We study a three-dimensional, incompressible, viscous, micropolar fluid with
anisotropic microstructure on a periodic domain. Subject to a uniform microtorque,
this system admits a unique nontrivial equilibrium. We prove that when the
microstructure is inertially oblate (i.e., pancake-like) this equilibrium is nonlinearly
asymptotically stable.
Our proof employs a nonlinear energy method built from the natural energy
dissipation structure of the problem. Numerous difficulties arise due to the
dissipative-conservative structure of the problem. Indeed, the dissipation fails
to be coercive over the energy, which itself is weakly coupled in the sense
that, while it provides estimates for the fluid velocity and microstructure
angular velocity, it only provides control of two of the six components of the
microinertia tensor. To overcome these problems, our method relies on a
delicate combination of two distinct tiers of energy-dissipation estimates,
together with transport-like advection-rotation estimates for the microinertia.
When combined with a quantitative rigidity result for the microinertia, these
allow us to deduce the existence of global-in-time decaying solutions near
equilibrium.