Pattern formation in the splay Freedericks transition of a nematic side-group polysiloxane

Abstract

Periodic director patterns in the magnetic-field induced splay Freedericks transition of a nematic side-group polysiloxane are reported. For this purpose liquid crystal cells (10 μm-500 μm) are studied by polarization microscopy as well as by deuteron NMR. Through this combinatin, the optically observed spatial dependence of the director can be quantitatively analyzed in terms of director distributions extracted from the NMR lineshape. In the equilibrium state of the Freedericks transition (static Freedericks effect), the director exhibits a one-dimensional periodicity perpendicular to the initial director orientation n0. The dynamics of the Freedericks transition involves a transient two-dimensional director pattern, representing convection rolls in which the nonlinear coupling between director rotation and viscous flow of the nematic (back-flow) leads to a reduction of the viscosity.

Instabilities in nematic elastomers in external electric and magnetic fields

“…The investigations have been performed by means of a linearized continuum model, and as a result, laterally homogeneous Frederiks-like instabilities as well as qualitatively different laterally undulated instabilities have been found. Whereas undulations in liquid crystalline polymers exposed to an external electric or magnetic field have already been observed and studied in the splay geometry (see, e.g., [26,27]), the undulatory instabilities found and investigated in reference [17] and in this paper are qualitatively different and to our knowledge have not been noted elsewhere for nematic SCLSCEs before. They are static and directly result from the coupling of the liquid crystalline subsystem to the elastic one, or, in other words, from the coupling of the macroscopic mesogenic orientational behavior to the macroscopic elastic behavior of the polymer network.…”

“…The case of a LMW nematic liquid crystal is included in this expression; we obtain it by taking the limits D 1 → 0 and D 2 → 0 in (21) and in abbreviations (11)- (16), (22)- (27). This way we recover the classical bend Frederiks instability with a critical wave vector q c = (q x,c , q z,c ) = (0, ± [11].…”

Deuterium Nuclear Magnetic Resonance Study of the Nematic Phase of Vanadium Pentoxide Aqueous Suspensions

“…In fact, for any initial angle Ω 0 > 60°, the reorientation takes place through a transient hydrodynamic instability which creates "zigzag" domains as was observed by polarized light microscopy. 9 This results in a distribution of nematic directors which is reflected in the increase of the half-height line width 15,19 (Figure 6, insert). The theoretical description of this instability is rather involved since elastic terms also need to be considered.…”

Linear analysis of pattern formation in nematics in oblique magnetic fields

“…The plot of l + (qz ) with the angle a as a control parameter, One solution of equation (15) is for a given magnetic ® eld, is shown in ® gure 2, for the q z = 0 . (16) viscoelastic parameters of PAA in the table. This shows the following behaviour: the system becomes unstable Solving equation (A7) for cos 2ac and inserting (13) and at a critical angle ac1 $ 46ß , obtained by inspection of (16) in it yields for the ® rst critical angle the curves of l + .…”

“…(16) viscoelastic parameters of PAA in the table. This shows the following behaviour: the system becomes unstable Solving equation (A7) for cos 2ac and inserting (13) and at a critical angle ac1 $ 46ß , obtained by inspection of (16) in it yields for the ® rst critical angle the curves of l + . But above this transition, for a certain range of values of the angle a, the maximum growth cos 2 ac1 = Õ K 2 (p/ d ) 2 / xa H 2 (17 a) rate is kept at q z = 0.…”