Interior regularity criterion for incompressible Ericksen-Leslie system

An interior regularity criterion of suitable weak solutions is formulated for the Ericksen-Leslie system of liquid crystals. Such a criterion is point-wise, with respect to some appropriate norm of velocity u and the gradient of d, and it can be viewed as a sort of simply sufficient condition on the local regularity of suitable weak solutions.

In this paper, we investigate the local regularity of weak solutions to the following 3D incompressible Ericksen-Leslie liquid crystal system:

with the initial boundary conditions

where \(u, d, P\) denote the velocity of the fluid, the uniaxial molecular direction, and the pressure, respectively, the \(i,j\)th element of \(\nabla d\odot\nabla d\) is \(\partial_{i}d^{k}\partial_{j}d^{k}\), \(d_{0}(x)\) is a unit vector, \(\Omega\subset \mathbb {R}^{3}\) is a smooth domain. Additionally, \(f(d)=\nabla F(d)\), and \(F(d)=\frac{1}{\zeta^{2}}( \vert d \vert ^{2}-1)^{2}, \zeta\) is a small number, formally speaking, as \(\zeta\to0, d\) tends to a unit vector.

The dynamic flows of liquid crystals have been successfully described by the Ericksen-Leslie theory [1–4]. System (1.1a)-(1.1c) is a coupled system of the Navier-Stokes equations with a parabolic system. It is Leray [5] and Hopf [6] that established the global existence of weak solutions to the 3D Navier-Stokes; however, the regularity of the weak solutions is still an open problem. Since the regularity of weak solutions to the 3D Navier-Stokes equations is hard to get, some related conditions or criteria for the regularity of the weak solutions are considered, such as the well-known Serrin type criterion [7] and the Beale-Kato-Majda type criterion [8]. Furthermore, based on the suitable weak solutions, some point-wise sufficient regularity criteria were imposed in [9–12].

The global existence of suitable weak solutions to system (1.1a)-(1.1c) was established in [13, 14] by Lin and Liu; however, noticing that system (1.1a)-(1.1c) contains the 3D Navier-Stokes equations as a subsystem, the uniqueness and regularity of these weak solutions are not known. In this paper, we would extend some point-wise sufficient conditions, which guarantee the local regularity of weak solutions for 3D Navier-Stokes equations, to the Ericksen-Leslie system (1.1a)-(1.1c). We would like to mention that when \(f(d)\) in system (1.1a)-(1.1c) is replaced by \(- \vert \nabla d \vert ^{2}d\), the global existence of weak solutions to the resulting system in three dimensions has only been known under the additional assumption that \(d_{3}\geq0\) or small initial data (see [15, 16]). Without these conditions, the general existence of weak solutions is still open. However, the Serrin type criterion and the Beale-Kato-Majda type criterion still hold true even for a weak solution (if it exists) (see [17, 18]).

The suitable weak solution established in [14] can be stated as below.

Definition 1.1

Suitable weak solutions in \(\Omega\times(0, T)\subset\mathbb {R}^{3}\times(0,\infty)\)

A pair \((u, d)\) is called a suitable weak solution to system (1.1a)-(1.1c) and (1.2) in an open set \(\mathcal {O}\subset\mathbb {R}^{3}\times(0,\infty)\) (we set \(\mathcal {O}_{t}=\mathcal {O}\cap(\mathbb {R}^{3}\times\{t\} )\)), if it satisfies the following properties:

• \((u, d)\) is a weak solution in the sense of distribution;

• \(u\in L^{\infty}(0,T;L^{2}(\Omega ))\cap L^{2}(0,T;H^{1}(\Omega)), d\in L^{\infty}(0,T;H^{1}(\Omega))\cap L^{2}(0,T;H^{2}(\Omega))\), or generally, there exist constants \(E_{1}, E_{2}\), such that

  $$\begin{aligned} &\int_{\mathcal {O}_{t}} \bigl[ \vert u \vert ^{2}+ \vert \nabla d \vert ^{2}+F(d) \bigr]\,\mathrm{ d}x< E_{1},\\ &\int\!\int_{\mathcal {O}} \bigl[ \vert \nabla u \vert ^{2}+ \bigl\vert \Delta d-f(d) \bigr\vert ^{2}+F(d) \bigr]\,\mathrm{ d}x\,\mathrm{ d}t< E_{2}; \end{aligned}$$

