Liquid Crystals
Introduction
Homogeneous, isotropic liquids have an average structure that is invariant under arbitrary rotations and translations.It has no long range order, and has the highest possible symmetry with maximum possible entropy.The crystalline state has long range translational and rotational order, with the lowest possible symmetry consistent with a regular filling of space. Between these two, there are systems which exhibit short range correlations in some directions and long range in others, and have symmetries intermediate between between those of liquids and crystals.
Among the materials that show intermediate order, the most widely studied are liquid crystals.Liquid crystals are usually made of strongly anisotropic organic molecules, either elongated (calamitic, rod-like molecules) or disk-like (discotic molecules). As a rule, the inner part of mesogenic molecules is rigid (e.g. phenyl groups) and the outer part flexible (aliphatic chains). This double character explains altogether the existence of steric interactions (between rod-like or disk-like cores) yielding orientational order and the fluidity of the mesomorphic phases. Typical examples are cyanobiphenyls and MBBA. These produce thermotropic mesophases,i.e. phases with a single component, whose phase transitions can be induced by a change in temperature.The other broad LC class is constituted by the lyotropic mesophases: they occur when anisotropic amphiphilic molecules (soaps, phospholipids, various types of surfactant molecules and biomolecules) are added to a solvent. Because amphiphiles have two distinct parts, a polar head and a non-polar tail,the building units of lyotropic phases are usually aggregates of many molecules(micelles) rather than single molecules. This microphase separation dominating the lyotropic behavior is partly present also in thermotropic LC, as for example in the smectic phases, where polar and non polar portions of the molecules form distinct alternatinig planes in the system.A typical example of lyotropics is a water solution of SDS, sodium dodecyl sulphate. For concentrations above the critical micellar concentration, cmc, these molecules form aggregates of different shapes, spherical or cylindrical micelles,bilayers, inverse cylinders, and inverse micelles.[1,2.3]
Classification of LC phases
LCs show many possible structures, which can belong to the same compound (polymorphism). There are four basic types of liquid crystalline phases, classified accordingly to the dimensionality of the translational correlations of building units: nematic (no translational correlations), smectic (1D correlation),columnar (2D) and various 3D-correlated structures, such as cubic phases.
Density Correlation and Structure Factor
A lot of information about the bulk structure of LCDs can be obtained via scattering of X-rays. let Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\boldsymbol{k}\rangle} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\boldsymbol{k}^{'}\rangle} be the incident and final plane wave state of the scattered particle with respective momenta Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hbar\boldsymbol{k}} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hbar\boldsymbol{k}^{'}} If the scattered particle interacts weakly with the scaterring medium via a sufficiently short-ranged interaction Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle U} , then by Fermi's Golden rule, the transition rate between Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\boldsymbol{k}\rangle} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\boldsymbol{k}^{'}\rangle} is proportional to the square of the matrix element,
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle M_{\boldsymbol{k},\boldsymbol{k^{'}}} = \langle\boldsymbol{k}|U|\boldsymbol{k^{'}}\rangle = \int d^{d}x e^{-i\boldsymbol{k}.\boldsymbol{x}}U(\boldsymbol{x})e^{i\boldsymbol{k^{'}}.\boldsymbol{x}} }
where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle U(\boldsymbol{x})} is the scattering potential in the coordinate representation of the scattered particle, and our plane wave states are unnormalized.
In multiparticle systems, the scattering potential is the sum of terms from individual atoms in the material:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle U(\boldsymbol{x}) = \sum_{\alpha} U_{\alpha}(\boldsymbol{x}-\boldsymbol{x^{'}})} where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{x^{'}}} is the position of the atom labeled Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \alpha} . the matrix element, therefore, is
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \langle\boldsymbol{k}|U|\boldsymbol{k^{'}}\rangle = \sum_{\alpha}\int d^{d}x e^{-i\boldsymbol{k}.\boldsymbol{x}}U_{\alpha}(\boldsymbol{x}-\boldsymbol{x_{\alpha}})e^{i\boldsymbol{k^{'}}.\boldsymbol{x}}}
To seperate the potential interaction and the interatomic correlation factors, we shift our centers to each Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x_{\alpha}} . Let Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{R}_{\alpha} = \boldsymbol{x}-\boldsymbol{x_{\alpha}}} .
