  We study the transverse spin angular momentum of fiber eigenmodes. As an
intrinsic property of fiber eigenmodes, it becomes considerable in optical
nanofibers and, especially, increases sharply in the evanescent field. We
derive analytical expressions of this angular momentum, and present its density
distribution inside andoutside nanofibers. Significantly, we find the optimal
ratio of the fiber core radius to the wavelength to obtain the maximum surface
transverse spin. For instance, the optimal fiber radius is one fourth of the
wavelength for the fundamental mode. Furthermore, we investigate the spin flow
of each guided mode on the nanofiber surface and the transverse mechanical
effects originated from the transverse spin on small particles.
