Biomedical subjects

Bing Wen

Publications and source records attributed to Bing Wen.

4 recordsLinked to original sources

PubMed2003

Continuous nematic anchoring transition due to surface-induced smectic order.

A continuous transition from tilted to homeotropic alignment at an interface is observed at a temperature T(a) for a nematic liquid crystal on cooling toward the nematic-smectic-A phase transition temperature. T(a) is found to depend on the treatment of the substrate. The behavior is examined theoretically in terms of a pair of competing easy axes (homeotropic and planar) and the tilt elasticity associated with the growth of surface-induced smectic order.

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PubMed2002

Depression of the nematic-isotropic phase transition temperature at nanopatterned surfaces.

A spatially varying herringbone pattern with an easy axis alternating in angle between 0 and psi and having a period of 200 nm was scribed into a polyimide-coated substrate. The depression of the nematic-isotropic transition temperature for a nematic layer at the patterned surface relative to its value at a uniformly rubbed surface was investigated as a function of psi for 15 < psi < 88 degrees. It was found that the depression of the transition temperature increases with psi, up to approximately 7 mK at psi=88 degrees. A simple model was developed that includes not only elasticity, but also anchoring effects at the polyimide. The model, which is used to calculate the thickness of the nematic layer, indicates that anchoring-rather than elastic-effects play the dominant role in the depression of the layer's transition temperature.

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PubMed2002

First-order Fréedericksz transition above the nematic-smectic-a phase transition.

The Fréedericksz transition is investigated for thin homeotropic cells of a negative dielectric anisotropy liquid crystal above its nematic-smectic-A phase transition temperature in a spatially uniform electric field. For very thin cells, a crossover from second- to first-order Fréedericksz behavior is observed; no additional fields are necessary. The results are consistent with the theory of Shi [Liq. Cryst. 29, 121 (2002)]].

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PubMed2002

Nematic liquid-crystal polarization gratings by modification of surface alignment.

The stylus of an atomic force microscope is used to scribe preferred directions for liquid-crystal alignment on a polyimide-coated substrate. The opposing substrate that comprises the liquid-crystal cell is rubbed unidirectionally, resulting in a twisted nematic structure associated with each micrometer-sized pixel. The polarization of light entering from the uniformly rubbed substrate rotates with the nematic director by a different amount in each pixel, and each of the two emerging polarization eigenmodes interferes separately. Two examples are discussed: a square grating that allows only odd-order diffraction peaks and a grating that combines rotation with optical retardation to simulate a blazed grating for circularly polarized light. The gratings can be electrically switched if used with semitransparent electrodes.

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