Eich, A., Wolf, B. A., Bennett, L. and Hess, S.

"Electro- and magneto-rheology of nematic liquid crystals: Experiment and non-equilibrium molecular dynamics (NEMD) computer simulation"

*Journal-of-Chemical-Physics*
**2000**,
*113*,
3829-3838.

**Abstract: **The viscosity of the nematic liquid crystal PCH-5
[4-(trans-4'-pentylcyclohexyl)-benzonitrile], measured in the presence of an
electric field parallel to the gradient of the velocity, shows a complex dependence
both on magnitude E of the electric field and on the shear rate . When plotted
versus E2/ , all data points fall unto a master curve. In nonequilibrium molecular
dynamics computer simulations, performed for a Gay-Berne model fluid, the corresponding
influence of magnetic field on the viscosity was calculated. In both cases,
the shape of the master curve for the viscosity can be computed by considering
the competition between the torques exerted by the velocity gradient and by
the orienting field. It involves the Leslie viscosity coefficients g1 and g2,
the Miesowicz and Helfrich viscosities h1, h2 and h12. Thus it should be possible
to extract these coefficients from the data. This is straightforward in the
magnetic case since the molecules in the fluid "see" the externally
applied field. The internal electric field, however, differs from the applied
field E. When this effect is taken into account, using the known experimental
values for dielectric coefficients e|| and e^, the electro-rheological master
curve agrees very well with the experimental data. Values for the viscosity
coefficients

preprint number: 211