Theory of chiral modulations and fluctuations in smectic-A liquid crystals under an electric field (original) (raw)
Related papers
Physical Review Letters, 2006
We demonstrate the homogeneous and permanent reversal of the chirality of a condensed phase by an applied electric field. Tilted chiral smectic layers exhibit a coupled polarization density and molecular orientation fields which reorient about the layer normal as couple of fixed handedness in response to small applied electric fields. Experiments on some bent-core smectics show that above a threshold field the induced rotation can occur instead about the molecular long axis and that, as a result, the handedness of the phase can be flipped. The effect is quantitatively described by a nonequilibrium dissipative model of chiral smectic dynamics with anisotropic rotational viscosities.
Smectic layer instabilities in liquid crystals
Soft matter, 2015
Scientists aspire to understand the underlying physics behind the formation of instabilities in soft matter and how to manipulate them for diverse investigations, while engineers aim to design materials that inhibit or impede the nucleation and growth of these instabilities in critical applications. The present paper reviews the field-induced rotational instabilities which may occur in chiral smectic liquid-crystalline layers when subjected to an asymmetric electric field. Such instabilities destroy the so-named bookshelf geometry (in which the smectic layers are normal to the cell surfaces) and have a detrimental effect on all applications of ferroelectric liquid crystals as optical materials. The transformation of the bookshelf geometry into horizontal chevron structures (in which each layer is in a V-shaped structure), and the reorientation dynamics of these chevrons, are discussed in details with respect to the electric field conditions, the material properties and the boundary ...
A Continuum Theory of Chiral Smectic C Liquid Crystals
Siam Journal on Applied Mathematics, 2008
We formulate a nonlinear continuum theory of flow of chiral smectic C liquid crystals (C*) involving molecular director, layer order parameter, polarization vector, flow velocity, and hydrostatic pressure fields. In addition to chiral orientational ordering, smectic C* phases also present positional ordering, with molecular centers of mass arranged in one dimensional layers. The nonzero tilt angle of the molecular director with respect to the layer normal together with the chirality is responsible for the ferroelectric nature of the phase. This results in a stronger coupling with applied electric fields than the dielectric nematic. We apply the model to study the molecular reorientation dynamics in homeotropic geometry under the influence of an applied electric field. The switching process between states with opposite polarization is understood by the traveling wave solution of the system. We prove existence and uniqueness of the traveling wave and show that the predicted switching time is smaller than that when the flow effect is neglected. We also obtain bounds on the speed of switching and an optimality condition on the parameters of the problem. Numerical simulations confirm the predictions of the analysis.
Fluctuations and clinicity in tilted smectic liquid crystals
Physical Review E, 2002
The overwhelming majority of tilted smectic liquid crystals exhibit synclinic (Sm-C) ordering ͑a uniform tilt direction in all smectic layers͒ rather than anticlinic (Sm-C A) ordering ͑a tilt direction that alternates from layer to layer͒. We propose that polar molecular-scale fluctuations of the interface between smectic layers provide a general entropic mechanism favoring synclinic ordering, and present evidence from simulations of the hard spherocylinder system in support of this hypothesis. We find that the entropy of the synclinic state of L/Dϭ5 spherocylinders is higher than that of the anticlinic state for large tilt angles, and show that this entropy difference can be directly traced to molecular-scale fluctuations of the layer interface. This entropic mechanism may be suppressed in materials exhibiting anticlinic ordering due to a bent molecular conformational preference that quenches interface fluctuations.
Physical Review E, 2009
Chiral smectic liquid crystals exhibit a series of phases, including ferroelectric, antiferroelectric and ferrielectric commensurate structures as well as an incommensurate Sm − C * α phase. We carried out an extension of the phenomenological model, recently presented by M. B. Hamaneh and P. L. Taylor, based upon the distorted clock model. The salient feature of this model is that it links the appearance of new phases to a spontaneous microscopic twist : i.e. an increment α of the azimuthal angle from layer to layer. The balance between this twist and an orientational order parameter J gives the effective phase. We introduce a second orientational order parameter I which physical meaning comes from the macroscopic polarization, the effect of an applied electric is also studied. We derive new phase diagrams and correlate them to our experimental results under field showing the sequence of phases versus temperature and electric field in some compounds.
Twist disclination loops in chiral smectic C liquid crystals
Czechoslovak Journal of Physics, 1988
The notion of an infinitesimal twist disclination loop known from solids is introduced into a chiral smectic C liquid crystal with parallel layers. Finite rectangular and straight twist disclinations are described as a synthesis of a planar distribution of infinitesimal loops. Then a small impurity which disturbs only the molecular arrangement in one smectic layer is modelled by a pair of infinitesimal twist disclination loops. The interaction between a 2n-twist disclination and a small impurity is estimated using the interaction energy between a straight 2n-twist disclination and an infinitesimal twist disclination loop.
Transitions of smectic A to tilted phases in thin free standing films of liquid crystal
2011
The transition of orthogonal smectic A (SmA) phase to the tilted phases, upon lowering the temperature, is explored with a discrete phenomenological model and the phase diagrams are presented. The results show that the transition of SmA to uniplanar structures can be affected by the effect of chirality. The areas showing the uniplanar phase in the phase diagrams diminish with the increase in effect of chirality.
Field control of the surface electroclinic effect in chiral smectic-A liquid crystals
Physical Review E, 2004
The surface electroclinic effect, which causes an azimuthal deviation of the layer normal from the surface rubbing direction in cells of chiral smectic A liquid crystals, can be eliminated (and even reversed) by applying an electric field during cooling from the isotropic phase. The observed dependence on field strength confirms the notion that the surface electroclinic tilt results from an effective surface electric field and suggests a general method for controlling the azimuthal layer alignment of chiral smectic cells.
Physical Review E, 2005
The director structure around topological defects and in 2 walls in the two-dimensional orientation field of thin freely suspended films of tilted chiral smectic liquid crystal is observed to minimize splay of the spontaneous polarization. Concentric ring patterns in the director field unwind more slowly in higher polarization films. These experiments confirm that polarization space charge increases the effective elasticity of static polarization-splay distortions and that it attracts ionic charge, leading to an increase in the effective orientational viscosity of the director field.
Physical Review E, 1998
Using a high resolution optical polarimeter, we have measured the temperature dependence of the birefringence and optical rotation in chiral smectic liquid crystals that exhibit antiferroelectric, ferroelectric, and intermediate phases. The temperature dependence of the magnitude of the tilt angle was determined from the birefringence of 4-͑1-methyl-heptyloxycarbonyl-phenyl͒ 4Ј-octylbiphenyl-4-carboxylate ͑MHPOBC͒ and of 4-͑1-methylheptyloxycarbonyl-phenyl͒ 4Ј-octylcarbonyloxybiphenyl-4-carboxylate ͑MHPOCBC͒. Both substances exhibit a crossover of the order parameter exponent from the classical value of ϭ0.5 close to the transition to the tricritical value ϭ0.25 far away. This stresses the importance of the sixth order terms in the Landau free-energy expansion for ferroelectric and antiferroelectric liquid crystals. In addition, a discontinuous behavior in the magnitude of the tilt is observed when crossing the smectic-C ␣ * -smectic-C* or smectic-C* -smectic-C A * transitions, whereas the smectic-A -smectic-C ␣ * transition is continuous. The simultaneously determined optical rotation is used to elucidate the structures and the nature of phase transitions. The results are well explained within the framework of a discrete phenomenological model with nearest and next nearest neighbor interactions between the smectic layers.