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Title: Magnetic and electric field induced bistability in nematics and cholesterics
Authors: Kini, U.D.
Issue Date: Nov-1991
Publisher: Taylor & Francis
Citation: Liquid Crystals, 1991, Vol.10, p597 - 621
Abstract: Continuum theory is used for setting up the differential equations governing bistable behaviour of the director field (n) in nematics having positive diamagnetic susceptibility anisotropy b.chia the bistability being induced by slowly rotating a magnetic field H away from n0, the initial uniform director orientation. The bistability width wb is the range of magnetic tilt angle over which two different equilibrium configurations can exist. Under the rigid anchoring hypothesis two simple cases are studied (viz. the splay/bend and twist geometries) in which n is described by a single orientational degree of freedom and H rotates about an axis which is either normal to or parallel to the sample planes. The splay/bend geometry can be generalized to uniformly tilted n0 and to non-symmetric director tilt at the boundaries; the twist geometry can be extended to include twisted nematics as well as chiral nematics (or cholesterics having equilibrium pitch of the order of the sample thickness). Bistability should be observable even in situations where a Freedericksz threshold does not exist. The width wb is found to depend on the geometry, H and on n0 (and also on the equilibrium pitch for the chiral systems). For rigid anchoring, scaling analysis shows that wb should depend only on the reduced field for given material parameters and tilt of n0 and suggests the need for experiments using different sample thicknesses. When the director anchoring at the sample planes is weakened, wb varies sensitively with anchoring strength. When the anchoring is weak enough and H strong enough there might occur a discontinuous transition from a deformed state to one in which n is oriented uniformly parallel to H. The simple scaling which is valid for rigid anchoring does not hold for weak anchoring. A transition between two distortion states should be accompanied by transient flow in the splay/bend geometry; in the twist case, a transition may occur even without accompanying flow. Scaling analysis indicates that the time of transition between two states might depend on sample thickness. Linear stability analysis shows that n has a propensity towards instability near the edges of the bistable region. The governing equations can be extended to the case of H rotating at a uniform rate, a situation which is of current experimental interest. An electric field, E, acting along the sample planes affects the scaling. The electric field as well as the magnitude of the dielectric anisotropy b.chia (> 0) now determine wb. Flexoelectricity, which appears even as a volume effect for large deformations, may have considerable influence on wb especially for weak anchoring when E is impressed. Magnetic field induced bistability should be observable in more complex configurations in which n is described by two orientational degrees of freedom; this suggests generalizations of the rotating field experiments. In this context the case of a b.chia <0 nematic is briefly studied.
Description: Restricted Access.
ISSN: 0267-8292
1366-5855 (Online)
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Copyright: 1991 Taylor & Francis
Appears in Collections:Research Papers (SCM)

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