Numerical investigation of heterogeneous soft particle pairs in inertial microfluidics

Inertial particle microfluidics is a relatively new technology that exploits inertial effects to manipulate and control particle dynamics. Under inertial flow conditions, particle-particle interactions can lead to the formation of pairs and trains of particles. We analyse the behaviour of two particles, modelled as capsules, with different softness in channel flow under moderate inertia. We employ an immersed-boundary-lattice-Boltzmann-finite-element solver to account for the fluid mechanics, the capsule dynamics, and their coupling. We find that the initial configuration and the heterogeneity of particle softness are important. Pairs are more likely to form when the leading particle is stiffer than the lagging particle. The lateral equilibrium position of the particles in the pair is determined by both the leading and lagging particle. However, the axial spacing in a stable pair is dominated by the softness of the lagging particle. Our study has important implications for real-world applications. Since pair formation is undesired for particle separation, it is generally important to know under which circumstances particles with different properties are able or unable to form a pair. Applications relying on precise inter-particle spacing, such as cytometry, might benefit from our findings as well.