107 research outputs found
Magneto-optic dynamics in a ferromagnetic nematic liquid crystal
We investigate dynamic magneto-optic effects in a ferromagnetic nematic
liquid crystal experimentally and theoretically. Experimentally we measure the
magnetization and the phase difference of the transmitted light when an
external magnetic field is applied. As a model we study the coupled dynamics of
the magnetization, M, and the director field, n, associated with the liquid
crystalline orientational order. We demonstrate that the experimentally studied
macroscopic dynamic behavior reveals the importance of a dynamic cross-coupling
between M and n. The experimental data are used to extract the value of the
dissipative cross-coupling coefficient. We also make concrete predictions about
how reversible cross-coupling terms between the magnetization and the director
could be detected experimentally by measurements of the transmitted light
intensity as well as by analyzing the azimuthal angle of the magnetization and
the director out of the plane spanned by the anchoring axis and the external
magnetic field. We derive the eigenmodes of the coupled system and study their
relaxation rates. We show that in the usual experimental set-up used for
measuring the relaxation rates of the splay-bend or twist-bend eigenmodes of a
nematic liquid crystal one expects for a ferromagnetic nematic liquid crystal a
mixture of at least two eigenmodes.Comment: 20 pages, 23 figures, 42 reference
Pattern formation in ferrogels: analysis of the Rosensweig instability using the energy method,”
Abstract We present a nonlinear description of the Rosensweig instability in isotropic magnetic gels based on the energy minimizing method used by Gailitis to describe the Rosensweig instability in typical ferrofluids. We extend his discussion to media with elastic degrees of freedom, assuming the shear modulus as a perturbation to the pure fluid case. We study the relative stability of the regular planforms of stripes, squares and hexagons as a function of the elastic shear modulus
Inverse Lehmann effects can be used as a microscopic pump
For cholesteric and chiral smectic liquid crystals a rotation of the helical superstructure can be induced for suitable boundary conditions for external fields such as temperature gradients and electric fields: The Lehmann effect. Here we predict that the inverse effect can lead to a pump for particles and ions on a length scale of microns: When a spatial pattern such as a phase winding pattern or a spiral is generated, for example, for a freely suspended smectic C * film, a concentration current arises. We also point out, that this concentration current is, under suitable experimental conditions, accompanied by a heat current and/or an electric current. Similar effects are expected for cholesterics, smectic F * and I * as well as for Langmuir monolayers, since all these systems share the property of macroscopic chirality
Macroscopic Properties of Smectic C G Liquid Crystals
Abstract. We discuss the macroscopic behavior of smectic CG liquid crystals. Smectic CG is the most general tilted smectic phase that is fluid in the layers. It is characterized by global C1 symmetry. Consequently, it is ferroelectric, pyroelectric and piezoelectric, opening up a number of possible applications for such a phase. As smectic CG-phase has a macroscopic hand due to its structure, it is a natural candidate to explain the recent experimental observations of left and right-handed helices in a system composed of achiral molecules. We also discuss critically to what extent smectic CG could be important for liquid crystalline phases formed by banana-shaped molecules. Phase transitions involving a smectic CG phase and defects of its in-plane director are briefly discussed. PAC
Influence of Sedimentation on Convective Instabilities in Colloidal Suspensions
We investigate theoretically the bifurcation scenario for colloidal
suspensions subject to a vertical temperature gradient taking into account the
effect of sedimentation. In contrast to molecular binary mixtures, here the
thermal relaxation time is much shorter than that for concentration
fluctuations. This allows for differently prepared ground states, where a
concentration profile due to sedimentation and/or the Soret effect has been
established or not. This gives rise to different linear instability behaviors,
which are manifest in the temporal evolution into the final, generally
stationary convective state. In a certain range above a rather high barometric
number there is a coexistence between the quiescent state and the stationary
convective one, allowing for a hysteretic scenario.Comment: to appear in Int. J. Bif. Chao
Convective Nonlinearity in Non-Newtonian Fluids
In the limit of infinite yield time for stresses, the hydrodynamic equations
for viscoelastic, Non-Newtonian liquids such as polymer melts must reduce to
that for solids. This piece of information suffices to uniquely determine the
nonlinear convective derivative, an ongoing point of contention in the rheology
literature.Comment: 4 page
The Amplitude Equation for the Rosensweig Instability in Magnetic Fluids and Gels
The Rosensweig instability has a special character among the frequently
discussed instabilities. One distinct property is the necessary presence of a
deformable surface, and another very important fact is, that the driving force
acts purely via the surface and shows no bulk effect. These properties make it
rather difficult to give a correct weakly nonlinear analysis. In this paper we
give a detailed derivation of the appropriate amplitude equation based on the
hydrodynamic equations emphasizing the conceptually new procedures necessary to
deal with the distinct properties mentioned above. First the deformable surface
requires a fully dynamic treatment of the instability and the observed
stationary case can be interpreted as the limiting case of a frozen-in
characteristic mode. Second, the fact that the driving force is manifest in the
boundary conditions, only, requires a considerable change in the formalism of
weakly nonlinear bifurcation theory. To obtain the amplitude equations a
combination of solubility conditions and (normal stress) boundary conditions
has to be invoked in all orders of the expansions.Comment: 46 pages; 4 figure
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