This study investigates the influence of the Kelvin-Helmholtz (KH) instability at the base of a mixed layer on the settling and dispersion of small inertia particles in comparison to their classic Stokes settling in a quiescent environment. We use direct numerical simulations of the flow field coupled with Lagrangian tracking of point particles to examine the settling and trajectories of heavy particles ranging between 20 and $100\mu \mathrm{m}$ in diameter. Particles are initially positioned slightly above the center of the shear layer and inserted into the simulation immediately before the onset of the roll-up of the billow. For this initial positioning of particles, we find that the settling of particles $\ge 40$ $\mu \mathrm{m}$ is slower than their Stokes settling because when trapped within the KH billow, these particles preferentially sample the upward moving regions of the flow. This effect diminishes for particles larger than 80 $\mu \mathrm{m}$ as the particle and flow interactions become less strong due to fast particle settling. Conversely, small particles (20 $\mu \mathrm{m}$) exhibit enhanced settling in the presence of KH billows; their instantaneous vertical velocities are up to seven times faster than Stokes settling because they preferentially sample the downward moving portion of the billow. Particle dispersion induced by the KH flow is strongly influenced by the duration for which particles remain within the mixed layer in relation to the timing of the different phases of the KH flow development and decay. As larger particles leave the mixed layer earlier, their interaction with the KH flow becomes more limited, and this encounter time is also influenced by the initial particle position. The spanwise dispersion of the particles, and the drift of particle trajectories from twodimensional, decreases with increasing particle size. Conversely, the vertical dispersion of the particles is larger for larger particles as their early settling through the two-dimensional growth phase of the KH flow facilitates sampling of more inhomogeneous vertical velocities. This research offers new insights into the interactions between the KH instability and settling Lagrangian particles, highlighting the significant role of the instability in altering sediment transport in sheared, stratified flows for a range of tested conditions. Further investigation is necessary to understand the interaction between shear instabilities and particles over a broader range of parameters.

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• Fluid-particle interactions
• Kelvin-Helmholtz instability
• Particle-laden flows
• Shear flows
• Shear layer turbulence
• Stratified turbulence
• Transition to turbulence
• Turbulent multiphase flows
• Vortex breakdown