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the mass conservation equations of a viscous fluid with unit density and with an external force
caused by the phase field ϕ. Equation (6) is the condition of incompressibility. Equation (7)
is a relaxed transport equation of ϕ with advection by the velocity field u. The right-hand
side of (7) is a regularization term which ensures the consistent dissipation of energy. Roughly
speaking, the system (5)–(7) is governed by the coupling of the hydrodynamic fluid flow and
the bending elastic properties of the vesicle membrane. The resulting vesicle membrane con-
figuration and the flow field reflect the competition and the coupling of the kinetic energy and
membrane elastic energies.
Local and global well-posedness of the system (5)–(7) with the no-slip boundary condition
for the velocity field u and the Dirichlet boundary condition for the phase field function ϕ
have been studied in [9, 14]. In [9], by using the modified Galerkin argument, Du, Li and Liu
proved global existence of weak solution, moreover, they also proved the weak solution is unique
under an additionally regularity assumption u ∈ L
8
(0, T ; L
4
(Q)). Similar results also hold for
periodic boundary case, see Theorem 2.1 in [15]. However, as for the conventional Navier-Stokes
equations, the question of regularity and uniqueness of weak solution of the system (5)–(9) in
three dimensional space is still an outstanding open problem. For some regularity criteria of
weak solutions, we refer the reader to see [15, 16]. In this paper, we are interested in finding
sufficient conditions for weak solutions of the system (5)–(9) such that they become unique.
Let us recall the definition of weak solution (for definitions of functional settings for periodic
problems we refer the reader to see Section 2).
Definition 1. Let u
0
∈ L
2
per
(Q) and ϕ
0
∈ H
2
per
(Q) with ∇ · u
0
= 0 and
Q
u
0
dx = 0. A
measurable pair of functions (u, ϕ ) is called a weak solution of (5)–(9) on (0, T ) × Q if it
satisfies the following conditions:
(i) u ∈ L
∞
(0, T ; L
2
per
(Q))∩ L
2
(0, T ; H
1
per
(Q)) and ϕ ∈ L
∞
(0, T ; H
2
per
(Q))∩ L
2
(0, T ; H
4
per
(Q)).
(ii) ∇ · u = 0 in the sense of distribution.
(iii) For any η ∈ C
∞(per)
0
([0, t] × Q), ∇ · η = 0, ξ ∈ C
∞(per)
0
([0, t] × Q) with 0 < t ≤ T , we have:
t
0
Q
u · ∂
t
η−µ∇u∇η+u · ∇η · u
(τ, x)dxdτ =−
t
0
Q
(
δE(ϕ)
δϕ
∇ϕ · η)(τ, x)dxdτ
+
Q
u(t, x)η(t, x)dx −
Q
u
0
(x)η(0, x)dx,
t
0
Q
ϕ · ∂
t
ξ + u · ∇ξ · ϕ
(τ, x)dxdτ = γ
t
0
Q
(
δE(ϕ)
δϕ
· ξ)(τ, x)dxdτ
+
Q
ϕ(t, x)ξ(t, x)dx −
Q
ϕ
0
(x)ξ(0, x)dx,
where f ∈ C
∞(per)
0
([0, t] × Q) means that f ∈ C
∞
0
([0, t] × Q) and f (t, x + e
i
) = f(t, x) for
all t ∈ [0, T ].
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