  We investigate the character of the van der Waals (vdW) torque and force
between two coplanar and dielectrically anisotropic topological insulator (TI)
slabs separated by a vacuum gap in the non-retardation regime, where the optic
axes of the slabs are each perpendicular to the normal direction to the
slab-gap interface and also generally differently oriented from each other. We
find that in addition to the magnetoelectric coupling strength, the anisotropy
can also influence the sign of the vdW force, viz., a repulsive vdW force can
become attractive if the anistropy is increased sufficiently. In addition, the
vdW force oscillates as a function of the angular difference between the optic
axes of the TI slabs, being most repulsive/least attractive (least
repulsive/most attractive) for angular differences that are integer
(half-integer) multiples of $\pi$. Our third finding is that the vdW torque for
TI slabs is generally weaker than that for ordinary dielectric slabs. Our work
provides the first instance in which the vector potential appears in a
calculation of the vdW interaction for which the limit is non-retarded or
static.
