameter and the temperature field. As Δ becomes smaller, there is a transition from steady-state growth to a diffusive regime. At the phase boundary, υ remains nonzero for p>pc; otherwise, it vanishes as (Δ - Δc)ν with ν = 1 for p

Shape Measurements of Faceted Smectic A Droplets

Abstract: We have measured profiles of smectic A droplets in air as a function of temperature. As the droplet is cooled below the nematic-smectic A transition temperature TNA, the facet radius is proportional to (TNA−T)α, with α = 0.45 ± 0.1, an exponent consistent with that measured for the layer compression modulus B. While the relaxation time for shape changes upon cooling is less than one minute, that for heating ranges from hours to days, depending on (TNA−T). An estimate of the energy barrier to nucleating new layers suggests that that process is forbidden and that another explanation of the relaxation-rate asymmetry must be found.