--- trunk/tengDissertation/LiquidCrystal.tex 2006/06/04 20:18:07 2786 +++ trunk/tengDissertation/LiquidCrystal.tex 2006/06/22 22:19:02 2880 @@ -38,7 +38,7 @@ Fig.~\cite{LCFig:SMCP}). packing arrangements (two conglomerates and two macroscopic racemates), which depend on the tilt direction and the polar direction of the molecule in adjacent layer (see -Fig.~\cite{LCFig:SMCP}). +Fig.~\ref{LCFig:SMCP}). \begin{figure} \centering @@ -184,11 +184,11 @@ ratio between \textit{end-to-end} well depth $\epsilon \caption[]{} \label{LCFig:BananaMolecule} \end{figure} -\begin{figure} -\centering -\includegraphics[width=\linewidth]{bananGB.eps} -\caption[]{} \label{LCFigure:BananaGB} -\end{figure} +%\begin{figure} +%\centering +%\includegraphics[width=\linewidth]{bananGB.eps} +%\caption[]{} \label{LCFigure:BananaGB} +%\end{figure} \begin{figure} \centering @@ -208,6 +208,94 @@ where $\epsilon _{fs}$ is the permittivity of free spa \end{equation} where $\epsilon _{fs}$ is the permittivity of free space. -\section{\label{liquidCrystalSection:methods}Methods} +\section{Computational Methodology} -\section{\label{liquidCrystalSection:resultDiscussion}Results and Discussion} +A series of molecular dynamics simulations were perform to study the +phase behavior of banana shaped liquid crystals. In each simulation, +every banana shaped molecule has been represented by three GB +particles which is characterized by $\mu = 1,~ \nu = 2, +~\epsilon_{e}/\epsilon_{s} = 1/5$ and $\sigma_{e}/\sigma_{s} = 3$. +All of the simulations begin with same equilibrated isotropic +configuration where 1024 molecules without dipoles were confined in +a $160\times 160 \times 120$ box. After the dipolar interactions are +switched on, 2~ns NPTi cooling run with themostat of 2~ps and +barostat of 50~ps were used to equilibrate the system to desired +temperature and pressure. + +\subsection{Order Parameters} + +To investigate the phase structure of the model liquid crystal, we +calculated various order parameters and correlation functions. +Particulary, the $P_2$ order parameter allows us to estimate average +alignment along the director axis $Z$ which can be identified from +the largest eigen value obtained by diagonalizing the order +parameter tensor +\begin{equation} +\overleftrightarrow{\mathsf{Q}} = \frac{1}{N}\sum_i^N % + \begin{pmatrix} % + u_{ix}u_{ix}-\frac{1}{3} & u_{ix}u_{iy} & u_{ix}u_{iz} \\ + u_{iy}u_{ix} & u_{iy}u_{iy}-\frac{1}{3} & u_{iy}u_{iz} \\ + u_{iz}u_{ix} & u_{iz}u_{iy} & u_{iz}u_{iz}-\frac{1}{3} % + \end{pmatrix}, +\label{lipidEq:po1} +\end{equation} +where the $u_{i\alpha}$ is the $\alpha$ element of the unit vector +$\mathbf{\hat{u}}_i$, and the sum over $i$ averages over the whole +collection of unit vectors. The $P_2$ order parameter for uniaxial +phase is then simply given by +\begin{equation} +\langle P_2 \rangle = \frac{3}{2}\lambda_{\text{max}}. +\label{lipidEq:po3} +\end{equation} +In addition to the $P_2$ order parameter, $ R_{2,2}^2$ order +parameter for biaxial phase is introduced to describe the ordering +in the plane orthogonal to the director by +\begin{equation} +R_{2,2}^2 = \frac{1}{4}\left\langle {(x_i \cdot X)^2 - (x_i \cdot +Y)^2 - (y_i \cdot X)^2 + (y_i \cdot Y)^2 } \right\rangle +\end{equation} +where $X$, $Y$ and $Z$ are axis of the director frame. + +\subsection{Structure Properties} + +It is more important to show the density correlation along the +director +\begin{equation} +g(z) =< \delta (z-z_{ij})>_{ij} / \pi R^{2} \rho +\end{equation}, +where $z_{ij} = r_{ij} \dot Z$ was measured in the director frame +and $R$ is the radius of the cylindrical sampling region. + +\subsection{Rotational Invariants} + +As a useful set of correlation functions to describe +position-orientation correlation, rotation invariants were first +applied in a spherical symmetric system to study x-ray and light +scatting\cite{Blum1971}. Latterly, expansion of the orientation pair +correlation in terms of rotation invariant for molecules of +arbitrary shape was introduce by Stone\cite{Stone1978} and adopted +by other researchers in liquid crystal studies\cite{Berardi2000}. + +\begin{eqnarray} +S_{22}^{220} (r) & = & \frac{1}{{4\sqrt 5 }}\left\langle {\delta (r +- r_{ij} )((\hat x_i \cdot \hat x_j )^2 - (\hat x_i \cdot \hat +y_j )^2 - (\hat y_i \cdot \hat x_j )^2 + (\hat y_i \cdot \hat +y_j +)^2 ) \right.\\ +& & \left.- 2(\hat x_i \cdot \hat y_j )(\hat y_i \cdot \hat x_j ) - +2(\hat x_i \cdot \hat x_j )(\hat y_i \cdot \hat y_j ))} +\right\rangle +\end{eqnarray} + +\begin{equation} +S_{00}^{221} (r) = - \frac{{\sqrt 3 }}{{\sqrt {10} }}\left\langle +{\delta (r - r_{ij} )((\hat z_i \cdot \hat z_j )(\hat z_i \cdot +\hat z_j \times \hat r_{ij} ))} \right\rangle +\end{equation} + +\section{Results and Conclusion} +\label{sec:results and conclusion} + +To investigate the molecular organization behavior due to different +dipolar orientation and position with respect to the center of the +molecule,