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \langle\boldsymbol{k}|U|\boldsymbol{k^{'}}\rangle = \sum_{\alpha}\int d^{d}x e^{-i\boldsymbol{k}.(\boldsymbol{x}_{\alpha}+\boldsymbol{R}_{\alpha})}U_{\alpha}(\boldsymbol{R}_{\alpha})e^{i\boldsymbol{k^{'}}.(\boldsymbol{x}_{\alpha}+\boldsymbol{R}_{\alpha})} } Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle = \sum_{\alpha}[\int d^{d}x e^{-i\boldsymbol{q}.\boldsymbol{R}_{\alpha}}U_{\alpha}(\boldsymbol{R}_{\alpha})]e^{-i\boldsymbol{q}.\boldsymbol{x}_{\alpha}}} Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle =\sum_{\alpha}U_{\alpha}(\boldsymbol{q})e^{-i\boldsymbol{q}.\boldsymbol{x}_{\alpha}}} Here the scattering wave vector is Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{q} = \boldsymbol{k} - \boldsymbol{k}^{'}} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle U_{\alpha}(\boldsymbol{q})} is the atomic form factor, which is nothing but the Fourier transform of the atomic potential. The differential cross-section is proportional to the matrix element squared: Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\langle\boldsymbol{k}|U|\boldsymbol{k^{'}}\rangle|^{2} = \sum_{\alpha,\alpha^{'}}U_{\alpha}(\boldsymbol{q})U_{\alpha^{'}}^{*}(\boldsymbol{q})e^{-i\boldsymbol{q}.\boldsymbol{x}_{\alpha}}e^{i\boldsymbol{q}.\boldsymbol{x}_{\alpha^{'}}}}
If the positions of atoms are rigidly fixed,then this expression gives the exact answer. However, in real materials, the particles move around,probing large regions of phase space consistent with statistical mechanics, and we need an ensemble average of the ideal cross-section. Under ergodic hypothesis and assuming the particles are identical,
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{\mathrm{d}^{2}\sigma }{\mathrm{d} \Omega}\sim |U_{\alpha}(\boldsymbol{q})|^{2}\boldsymbol{I}(\boldsymbol{q})} where the function
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{I}(\boldsymbol{q}) = \left \langle\sum_{\alpha,\alpha^{'}}e^{-i\boldsymbol{q}.(\boldsymbol{x}_{\alpha}-\boldsymbol{x_{\alpha^{'}}})}\right \rangle} is called the structure function. As intensive version of the structure function is called the structure factor.
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S(\boldsymbol{q}) = \frac{\boldsymbol{I}(\boldsymbol{q})}{N} } or Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S(\boldsymbol{q}) = \frac{\boldsymbol{I}(\boldsymbol{q})}{V}}
Two point density correlation function and its relation with the structure factor
The number density operator is defined as Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n(\boldsymbol{x}) = \sum_{\alpha}\delta(\boldsymbol{x}-\boldsymbol{x}_{\alpha})}
The two point density density correlation function is defined as Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle C_{nn}(\boldsymbol{x}_{1},\boldsymbol{x}_{2}) = \left \langle n(\boldsymbol{x}_{1})n(\boldsymbol{x}_{2})\right \rangle} ) Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle =\left \langle \sum_{\alpha,\alpha^{'}}\delta(\boldsymbol{x}_{1}-\boldsymbol{x}_{\alpha})\delta(\boldsymbol{x}_{2}-\boldsymbol{x}_{\alpha^{'}})\right \rangle}
The structure function is simply a Fourier transform of this function:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{I}(\boldsymbol{q})= \left \langle n(\boldsymbol{q})n(-\boldsymbol{q})\right \rangle} where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n(\boldsymbol{q}) = \int d^{d}x e^{-i\boldsymbol{k}.\boldsymbol{x}}n(\boldsymbol{x} = \sum_{\alpha}e^{-i\boldsymbol{k}.\boldsymbol{x}}} is the Fourier transform of the density. In a statistical ensemble, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left \langle n(\boldsymbol{q})\right \rangle = \int d^{d}x e^{-i\boldsymbol{k}.\boldsymbol{x}}\left \langle n(\boldsymbol{x})\right \rangle = \left \langle\sum_{\alpha}e^{-i\boldsymbol{k}.\boldsymbol{x}} \right \rangle} So scattering measures density-density correlation.
Isotropic, nematic and cholesteric phases[4]
In isotropic liquids, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left \langle n(\boldsymbol{x})\right \rangle = \left \langle n \right \rangle} , a constant, so Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left \langle n(\boldsymbol{q}\right \rangle = V \left \langle n \right \rangle \delta _{q,0} } is nonzero only for Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{q} = 0} , and there is a contribution to the structure factor only for forward scattering.
In a regularly periodic crystal, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left \langle n(\boldsymbol{x})\right \rangle = \sum_{\boldsymbol{G}}\left \langle n_{\boldsymbol{G}} \right \rangle e^{-i\boldsymbol{G}.\boldsymbol{x}}} So there is a peak corresponding to each wavevector Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{G}} in the reciprocal space.
In an Isotropic LCD, the structure factor is isotropic, but shows liquid-like rings at wave numbers corresponding to the two characteristic lengths of the individual molecules-their length l and diameter a.