• for any \(\varphi\in C_{c}^{\infty}(\mathcal {O})\), more specifically, for any \(\varphi\in C_{c}^{\infty}(B(x_{0}, R)\times(t_{0}-R^{2}, t_{0}))\), the following generalized energy inequality holds

  $$\begin{aligned} &\int_{B(x_{0}, R)} \bigl( \vert u \vert ^{2}+ \vert \nabla d \vert ^{2} \bigr) \varphi\,\mathrm{ d}x+2 \int _{t_{0}-R^{2}}^{t} \int_{B(x_{0}, R)} \bigl( \vert \nabla u \vert ^{2}+ \bigl\vert \nabla^{2} d \bigr\vert ^{2} \bigr)\varphi\,\mathrm{ d}x \,\mathrm{ d}\tau \\ &\quad\leq \int_{t_{0}-R^{2}}^{t} \int_{B(x_{0}, R)} \bigl\{ \bigl( \vert u \vert ^{2}+ \vert \nabla d \vert ^{2} \bigr) (\varphi _{t}+\Delta \varphi)+ \bigl( \vert u \vert ^{2}+ \vert \nabla d \vert ^{2}+2P \bigr)u\cdot\nabla\varphi \bigr\} \,\mathrm{ d}x\,\mathrm{ d}\tau \\ &\qquad{}+2 \int_{t_{0}-R^{2}}^{t} \int_{B(x_{0}, R)} \bigl((u\cdot\nabla) d\nabla d\nabla \varphi-\nabla f(d):\nabla d\varphi \bigr)\,\mathrm{ d}x\,\mathrm{ d}\tau. \end{aligned}$$

  (1.3)

In the following, we can take \(Q((x_{0}, t_{0}), R)\equiv B(x_{0}, R)\times(t_{0}-R^{2}, t_{0})\), \(B(x_{0}, R)\equiv\{y\in\mathbb {R}^{3}| \vert y-x_{0} \vert < R\}, z_{0}\equiv(x_{0}, t_{0})\) for simplicity.

We now state our main result of this paper.

Theorem 1.2

Let \((u, d)\) be a suitable weak solution to liquid crystal system (1.1a)-(1.1c) in \(Q(z_{0}, R)\). The real numbers \(l\geq1\) and \(s\geq1\) satisfy

Then there is a positive number \(\varepsilon=\varepsilon(s,l)\), such that if

then \(z_{0}\) is a regular point of \((u,\nabla d)\), i.e. \((u,\nabla d)\) is Hölder continuous in \(Q(z_{0},r)\), for some \(r\in(0, R]\).

Throughout this paper, we use c to denote a generic positive constant which can be different from line to line.

As the preparation for proving Theorem 1.2, we first give two auxiliary lemmas.

Lemma 2.1

We have

where

Proof

Step 1. For (1.1a), we choose the test function \(w=\chi\nabla q\), for any \(\chi\in C_{c}^{\infty}((t_{0}-\rho^{2}, t_{0})), q\in C_{c}^{\infty}(B(x_{0}, \rho))\), then it yields

It follows from \(\nabla\cdot u=0\) that

Therefore, for a.e. \(t\in(t_{0}-\rho^{2}, t_{0})\), we have

Step 2. Approximate p with \(p_{1}\) by confining q in \(W^{2, 3}(B(x_{0}, \rho))\).

Set \(p_{1}\in L^{\frac{3}{2}}(Q(z_{0}, \rho))\) such that, for a.e. \(t\in(t_{0}-\rho ^{2}, t_{0})\),

for any \(q(\cdot, t)\in W^{2, 3}(B(x_{0}, \rho))\), and \(q(\cdot, t)=0 \text{ on } \partial B(x_{0}, \rho)\). The existence of \(p_{1}\) is established due to the Lax-Milgram theorem with appropriate approximating process on u and d (see [11]).

Next, choose \(q_{0}(\cdot, t)\in W^{2, 3}(B(x_{0}, \rho))\), such that, for a.e. \(t\in(t_{0}-\rho^{2}, t_{0})\),

Then, by the Calderon-Zygmund inequality, it yields

Therefore, it follows from (2.3) and the Hölder inequality that

which yields \(\int_{Q(z_{0}, \rho)} \vert p_{1}(\cdot, t) \vert ^{\frac{3}{2}}\,\mathrm{ d}z\leq c\rho^{2}C(z_{0}, \rho; u, \nabla d)\).

Step 3. Estimates for the remainder \(p-p_{1}\).

For a.e. \(t\in(t_{0}-\rho^{2}, t_{0})\), let \(p_{2}=p-p_{1}\), then from (2.2)-(2.3) one infers that

By the harmonic property, one can get

while

Step 4. Estimates for p.

