  We develop a microscopic model of mutual friction represented by the
dissipative dynamics of a normal fluid flow which interacts with the helical
normal modes of vortices comprising a lattice in thermal equilibrium. Such
vortices are assumed to interact with the quasiparticles forming the normal
fluid through a pseudomomentum-conserving scattering Hamiltonian. We study the
approach to equilibrium of the normal fluid flow for temperatures below 1 K,
deriving an equation of motion for the quasiparticle pseudomomentum which leads
to the expected form predicted by the HVBK equations. We obtain an expression
for the mutual friction coefficient $B$ in terms of microscopic parameters,
which turns out to be practically independent of the vortex mass for values
arising from diverse theories. By comparing our expression of $B$ with previous
theoretical estimates, we deduce interesting qualitative features about the
excitation of Kelvin modes by the quasiparticle scattering.
