Due to increased global concern over clinical research and public healthcare over the past few years, microfluidic-based bioparticle sorting and separation has gained substantial attention as the first step in most diagnostic and therapeutic procedures [1]. Although a number of studies have focused on the applications and mechanics of the elasto-inertial focusing [2,3], lack of combined numerical and experimental investigation, specifically at higher Wi numbers (Wi is a dimensionless number that measures the ratio of elastic to viscous forces) leads to incomplete understanding of the focusing mechanics due to challenges of achieving convergence for Wi > 3. Herein, we numerically and experimentally study the focusing mechanisms at Wi = 3.6 and Wi = 18 and explore the effects of particle size, shear-thinning and N2-induced secondary flow on the migration and focusing of 4 and 7 µm particles.
COMSOL Multiphysics 5.6® is used to solve the momentum and mass conservation equations inside the microchannel. Simulation domain consists of a rectangular duct with the particle present as a spherical hole. Model schematics, mesh configuration details and viscoelastic phenomenon due to first and second normal stress differences (N1 and N2) in a Giesekus fluid are presented in
In order to predict focusing position of particle and explore combined effects of inertia and elasticity on the focusing patterns, elastic, inertial, and total force curves were plotted along the Y-midline of the channel [Fig. 2(af)]. At Wi = 3.6, FEL and FSF push the particles towards the channel center. At higher flowrates, pronounced FSG alongside with FEL will cause particle migration away from the channel center, leading to focusing positions at YP = 17, and YP = 14 for the 7 and 4 µm particles, respectively. Top and side view fluorescent streak images of the focusing position of the same two particle sizes support our simulation results [Fig. 2(g,h)]. Lastly, we showed that the predicted focusing position can be a strong function of the mobility factor in the Giesekus equation, i.e. the extent of shear-thinning behavior of the fluid [Fig. 2(i)]. Particles are shown to focus closer to the channel center when 𝛼 = 0.4, and no focusing is predicted when 𝛼 = 0.1.
We reported that elasto-inertial focusing of particles in rectangular microchannels is achieved by the interplay of FSG, FSF, FEL lift forces. Additionally, our results suggest that the correct prediction of focusing patterns along the Y-midline of the channel is directly affected by the appropriate estimation of rheological properties of the fluid, specifically the extent of shear-thinning behavior. We believe that our results can enhance the existing knowledge on the mechanics of elasto-inertial focusing and pave the way for easier and more accurate design of microfluidic sorting and separation devices.