We have

 □

We denote

Lemma 2.2

Under the assumptions of Theorem 1.2, we have

where \(q=2l(\frac{3}{s}+\frac{2}{l}-\frac{3}{2})\), and \(q'=\frac{q}{q-1}\).

Proof

With the help of the Hölder and Sobolev embedding inequalities, one gets

where \(\lambda s+2\mu+6\gamma=3, \lambda+\mu+\gamma=1\). Substituting v by u and ∇d, respectively, then one can get the summation

Therefore, by choosing appropriate parameters \(\lambda=\frac{1}{2s(\frac{3}{s}+\frac{2}{l}-\frac{3}{2})}\), \(\mu=\frac {\frac{3}{s}+\frac{3}{l}-2}{2(\frac{3}{s}+\frac{2}{l}-\frac{3}{2})}\), \(\gamma= \frac{\frac{2}{s}+\frac{1}{l}-1}{2(\frac{3}{s}+\frac{2}{l}-\frac{3}{2})}\), and integrating from \(t_{0}-\rho^{2}\) to \(t_{0}\) with the variable t, it follows from the Hölder and Young inequalities that

where \(\kappa=\frac{3l}{s}+2-l\) as in Theorem 1.2, and in the last step, we used the fact that \(\mu q\leq1, H(\rho)\leq A(\rho)\). □

Due to the induction argument as Proposition 2.6 in [10] or Lemma 2.2 in [19] (the parabolic version of the Campanato criterion), to get the desired consequence, it suffices to prove \(C(\theta^{k})+D(\theta^{k})< \epsilon_{0}\) for some small \(\epsilon_{0}\). Here θ is a small number, which will be chosen later.

From the generalized energy inequality, it is easy to check that, for \(\rho\in(0, R]\),

Denoting \(G(\rho)=A(\rho)+E(\rho)+D(\rho)\), due to Lemmas 2.1-2.2, and the fact that \(C(2\theta\rho)\leq\frac{1}{4\theta^{2}}C(\rho)\), we can get

where in the last step we have used \(\frac{\epsilon^{\frac{2}{3q}}}{\theta^{\frac{4}{3}}}(G(\rho)+1)^{\frac{2}{3}}\leq c[\epsilon^{\frac{1}{q}}+\frac{\epsilon^{\frac{1}{2q}}}{\theta^{2}}(G(\rho)+1)]\). Now choosing θ and ϵ such that \(c\theta<\frac{1}{4}\) and \(c\frac{\epsilon^{\frac{1}{2q}}}{\theta^{2}}<\frac{1}{4}\), then it yields \(G(\theta\rho)\leq\frac{1}{2}G(\rho)+c\frac{\epsilon^{\frac{1}{2q}}}{\theta^{2}}\). Iterating the above process, we obtain \(G(\theta^{k} \rho)\leq\frac{1}{2^{k}}G(\rho)+c\frac{\epsilon^{\frac{1}{2q}}}{\theta^{2}}\), therefore,

For \(C(\theta^{k}\rho)\), by Lemma 2.2, we have

where in the last step we use the fact that \(\epsilon^{\frac{1}{q}}\leq \frac{\epsilon^{\frac{1}{2q}}}{\theta^{2}}\) for ϵ small enough. With these inequalities in hand, for fixed ρ and \(\epsilon_{0}\), we can choose \(k_{0}\) large enough such that \(c\frac{1}{2^{k_{0}}}G(\rho)<\frac{\epsilon_{0}}{4}\), and choose ϵ small enough, such that \(c\frac{\epsilon^{\frac{1}{2q}}}{\theta^{2}}<\frac{\epsilon_{0}}{4}\). With these prerequisites and (3.1)-(3.2), it follows that \(D(\theta^{k}\rho)+C(\theta^{k}\rho)<\epsilon_{0}\).

The authors thank Dr. Huajun Gong and Dr. Jinkai Li for helpful discussions and suggestions. Ma is supported by Fostering Talents of NSFC-Henan Province (U1404102) and NSFC (No. 11501174, 11626090). Feng is supported by NSFC (No. 61401283, 11601342, 61472257), GDPSTPP (No. 2013B040403005) and (No. GCZX-A1409).

Affiliations

1. College of Information and Management Science, Henan Agricultural University, Zhengzhou, 450046, P.R. China

   • Wenya Ma

2. College of Mathematics and Statistics, Shenzhen University, Shenzhen, P.R. China

   • Jiqiang Feng

Corresponding author

Correspondence to Jiqiang Feng.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

Both authors contributed equally in writing this paper. They both read and approved the final manuscript.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Reprints and Permissions