  Spontaneous growth of long-wavelength deformations is a defining feature of
active fluids with orientational order. We investigate the effect of biaxial
rectangular confinement on the instability of initially shear-aligned 3D
isotropic active fluids composed of extensile microtubule bundles and kinesin
molecular motors. Under confinement, such fluids exhibit finite-wavelength
self-amplifying bend deformations which grow in the plane orthogonal to the
direction of the strongest confinement. Both the instability wavelength and the
growth rate increase with weakening confinement. These findings are consistent
with a minimal hydrodynamic model, which predicts that the fastest growing
deformation is set by a balance of active driving and elastic relaxation.
Experiments in the highly confined regime confirm that the instability
wavelength is set by the balance of active and elastic stresses, which are
independently controlled by the concentration of motors and non-motile
crosslinkers.
