Biomedical subjects

J V Selinger

Publications and source records attributed to J V Selinger.

6 recordsLinked to original sources

PubMed2002

Acoustic realignment of nematic liquid crystals.

In a nematic liquid-crystal cell, the molecules can be realigned by an ultrasonic wave, leading to a change in the optical transmission through the cell. We present a model for this acousto-optic effect, and show that the magnitude of this effect is controlled by a director-density coupling. We then measure the optical transmission as a function of acoustic intensity for three liquid-crystal materials, and confirm that the data fit the functional form of the theoretical prediction. This fit gives the value of the director-density coupling, which varies greatly from material to material.

Journal Article↗

PubMed2001

Field-dependent tilt and birefringence of electroclinic liquid crystals: theory and experiment.

An unresolved issue in the theory of liquid crystals is the molecular basis of the electroclinic effect in the smectic-A phase. Recent x-ray scattering experiments suggest that, in a class of siloxane-containing liquid crystals, an electric field changes a state of disordered molecular tilt in random directions into a state of ordered tilt in one direction. To investigate this issue, we measure the optical tilt and birefringence of these liquid crystals as functions of field and temperature, and we develop a theory for the distribution of molecular orientations under a field. A comparison of theory and experiment confirms that these materials have a disordered distribution of molecular tilt directions that is aligned by an electric field, giving a large electroclinic effect. It also shows that the effective dipole moment, a key parameter in the theory, scales as a power law near the smectic-A--smectic-C transition.

Journal Article↗

PubMed1999

Monte Carlo simulation of smectic liquid crystals and the electroclinic effect: the role of molecular shape.

Using Monte Carlo simulation methods, we explore the role of molecular shape in the phase behavior of liquid crystals and the electroclinic effect. We study a "bent-rod" mesogen shaped like the letter Z, composed of seven soft spheres bonded rigidly together with no intramolecular degrees of freedom. For strongly angled molecules, we find that steric repulsion alone provides the driving force for a smectic-C phase, even without intermolecular dipole-dipole interactions. For weakly angled (nearly rodlike) molecules, we find a stable smectic-A (SmA) phase and a strong electroclinic effect with a saturation tilt angle of about 19 degrees. In the SmA phase we find evidence of vortexlike point defects. We also observe a field-induced nematic-smectic phase transition.

Journal Article↗

PubMed1998

Cosmic chirality.

Explore the source record for details and available documents.

Amino Acids↗

PubMed1997

Cooperative chiral order in the B-Z transition in random sequences of DNA.

We present a theory for cooperative chiral order in the transition between right-handed B-DNA and left-handed Z-DNA. This theory, based on the random-field Ising model, predicts the characteristic length scale of Z-DNA segments. This length scale depends on whether the DNA is a homopolymer or a random sequence: it is approximately 4000 nucleotides in a homopolymer but only approximately 25 nucleotides in a random sequence. These theoretical results are consistent with experiments on DNA homopolymers and random sequences.

Base Sequence↗

PubMed1996

Chiral molecular self-assembly of phospholipid tubules: a circular dichroism study.

We report on spectroscopic studies of the chiral structure in phospholipid tubules formed in mixtures of alcohol and water. Synthetic phospholipids containing diacetylenic moieties in the acyl chains self-assemble into hollow, cylindrical tubules in appropriate conditions. Circular dichroism provides a direct measure of chirality of the molecular structure. We find that the CD spectra of tubules formed in mixtures of alcohol and water depends strongly on the alcohol used and the lipid concentration. The relative spectral intensity of different circular dichroism bands correlates with the number of bilayers observed using microscopy. The results provide experimental evidence that tubule formation is based on chiral packing of the lipid molecules and that interbilayer interactions are important to the tubule structure.

Alcohols↗