Del Giudice, Francesco (2015) Microparticle manipulation in viscoelastic liquids: towards a modular microfluidics. [Tesi di dottorato]

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Microfluidics have received great attention in the last decades due to its wide applicability in molecular analysis, biodefense, molecular biology and microelectronics. In the molecular analysis field, in particular, several ap- plications such as particles/cells counting and separation are explored. For these applications, particles or cells are commonly suspended in Newtonian liquids. 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 of the above mentioned operations by using simpler apparati as compared to the case with a Newtonian liquid. In most applications, microchannels with a square cross-section are used, due to the ease of fabrication. In this thesis, we study the effect of fluid rheology on the suspended par- ticle motion, in square-shaped microchannels. To this aim, several fluids are considered. Elastic, constant-viscosity fluids promote particle migration towards the channel centreline, achieving the so-called 3D focusing. For shear-thinning fluid, in contrast, the migration phenomenon is more com- plex: at low flow rates, i.e. in the constant viscosity region, particles still migrate towards the channel centreline, while at high flow rates, i.e. in the shear-thinning region, the migration reverts direction, and the particles are driven towards the corners of the channel cross-section. In addition, our re- sults highlight the weak effect of inertia on particle migration as compared to viscoelastic effects, even for low elasticity suspending liquids. In the second part of this thesis, we exploit the viscoelasticity-induced parti- cle migration to design a microrheometer capable of estimating the relaxation time of viscoelastic fluids, down to milliseconds. A remarkable improvement in the accuracy of the measure of the relaxation time is found, when comparing our results with experimental data obtained from shear or elongational experiments available in literature. Good agreement of our results with available theoretical predictions is also found. Finally, we design and fabricate a microfluidic device for deflection of mag- netic particles from a contaminated streamflow into a clean one. This device is made of two modules: a first one (a straight channel), where magnetic particles are 3D focused on the channel centreline; a second one (a H-shaped channel), where a permanent magnet is used to displace the magnetic beads from the original to the buffer stream. When viscoelastic focusing and magnetophoresis are combined, by applying the two moduli in series, a deflection efficiency of ∼ 96% can be achieved.

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