When the isotropic liquid is cooled, the first phase rhat condenses in the nematic (N)phase in which long molecules align so that they are on an average parallel to a particular direction specified by a unit vector Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}}
called the directorThe positions of the molecular centers of mass remains randomly distributed, like in an isotropic fluid. So the nematic phase breaks rotational invariance but maintains translational invariance. It is easy to visualize that in a statistical ensemble, rotations about Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}}
leaves the phase unchanged, while rotation in a plane perpendicular to Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}}
does not. The number density of the nematic phase has two peaks. The structure factor of the nematic phase is axially symmetric in any phase perpendicular to Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}}
but has only two-fold symmetry in any plane containing Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}}
. Two possible forms for the X-ray intensity are shown in figures ?b and ?c. In the first there are diffuse spots at Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{q}=q_{0}\boldsymbol{n}}
with Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle q_{0} = 2\pi/l}
, and in the second there are diffuse rings cetered at the same values of q.
Smectics-A and -C
As temperature is further reduced, molecules begin to segregate into planes giving rise to a smectic-A (Sm-y4) liquid crystal. The usual picture of this smectic phase is one with molecules situated in well-defined layers with a spacing that is essentially the rod length as shown in Fig. ?c. There is liquid-like motion of the rods in each layer and no correlation of the positions of the molecules from one layer to the next. In smectic-,4 liquid crystals, molecules are aligned perpendicular to the layers. The introduction of the layering indicates the presence of a mass density wave perpendicular to the layers. There is, therefore, positional correlation in the system which can be described as a sinusoidal modulation of the average molecular number-density,
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left \langle n(\boldsymbol{x})\right \rangle = n_{0} + 2n_{q_{0}}\cos({q_0}z)}
where the z-axis is along the layer normals and parallel to Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}} . From this, we get
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S(\boldsymbol{q}) = |\left \langle n_{q_{0}}\right \rangle|^{2}(2\pi)^{3}[\delta(\boldsymbol{q}_{z} - q_{0}\boldsymbol{e}_{z}) + \delta(\boldsymbol{q}_{z}+q_{0}\boldsymbol{e}_{z})]}
as can be seen in Fig?
In some systems, molecules align along an axis tilted relative to the smectic planes as shown in Fig. ?d. This is the smectic-C phase. It has a lower symmetry than the smectic-,4 phase because the tilted molecules pick out a special direction in the smectic plane, i.e., their projections in the xy-plane align,like the molecules in a nematic, along a common direction denoted by a unit vector Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{e}} , called the e-director. There are in fact transitions from the smectic-A to the smectic-C phase in which the tilt angle grows continuously from zero. The structure factor of the nematic phase just above a smectic-C phase has diffuse rings rather than diffuse spots as shown in Fig. ?c.
Hexatic Phases
When smectic-A phases are cooled, they condense into what was historically called a smectic-B phase. It is now known that a smectic-B phase can be a crystalline-B phase with a three-dimensional crystal structure and Bragg scattering at points in a three-dimensional reciprocal lattice. In some cases, however, the smectic-B phase is not a true crystal. It has hexagonal orientational order manifested by the development of a six-fold modulation in the intensity of the diffuse ring at q = 2n/a in the X-ray scattering intensity of the smectic-,4 phase. Whereas the smectic-,4 phase is invariant with respect to arbitrary rotations about Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}} ,this hexatic-B phase is invariant only with respect to rotations of 2n/6 about Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}} .
Discotic and Columnar Phases
The nematic and smectic phases just discussed are generally found in materials consisting of rod-like molecules. Plate-like rather than rod-like molecules such as those shown in Fig. ? can form discotic nematic phases, in which the plate normals are aligned, as well as phases with crystalline order in two dimensions and liquid-like order in the third direction (Fig. 2.7.11). The latter are called columnar. The plate-like molecules segregate into columns with the plate normals either parallel to or tilted at an angle to the columnar axes. There is no long-range positional order within a column. The columns themselves form any of the two-dimensional crystals.
The First Order Nematic-to-Isotropic Transition
Suppose that the liquid crystal is composed of rod-like molecules in which (i) the distribution function is cylindrically symmetric about the axis of preferred orientation n and (ii) the directions Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \boldsymbol{n}} and are fully equivalent, i.e., the preferred axis is non-polar. Subject to these two symmetry properties, and assuming the rods to be cylindrically symmetric, the simplest way of defining the degree of alignment is by the parameter s, first introduced by Tsvetkov[5]
where is the angle which the long molecular axis makes with . For perfectly parallel alignment , while for random orientations
The next task is to create an order parameter. Although it is tempting to associate the order parameter with an unit vector along the long axis direction of molecule located at .However, we note that a rotation by leaves the phase unchanged, and so the order prameter must be an even function of . So we look for the next option, which is a symmetric traceless tensor, which, as needed, yields zero when averaged over all directions
Where the unit vector n is the Frank director, and where is the angle between the molecular axis and the director n.
Now the Landau free energy must be invariant under all rotations. Since Q is a tensor, the only way, to the fourth order, is
