30 research outputs found
Energy Dissipation and Regularity for a Coupled Navier-Stokes and Q-Tensor System
We study a complex non-newtonian fluid that models the flow of nematic liquid
crystals. The fluid is described by a system that couples a forced
Navier-Stokes system with a parabolic-type system. We prove the existence of
global weak solutions in dimensions two and three. We show the existence of a
Lyapunov functional for the smooth solutions of the coupled system and use the
cancellations that allow its existence to prove higher global regularity, in
dimension two. We also show the weak-strong uniqueness in dimension two
Colloquium: Mechanical formalisms for tissue dynamics
The understanding of morphogenesis in living organisms has been renewed by
tremendous progressin experimental techniques that provide access to
cell-scale, quantitative information both on theshapes of cells within tissues
and on the genes being expressed. This information suggests that
ourunderstanding of the respective contributions of gene expression and
mechanics, and of their crucialentanglement, will soon leap forward.
Biomechanics increasingly benefits from models, which assistthe design and
interpretation of experiments, point out the main ingredients and assumptions,
andultimately lead to predictions. The newly accessible local information thus
calls for a reflectionon how to select suitable classes of mechanical models.
We review both mechanical ingredientssuggested by the current knowledge of
tissue behaviour, and modelling methods that can helpgenerate a rheological
diagram or a constitutive equation. We distinguish cell scale ("intra-cell")and
tissue scale ("inter-cell") contributions. We recall the mathematical framework
developpedfor continuum materials and explain how to transform a constitutive
equation into a set of partialdifferential equations amenable to numerical
resolution. We show that when plastic behaviour isrelevant, the dissipation
function formalism appears appropriate to generate constitutive equations;its
variational nature facilitates numerical implementation, and we discuss
adaptations needed in thecase of large deformations. The present article
gathers theoretical methods that can readily enhancethe significance of the
data to be extracted from recent or future high throughput
biomechanicalexperiments.Comment: 33 pages, 20 figures. This version (26 Sept. 2015) contains a few
corrections to the published version, all in Appendix D.2 devoted to large
deformation
Flow and reorientation in the dynamics of nematic defects
We propose a simple phenomenological model for hydrodynamic defect dynamics in nematic liquid crystals, inspired by the Ericksen-Leslie theory. We identify the main forces that govern both fluid and defect motion and we comment on their symmetry. As shown for two annihilating disclinations, our model is predictive for arbitrary length scales and topological charges
Dissipative Ordered Fluids: Theories for liquid crystals
This is a book on the dissipative dynamics of ordered fluids, with a particular focus on liquid crystals. It covers a whole range of different theories, mainly concerned with nematic liquid crystals in both their chiral and nonchiral variants. The authors begin by giving a detailed account of the molecular origins of orientational order in fluids
Dynamics of dissipative ordered fluids
A variational principle is proposed that allows to derive the equations of motion for a fluid with a general microstructure described by a tensorial order parameter. The only constitutive ingredients are the densities of the free energy and the dissipation, both subject to appropriate invariance requirements. As an illustration, it is shown how the hydrodynamic theory for uniaxial nematic liquid crystals can be derived within this setting
Reorientational dynamics of conjugated nematic point defects
To appreciate the universal qualitative features of defect annihilation in nematic liquid crystals, we study how the viscous force of reorientational dynamics behaves under a transformation that reverses the sign of the defect's topological charge. As an illustration of our general results, we consider a class of point defects that were first studied by A. Saupe. The reorientational viscous forces acting on them differ dramatically from those acting on line defects
Steric effects in dispersion forces interactions
Classically, there have been two different ways to obtain mean-field theories for liquid crystals. One is based on short-range repulsive steric forces and the other on long-range attractive dispersion forces. In the former approach, it is the anisotropic shape of the molecules that leads to the anisotropic interaction, and in the latter it is the anisotropy of the molecular polarizability. In real molecules both causes of anisotropy can be expected to contribute to the effective interaction, and so it is desirable to assess the combined effect of anisotropic long-range attraction and short-range repulsion. Here we present an avenue to this end. We start from dispersion forces interactions and combine them with hard-core repulsions in a formal theory, whose crucial element is the steric tensor, a fourth-rank tensor depending on the anisotropy of the interacting molecules. This tensor can be determined analytically for a special class of molecular shapes
Fluids with dissipative microstructure
A variational principle is proposed which allows to derive the equations of motion for a dissipative fluid with general microstructure. The only constitutive ingredients are the densities of the free energy and the dissipation, both subject to appropriate invariance requirements. The strict interplay between the microstructures considered here and those studied by Capriz is also examined in some detail
Elastic forces on nematic points defects
We present a general method to compute the elastic forces exerted on nematic point defects: these are the forces that set defects in motion, and so they are especially relevant to defect dynamics, a subject which is recently attracting much interest. These forces are mediated by the distortion introduced by defects in the director field; for example, they are responsible for the attraction between defects of opposite topological charge