Inertial microfluidics (Reynolds number Re ∼ O(10−100)) emerged in the late 2000s and has contributed to wide applications in biomedical engineering [1]. One key application is label-free, high-throughput separation of target particles from heterogeneous suspensions (e.g. blood) for diagnosis purposes. Such separation relies on the migration of different particle types to their own focusing positions determined by the balance of inertial and drag forces within microchannel cross-sections. However, current inertial microfluidic devices are mostly limited to handling diluted samples (concentration <5%), due to a lack of understanding of the interplay of inertial particle migration and particle-particle interactions in dense suspensions, which are both experimentally and numerically challenging. Using an in house 3D lattice-Boltzmann code with incorporated immersed-boundary and finite-element methods [2], we perform numerical simulations to elucidate the mechanisms for inertial particle motion in heterogeneous suspensions. A range of effects including flow inertia, sample concentration, mixture fraction and particle size/softness heterogeneity are investigated. We demonstrate that in homogeneous suspensions with increasing concentration, the hydrodynamic interactions between particles introduce substantial fluctuations to their motion and result in weakened focusing behaviour. For heterogeneous suspensions, the interparticle interactions are more complex, affecting the lift velocities and collective motion of particles distinctly based on their absolute and relative size. Subject to appropriate particle confinement and mixture fraction, satisfactory focusing or enrichment of target particles can be achieved in dense suspensions (e.g. 20%) under high inertia conditions (Re >100). This work furthers our understanding of the inertial lift of particles in concentrated samples and provide evidence for the design and optimisation of microfluidic devices aimed at particle separation from dense heterogeneous suspensions. REFERENCES [1] J.M. Martel and M. Toner, Inertial Focusing in Microfluidics. Ann. Rev. Biomed. Eng., 16(1): 371–396, 2014. [2] B. Owen, K. Kechagidis, S.R. Bazaz, R. Enjalbert, E. Essmann, C. Mallorie, F. Mirghaderi, C. Schaaf, K. Thota, R. Vernekar, Q. Zhou, M.E. Warkiani, H. Stark and T. Krüger, Lattice-Boltzmann modelling for inertial particle microfluidics applications - a tutorial review. Adv. Phys. X, 8(1):2246704, 2023.