Particle dynamics in microfluidics: slip and deformability

In the last decades there has been a growing interest in microfluidics, i.e. a technology characterized by the engineered manipulation of fluids at the sub-millimeter scale, due to the advantages of a fine tuning of flow and transport conditions and the capability to handle micrometric particles, which are fundamental in both synthesis and analysis. More recently, an increasing interest in using viscoelastic fluids in microfluidic devices is observed. Indeed, it has been recently proven that the non-Newtonian rheological properties of the fluid allow to perform several operations by using simpler apparatuses as compared to the case with a Newtonian liquid. However, at small scale and with complex fluids some peculiar phenomena can occur, such as the boundary slip, which can affect both the fluid velocity profile and the fluid-particle interactions. In this thesis, we study the effects of boundary slip on the dynamics of a spherical particle suspended in an inertialess Newtonian or viscoelastic shear-thinning fluid, under shear or Poiseuille flow, by means of 3D Arbitrary Lagrangian Eulerian (ALE) Finite Element Method (FEM) direct numerical simulations. In particular, we investigate separately on the effect of wall and particle slip on the cross-stream migration induced by fluid viscoelasticity. Furthermore, many particles that are treated in microfluidics are deformable, especially in biological and medical applications (from microgels to cells). Hence, in the second part of this thesis, we address the dynamics of an elastic particle suspended in an inertialess Newtonian fluid flowing through a channel with an orthogonal side branch (asymmetric T-shaped bifurcation) by means of 2D ALE FEM simulations. The effects of the elastic capillary number and the confinement ratio on the particle trajectory and deformation near the bifurcation are investigated. We discuss how this device can be exploited for separating particles based on their elasticity.