The small Deborah number limit of the Doi-Onsager equation without hydrodynamics

TL;DR

This paper analyzes the limit of the Doi-Onsager equation for nematic liquid crystals as the Deborah number approaches zero, showing convergence to a harmonic map heat flow into the sphere.

Contribution

It establishes the strong convergence of solutions to a local equilibrium described by a harmonic map heat flow, without considering hydrodynamics.

Findings

Solutions converge strongly to local equilibrium distributions

The limit flow is a harmonic map heat flow into ^2

The proof involves compactness of the second moment and energy estimates

Abstract

We study the small Deborah number limit of the Doi-Onsager equation for the dynamics of nematic liquid crystals without hydrodynamics. This is a Smoluchowski-type equation that characterizes the evolution of a number density function, depending upon both particle position $x\in \mathbb{R}^d(d=2,3)$ and orientation vector $m\in\mathbb{S}^2$ (the unit sphere). We prove that, when the Deborah number tends to zero, the family of solutions with rough initial data near local equilibria will converge strongly to a local equilibrium distribution prescribed by a weak solution of the harmonic map heat flow into $\mathbb{S}^2$. This flow is a special case of the gradient flow to the Oseen-Frank energy functional for nematic liquid crystals. The key ingredient is to show the strong compactness of the family of number density functions and the proof relies on the strong compactness of the corresponding second moment (or the $Q$-tensor), a spectral decomposition of the linearized operator near the limit local equilibrium distribution, as well as the energy dissipation estimate.