Nematic liquid crystals are well modeled as a fluid of rigid rods. Startingfrom this model, we use a Poisson-bracket formalism to derive the equationsgoverning the dynamics of nematic liquid crystals. We treat the spin angularmomentum density arising from the rotation of constituent molecules about theircenters of mass as an independent field and derive equations for it, the massdensity, the momentum density, and the nematic director. Our equations reduceto the original Leslie-Ericksen equations, including the inertial director termthat is neglected in the hydrodynamic limit, only when the moment of inertiafor angular momentum parallel to the director vanishes and when a dissipativecoefficient favoring locking of the angular frequencies of director rotationand spin angular momentum diverges. Our equations reduce to the equations ofnematohydrodynamics in the hydrodynamic limit but with dissipative coefficientsthat depend on the coefficient that must diverge to produce the Leslie-Ericksenequations.
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