Flow properties of nematics

The flow properties of nematic LCPs are often extraordinary and are only partially understood. Furthermore, these properties vary considerably from one LCP to another, and even a single LCP sample may behave very differently in different regimes of shear rate. Despite their complexity, there are some typical features, described in the next sections. 

T. Carlsson, Theoretical investigations of the flow properties of nematic liquid crystals , thesis, Chalmers University of Technology, Goteborg (1984). 

Conditions on the Flow Properties of Nematic-Filled Cells and Capillaries... 

In this section we study the flow properties of nematic-filled cells under shear flow. The cell geometry is important because many micro-fluidic devices are designed with channellike shapes and its mathematical treatment is simpler as compared to the case of capillaries. 

In this section we study the flow properties of nematic-fUled capillaries under the action of an electric field for two different flow conditions. In first place we treat the case of capillaries subjected to a pressure gradient and in second place we consider the case of a Couette flow. 

The flow properties of other liquid crystals, such as chiral nematics (i.e.. cholesterics). smectics C, and hexagonal phases, are even more poorly understood. 

When De 1 and molecular elasticity is negligible, the flow properties of polymeric nematics can, in principle, be described by the Leslie-Ericksen equations (see Section 10.2.3). However, at moderate and high De, the Leslie-Ericksen continuum theory fails, and a molecular theory is required to describe the effect of flow on the distribution of molecular orientations. 

Under steady shearing, these trapped disclinations should play the role of an anchoring condition, much like the role solid walls play in the flow properties of small-molecule nematics. A scaling analysis of this problem in Section 10.2.5 gives an equation, (10-28), for the steady-state shear viscosity for flow between surfaces with strong, homeotropic anchoring ... 

Sarman and Evans [24, 32] performed a comprehensive study of the flow properties of a variant of the Gay-Beme fluid. In order to make the calculations faster the Lennard-Jones core of the Gay-Beme potential was replaced by a 1/r core. This makes the potential more short ranged thereby decreasing the number of interactions and making the simulation faster. The viscosity coefficients were evaluated by EMD Green-Kubo methods both in the conventional canonical ensemble and in the fixed director ensemble. The results were cross checked by shear flow simulations. The studies covered nematic phases of both prolate ellipsoids with a length to width ratio of 3 1 and oblate ellipsoids with a length to width ratio of 1 3. The complete set of potential parameters for these model systems are given in Appendix II. 

There has also been a study of the flow properties of a version of the Gay-Berne fluid that can form smectic A liquid crystals [36]. It becomes flow unstable close to the nematic-smectic A (N-S ) transition point. This is in agreement with the theory by Brochard and Jahnig [37]. They predicted that the twist viscosity would diverge at this transition. Therefore the correlation function P (r) P"(0))g. i2 must also diverge. This means that the equality... 

Hydrodynamic instabilities 3.11.1 Homogeneous instability in shear flow The anisotropic properties of nematics give rise to certain novel instability mechanisms that are not encountered in the classical problem of hydro-dynamic instability in ordinary liquids. Theoretical work on electro-hydrodynamic instability stimulated systematic studies on two other types of convective processes, viz, thermal and hydrodynamic instabilities, and it was soon established that the basic mechanisms involved in all three cases are closely similar. " In this section we shall examine the problem of hydrodynamic instabilities in nematics. 

The flow properties of a cholesteric liquid crystal are surprisingly different from those of a nematic. Its viscosity increases by about a million times as the shear rate drops to a very low value (fig. 4.5.1). One of the difficulties in interpreting this highly non-Newtonian behaviour is the uncertainty in the wall orientation which cannot be controlled as easily as in the nematic case. Some careful measurements of the apparent viscosity // pp in Poiseuille flow have been made by Candau, Martinoty and Debeauvais of a... 

Electroconvection in nematics is certainly a prominent paradigm for nonequilibrium pattern-forming instabilities in anisotropic systems. As mentioned in the introduction, the viscous torques induced by a flow field are decisive. The flow field is caused by an induced charge density p i when the director varies in space. The electric properties of nematics with their quite low electric conductivity 10 (fl m) ] are well described within the electric quasi-static approximation, i.e. by charge conservation and Pois-... 

We shall now study the flow properties of the nematic phase. From the microscopic viewpoint, no spedal consideration may seem necessary sinoe the constitutive equation (10.75) and eqn (10.78) applies both for the isotropic and the nematic states, llns is not tte case. The rheological properties of solutions of todlike polymers are dian d entirely when the tem becomes nematic. 

Though the elastic energy equation (10.140) is important in many nonlinear flow properties of low-molecular-weight nematics, its effect is less important in polymeric nematics since the stress is usually dominated by the viscosity in polymeric nematics. 

Although polymers and wormlike micelles are very different in nature, the flow properties of their nematic phases are similar. This property was ascribed to the existence of textures at a mesoscopic scale, and to the fact that the dynamics of the textures dominate the mechanical responses of these fluids [310]. 

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