Pattern Forming Instability in Homeotropically Aligned Liquid Crystals

Abstract

The electrohydrodynamic pattern forming instability (EHC) driven by an ac voltage applied to a homeotropically aligned nematic liquid crystal layer is experimentally studied near onset. By controlling an external magnetic field, many different scenarios become accessible. For finite fields various regular convective structures are observed, which we call, for example, wavy, chain, and bamboo-chevron patterns. The various types are documented with the help of a phase diagram governed by the frequency and the strength of the applied ac voltage. In addition the stability regimes in the voltage-wavenumber plane (the "Busse balloon") are mapped out. One finds significant differences from the conventional planar case, for which a theoretical analysis is lacking so far. For zero magnetic field, on the other hand, no regular structure is observed, even immediately above the onset of EHC. A new spatiotemporally chaotic pattern called the soft mode turbulence directly appears via a supercritical bifurcation from the nonconvective state. This is due to the presence of the Goldstone mode related to the spontaneously broken rotational symmetry associated with the director component in the plane of the layer.

Convective Patterns in Liquid Crystals Driven by Electric Field

“…9a) or normal rolls (Fig. 9b) depending on the frequency -are disordered and correspond to a special manifestation of spatio-temporal chaos, the soft mode turbulence [37][38][39].…”

Compressed exponential relaxation as superposition of dual structure in pattern dynamics of nematic liquid crystals

“…With further increasing V , the electrohydrodynamic convection occurs at the electroconvective threshold voltage V c . The nonlinear coupling between the convective mode q(x, y) and the Nambu-Goldstone one C(x, y) induces spatially and temporally disordered pattern called soft-mode turbulence (SMT) [9][10][11]. SMT is regarded as an experimentally observed spatiotemporal chaos.…”

“…There are two types of SMT pattern; oblique rolls in f < f L and normal rolls in f > f L , where f L denotes the Lifshitz frequency [9,10]. The interaction between the convective and Goldstone modes are different between oblique and normal rolls [45,46]; the convective wave vector in SMT tend to become parallel to the C-director in the normal roll and oblique in the oblique roll.…”

Electroconvection in homeotropic nematic liquid crystals

“…Consequently the EC pattern is spatially disordered even at the onset. Moreover, oblique roll patterns always vary with time due to the presence of an inherent torque acting on the rolls; for normal rolls this dynamic disorder starts slightly above V c [10][11][12][13][14]. This behaviour is a special manifestation of spatio-temporal chaos, called soft mode turbulence in order to distinguish it from the usual route to chaos marked by the generation and violent motion of increasing number of defects as found in the planar EC.…”