  This work investigates the effects of the choice of momentum diffusion
operator on the evolution of multiphase fluid systems resolved with Meshless
Lagrangian Methods (MLM). Specifically, the effects of a non-zero viscosity
gradient at multiphase interfaces are explored. This work shows that both the
typical Smoothed Particle Hydrodynamics (SPH) and Generalized Finite Difference
(GFD) diffusion operators under-predict the shear divergence at multiphase
interfaces. Furthermore, it was shown that larger viscosity ratios increase the
significance of this behavior. A multiphase GFD scheme is proposed that makes
use of a computationally efficient diffusion operator that accounts for the
effects arising from the jump discontinuity in viscosity. This scheme is used
to simulate a 3D bubble submerged in a heavier fluid with a density ratio of
2:1 and a dynamic viscosity ratio of 100:1. When comparing the effects of
momentum diffusion operators, a discrepancy of 57.2% was observed in the
bubble's ascent velocity.
