Biomedical subjects

ME Neubert

Publications and source records attributed to ME Neubert.

18 recordsLinked to original sources

PubMed2000

Freedericksz transition in an anticlinic liquid crystal

It is shown experimentally that a very-long-pitch, surface-stabilized, anticlinic liquid crystal undergoes a two-step electric-field-induced transition to the synclinic phase. The liquid crystal remains undistorted below a threshold field E(th). For E>E(th), a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal. At higher fields a transition to the synclinic phase occurs via solitary waves.

Journal Article↗

PubMed2000

Bend elasticity of mixtures of V-shaped molecules in ordinary nematogens

Freedericksz measurements were performed on mixtures of a nematic mesogen and a V-shaped molecule. The bend elastic constant was found to decrease significantly with increasing concentration of the V-shaped molecule. The results are discussed theoretically, taking into account the detailed structure of the molecules.

Journal Article↗

PubMed2000

Structural study of the smectic-I to smectic-F transition in freely suspended films

The smectic-I (S(I)) to smectic-F (S(F)) phase transition in terephthal-bis-(4n)-decylaniline (TB10A) has been examined for a possible continuous transition via the intermediate smectic-L (S(L)) phase. X-ray diffraction measurements of thick, single-domain, freely suspended films are used to classify the phases and to determine the hexatic order parameter C6 and its harmonics. Instead of the continuous transition suggested in the literature we find a first-order S(I)-->S(F) transition with a discontinuous change in the direction of the bond-orientational order relative to the molecular tilt. The tilt of the molecular form factor in the hexatic phases is inconsistent with the tilt estimated from published layer spacing measurements, suggesting that the hexatic phases of TB10A must have the molecular cores oriented at an angle relative to the tails. This result taken together with published results on the S(C) phase suggests that the S(C)-->S(I)-->S(F) transitions are driven by changes in the conformation of the hydrocarbon tails. Examination of the harmonic scaling relation between the hexatic order parameters shows mean-field behavior in the S(I) phase. This result will make binary mixtures of TB10A with other materials a practical system for the study of the crossover from mean field to the XY behavior seen in other hexatic systems.

Journal Article↗

PubMed2000

Freedericksz transition in an anticlinic liquid crystal

The Freedericksz geometry is used to show experimentally that a very-long-pitch, surface stabilized, anticlinic liquid crystal undergoes a two-step electric-field-induced transition to the synclinic phase. The liquid crystal remains undistorted below the threshold field E(th). For E>E(th), a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal, resulting in a nonzero polarization that couples to the applied field. Measurements of E(th) as a function of temperature are reported. Related quasielastic light scattering measurements demonstrate that acoustic Goldstone mode fluctuations are quenched by a dc electric field E>E(th). At high fields a transition to the synclinic phase occurs via solitary waves.

Journal Article↗

PubMed2000

Electric field-induced acoustic-optic mode coupling in an anticlinic liquid crystal

A dc electric field was applied perpendicular to the tilt plane of a pitch-compensated (unwound helix) anticlinic liquid crystal. By means of quasielastic light scattering, the field was found to couple the acoustic and optic Goldstone modes, resulting in an increase of the relaxation time tau(beta) of the acousticlike eigenmode. Elastic constants were estimated from the relaxation time data.

Journal Article↗

PubMed1995