Milind P. Mahajan

Ultrahigh-resolution liquid crystal display with gray scale

An atomic force microscope was used to write planar alignment patterns on a polyimide-coated glass substrate. Paired with a substrate treated for homeotropic alignment, the resulting hybrid liquid crystal cell produced fixed gray scale images with pixel sizes of order 1 μm. The physics and efficacy of this architecture are compared to a cell having planar alignment at both substrates.

Correlation between rub-induced grooves in a polyimide-treated substrate and microstructure of rubbing fiber: An atomic force microscopy study

Rubbed polyimide surfaces, which are used for liquid crystal alignment, generally exhibit microscopic grooves which lie parallel to the rubbing direction. Using atomic force microscopy we examined both the grooves and the fibers that create the grooves. We find that for a wide range of rubbing strengths, the microstructure of the grooves, as determined by their radii of curvature, correlates well with the microscopic topography of the fibers. This result indicates that the rubbing-induced topography depends on not only the characteristic rubbing strength, but on the structure of the rubbing fiber as well.

History-dependent orientational order of rubbed polyimide for liquid-crystal alignment

A polyimide film that was spin coated onto a glass substrate was multiply rubbed along different directions and studied using ellipsometry and atomic force microscopy. The data show a minimum required rubbing strength for the onset of orientational order in the polyimide. When over rubbed along an axis perpendicular to the first rubbing direction, a smaller rubbing strength was required for the onset of order along this direction. This behavior indicates that the polyimide had been partially disentangled by the initial rubbing, needing only weaker rubbing to be reoriented by the second rubbing.

Liquid crystal bridges

The liquid crystalline material octylcyanobiphenyl was studied in the form of bridges spanning the space between two solid supports in an immiscible water bath. In the nematic phase the bridge collapses above a certain length-to-diameter ratio, consistent with the behaviour of ordinary Newtonian liquid bridges. The smectic A phase, however, exhibited the formation of very long, stable columns as a consequence of its non-Newtonian behaviour.

Paramagnetic liquid bridge in a gravity-compensating magnetic field

Magnetic levitation was used to stabilize cylindrical columns of a paramagnetic liquid in air between two solid supports. The maximum achievable length to diameter ratio Rmax was ∼(3.10±0.07), very close to the Rayleigh–Plateau limit of π. For smaller R, the stability of the column was measured as a function of the Bond number, which could be continuously varied by adjusting the strength of the magnetic field.

Stability of liquid crystalline bridges

The stability of cylindrical bridges of the liquid crystal octylcyanobiphenyl in an immiscible liquid bath was investigated in the nematic and smectic A phases. In the nematic phase the bridge was found to destabilize at a length-to-diameter (slenderness) ratio R similar to that of ordinary Newtonian fluids. On the other hand, the Bingham behavior of the smectic A phase, i.e., an apparent yield stress, enabled the formation of stable columns with R well in excess of π.

Magnetic levitation of liquid crystals

The principle of magnetic levitation is demonstrated using a large magnetic field gradient to elevate a polycrystalline sample of dodecyloxycyanobiphenyl against gravity. Additionally, a nematic droplet of pentylcyanobiphenyl clinging to a vertically oriented wire is elevated against gravity. The contact angle and length of the droplet are extracted from the droplet shape in the context of a gravitation-free model.

Ultrafast, electric field-induced fingers in an anticlinic liquid crystal

Propagating fingers of synclinic liquid crystalline phase were observed to invade the anticlinic phase for applied electric fields E larger than a characteristic threshold field Eth. The front velocity was found to be highly non-linear in E, with enormous velocities of at least 10 cm s-1, and perhaps as high as 400 cm s-1 for the maximum applied field. These are by far the largest velocities ever observed for a liquid crystal. The results are discussed theoretically, including the possibilities of a field-dependent molecular interaction coefficient and shear thinning.