1,721,467 research outputs found
Reexamination of the Helfrich-Hurault effect in smectic-A liquid crystals
No full textThe Helfrich-Hurault effect is a phase transition that occurs in samples of cholesteric or smectic liquid
crystals subject to external electric or magnetic fields. In this paper we analyze the Helfrich-Hurault effect of
smectic-A liquid crystals in an electrostatic field taking into account the complete electromechanical coupling.
A comparison is made with the results already obtained for the partially coupled case where one takes into
account only the effect of the field on the crystal configuration and considering that field unaffected
The Slight Distortions Induced by an Electrostatic Field on Finite Samples of Smectic-A Liquid Crystals
No full textThis paper derives governing equations of the interaction of finite samples of smectic-A liquid crystals with an electrostatic field. Our model takes into account effects of the electric field on the layers distortion and vice-versa. The governing equations are obtained by the principle of virtual work. They are adapted for a particular problem in order to analyze the phase transition of Helfrich-Hurault type induced by an electrostatic field on a finite sample of smectic.This paper derives governing equations of the interaction of finite samples of smectic-A liquid crystals with an electrostatic field. Our model takes into account effects of the electric field on the layers distortion and vice-versa. The governing equations are obtained by the principle of virtual work. They are adapted for a particular problem in order to analyze the phase transition of HelfrichHurault type induced by an electrostatic field on a finite sample of smectic
Erratum: Reexamination of the Helfrich-Hurault effect in smectic-A liquid crystals (Physical Review E (2005) 72 (041708))
No full textPeriodic splay-twist Fréedericksz transition for nematics confined between two concentric cylinders
No full textcrystals confined between two infinite concentric cylinders. The calculation of Lonberg and Meyer Phys. Rev. Lett. 55, 718 1985 , for nematics sandwiched between two infinite planes, is extended to annular domains.
The phase transition is triggered by an applied voltage between the outer and the inner delimiting walls. The critical threshold behavior is analyzed via the linearized Euler-Lagrange equations related to the Frank's free
energy. It is found that, the threshold depends on both the ratio between the twist and the splay elastic constants, and the sample radii ratio. Results for planar samples are recovered in the thin cell limit. With
respect to the planar geometry, our analysis predicts that for annular geometries the periodic Fréedericksz
transition is also allowed for elastic anisotropies K2 / K1 > 0.303
Surface tension controls the onset of gyrification in brain organoids
No full textUnderstanding the mechanics of brain embryogenesis can provide insights on pathologies related to brain development, such as lissencephaly, a genetic disease which causes a reduction of the number of cerebral sulci. Recent experiments on brain organoids have confirmed that gyrification, i.e. the formation of the folded structures of the brain, is triggered by the inhomogeneous growth of the peripheral region. However, the rheology of these cellular aggregates and the mechanics of lissencephaly are still matter of debate. In this work, we develop a mathematical model of brain organoids based on the theory of morpho-elasticity. We describe them as non-linear elastic bodies, composed of a disk surrounded by a growing layer called cortex. The external boundary is subjected to a tissue surface tension due the intercellular adhesion forces. We show that the resulting surface energy is relevant at the small length scales of brain organoids and affects the mechanics of cellular aggregates. We perform a linear stability analysis of the radially symmetric configuration and we study the post-buckling behaviour through finite element simulations. We find that the process of gyrification is triggered by the cortex growth and modulated by the competition between two length scales: the radius of the organoid and the capillary length generated by surface tension. We show that a solid model can reproduce the results of the in-vitro experiments. Furthermore, we prove that the lack of brain sulci in lissencephaly is caused by a reduction of the cell stiffness: the softening of the organoid strengthens the role of surface tension, delaying or even inhibiting the onset of a mechanical instability at the free boundary
Mielopatia compressiva cervicale da discopatia nel cane di razza dalmata: rilievi clinici, diagnostici e chirurgici in 12 casi (2005-2007).
No full textQuantum-state transfer between tripod atoms over a dark fiber
No full textIn this work we introduce a model for quantum-state transfer between tripod atoms over a dark fiber. Two tripod atoms are confined in separate cavities linked by an optical fiber. The cavities and the fiber sustain two
optical modes of opposite circular polarization. For each atom, the two ground states encode the quantum state to be transferred and are coupled to a common excited state by the cavity modes of opposite polarization. The
remaining transition for each atom is used to control the transfer process. We demonstrate that by using laser pulses the dynamics of the system can be confined within a degenerate dark state subspace, with the different
dark states interacting via nonadiabatic couplings. We solve analytically the dynamics in the dark state subspace, and determine the conditions on the pulse shape for the implementation of the quantum transfer. We identify
a possible pulse shape which satisfies the required conditions, and demonstrate the quantum-state transfer via numerical simulations
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