  We investigate the effect of particle inertia on the merger of co-rotating
dusty vortex pairs at semi-dilute concentrations. In a particle-free flow, the
merger is triggered once the ratio of vortex core size to vortex separation
reaches a critical value. The vortex pair separation then decreases
monotonically until the two cores merge together. Using Eulerian-Lagrangian
simulations of co-rotating particle-laden vortices, we show substantial
departure from the vortex dynamics previously established in particle-free
flows. Most strikingly, we find that disperse particles with moderate inertia
cause the vortex pair to push apart to a separation nearly twice as large as
the initial separation. During this stage, the drag force exerted by particles
ejected out of the vortex cores on the fluid results in a net repulsive force
that pushes the two cores apart. Eventually, the two dusty vortices merge into
a single vortex with most particles accumulating outside the core similar to
the dusty Lamb-Oseen vortex described in Shuai & Kasbaoui (J. Fluid Mech., vol
936, 2022, A8) For weakly inertial particles, we find that the merger dynamics
follow the same mechanics as those of a single-phase flow, albeit with a
density that must be adjusted to match the mixture density. For highly inertial
particles, the feedback force exerted by the particles on the fluid may stretch
the two cores during the merger to a point where each core splits into two,
resulting in inner and outer vortex pairs. In this case, the merger occurs in
two stages where the inner vortices merge first, followed by the outer ones.
