  Context: Magnetic null points are associated with high-energy coronal
phenomena such as solar flares and are often sites of reconnection and particle
acceleration. Dynamic twisting of a magnetic null point can generate a
Kelvin-Helmholtz instability (KHI) within its fan plane and, under continued
twisting, can instigate spine-fan reconnection and an associated collapse of
the null point.
  Aim: This article aims to compare the effects of isotropic and anisotropic
viscosity in simulations of the KHI and collapse in a dynamically twisted
magnetic null point.
  Methods: Simulations were performed using the 3D magnetohydrodynamics code
Lare3d with a custom anisotropic viscosity module. A pair of high resolution
simulations was performed, one using isotropic viscosity and another using
anisotropic viscosity, keeping all other factors identical, and the results
analysed in detail. A further parameter study was performed over a range of
values for viscosity and resistivity.
  Results: Both viscosity models permit the growth of the Kelvin-Helmholtz
instability and the eventual collapse of the null point. Over all studied
parameters, anisotropic viscosity allows a faster growing instability, while
isotropic viscosity damps the instability to the extent of stabilisation in
some cases. Although the viscous heating associated with anisotropic viscosity
is generally smaller, the ohmic heating dominates and is enhanced by the
current sheets generated by the instability, leading to a greater overall
heating rate when using anisotropic viscosity. The collapse of the null point
occurs significantly sooner when anisotropic viscosity is employed.
