Pretransitional anomalies in the shear flow near a second-order nematic-smectic- a phase change

Electrically driven nematic flow in microfluidic devices containing a temperature gradient

Fluid pumping principle has been developed utilizing the interaction, on the one hand, between the electric field E and the gradient ∇n[over ̂] of the director's field, and, on the other hand, between the ∇n[over ̂] and the temperature ∇T gradient arising in a homogeneously aligned liquid crystal (HALC) microfluidic channel. Calculations, based upon the nonlinear extension of the classical Ericksen-Leslie theory, with accounting the entropy balance equation, show that due to the coupling among the ∇T, ∇n,[over ̂] and E in the HALC microfluidic channel the horizontal flow v=v_{x}i[over ̂]=ui[over ̂] may be excited. The direction and magnitude of v is influenced both by the heat flux q across the microfluidic channel and the strength of the electric field E. The results of calculations showed that the dependence of the maximum value of the equilibrium velocity distribution |u_{max}(E/E_{th})| across the LC channel versus electric field E/E_{th} is characterized by maximum value at E/E_{th}=2.0. In the case when the electric field E≫E_{th}, the horizontal flow of the LC material completely stops and a novel mechanism of converting of the electric field in the form of the kinklike wave reorientation of the director field n[over ̂] can be excited in the LC channel.

Effect of electric field on the rheological and dielectric properties of a liquid crystal exhibiting nematic-to-smectic-A phase transition

We report simultaneous measurements of shear viscosity (η) and dielectric constant (ε) of octyloxy cyanobiphenyl (8OCB) in the nematic (N) and smectic-A (SmA) phases as functions of temperature and electric field. With increasing electric field η increases in the N phase whereas it decreases in the SmA phase and saturates beyond a particular field in both the phases. The flow curves in the intermediate-field range show two Newtonian regimes in the N phase. The temperature-dependent behavior of η and ε at zero or at small electric field suggests the occurrence of several structures that results from precessional motion of the director along the neutral direction as reported in similar other system. We show that the precessional motions are gradually suppressed with increasing electric field and the effective viscosity resembles with the Miesowicz viscosity η1 at high enough electric field. In the intermediate field range the temperature-dependent η exhibits anomalous behavior across the N-SmA phase transition which is attributed to the large contribution of Leslie coefficient α1.

Pretransitional anomalies in the orientational dynamics induced by temperature gradient in nematic hybrid-oriented cells

We have carried out a numerical study of the system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across a hybrid-oriented liquid crystal cell (HOLCC), where the nematic sample is confined by two parallel horizontal surfaces, a temperature gradient is applied and directed normal to the boundaries, and the temperature on the lower cooler boundary is reduced towards T(NA). Calculations show that the temperature gradient causes the investigated system to settle down to a stationary flow regime in the horizontal direction. Presmectic anomalies in the vicinity of the lower boundary have been investigated as well, taking into account the fluctuations of the local smectic order parameter above T(NA). It has been shown that these fluctuations influence both magnitude and direction of the hydrodynamic flow, produced by the temperature gradient. We have also investigated the role of hydrodynamic flow in the resulting relaxation processes, for a number of dynamic regimes in a HOLCC containing 4-cyano-4'-octylbiphenyl.

Small-angle x-ray scattering study of flow alignment of a thermotropic liquid crystal in the nematic and smectic phases

The capillary flow alignment of the thermotropic liquid crystal 4-n-octyl-4'-cyanobiphenyl in the nematic and smectic phases is investigated using time-resolved synchrotron small-angle x-ray scattering. Samples were cooled from the isotropic phase to erase prior orientation. Upon cooling through the nematic phase under Poiseuille flow in a circular capillary, a transition from the alignment of mesogens along the flow direction to the alignment of layers along the flow direction (mesogens perpendicular to flow) appears to occur continuously at the cooling rate applied. The transition is centered on a temperature at which the Leslie viscosity coefficient alpha3 changes sign. The configuration with layers aligned along the flow direction is also observed in the smectic phase. The transition in the nematic phase on cooling has previously been ascribed to an aligning-nonaligning or tumbling transition. At high flow rates there is evidence for tumbling around an average alignment of layers along the flow direction. At lower flow rates this orientation is more clearly defined. The layer alignment is ascribed to surface-induced ordering propagating into the bulk of the capillary, an observation supported by the parallel alignment of layers observed for a static sample at low temperatures in the nematic phase.