Elastic boost: an ideal non-linear rheology for particle manipulation and sorting

Particle sorting and manipulation have applications in a wide range of fields, from pharmaceutical sciences to biomedical and material engineering. It has been shown that the non-linearity in the background fluid flow, either coming from the fluid inertia or from the non-Newtonian rheology of the carrier fluid, can be used for this purpose. The latter is more efficient when a more controlled process is required since it can be performed even in the low inertia regime. For example, the viscoelasticity of the fluid has been introduced to induce particle sorting. Our preliminary studies [1,2] have suggested that yield-stress behaviour could provide yet another alternative for particle sorting. The synergy between elasticity and plasticity of the suspending fluid (elastoviscoplasticity) [3,4] is demonstrated to trigger particle-focusing in the core unyielded region inside the flow field which can keep the particles quiescent relative to the background flow while being purely transported (i.e. without any severe straining in case of soft particles/biomedical applications due to the semi-solid behaviour of the core unyielded region). Very recently, we investigated more realistic conditions to take one further step in implementing these concepts efficiently in practice [5]. [1] Chaparian & Tammisola, J. Fluid Mech. 885 (2020) A45. [2] Chaparian et al., J. Non-Newtonian Fluid Mech. 284 (2020) 104376. [3] Chaparian & Tammisola, J. Non-Newtonian Fluid Mech. 271 (2019) 104148. [4] Villalba et al., J. Non-Newtonian Fluid Mech. 319 (2023) 105078. [5] Habibi et al., J. Fluid Mech. 1007 (2025) A36.