| Summary: |
Rapid mixing is an essential requirement of many liquid microfluidic systems used in biochemistry analysis, drug delivery and sequencing and synthesis of nucleic acids. It is the case of such biological processes as cell activation, enzyme reactions, protein folding or lab lab-on on-chip platforms for complex reactions. Since the flows in these systems are characterized by low Reynolds numbers, in contrast to macrosystems where mixing is often very quick due to turbulence, mixing at the micro scale requires extremely long times, unless other nonlinearities are brought into effect to compensate for the lack of turbulence. Fortunately, in these microsystems many fluids contain additives that make them non non-Newtonian and in particular viscoelastic. The nonlinearities associated with viscoelasticity lead to secondary flows, thus constituting a convenient way to passively promote stirring by chaotic advection and the more so because often these secondary flows lead to flow instabilities which further enhance mixing. This combination of effects for viscoelastic fluids remains to be investigated at microscales and is the motivation for the present research proposal. Here, we wish to investigate in detail, both experimentally and numerically, the flow of viscoelastic fluids in a number of fundamental geometries in order to assess the role of rheology upon mixing mechanisms, in order to propose and possibly patent new efficient micromixers for non non-Newtonian fluids.
The simplest mixer is the 90° T mixer and this research project is based on this simple geometry. Two colored streams of the same fluid will be mixed in the mixer, using fluorescent dyes and microspheres, and from the probability density function of the intensity images the concentration of dye, and hence the mixing efficiency, can be quantified. The following typified geometries will be investigated, as well as some combined geometries: (1) the base case is the 90° mixer with a square outer cross sec  |
Summary
Rapid mixing is an essential requirement of many liquid microfluidic systems used in biochemistry analysis, drug delivery and sequencing and synthesis of nucleic acids. It is the case of such biological processes as cell activation, enzyme reactions, protein folding or lab lab-on on-chip platforms for complex reactions. Since the flows in these systems are characterized by low Reynolds numbers, in contrast to macrosystems where mixing is often very quick due to turbulence, mixing at the micro scale requires extremely long times, unless other nonlinearities are brought into effect to compensate for the lack of turbulence. Fortunately, in these microsystems many fluids contain additives that make them non non-Newtonian and in particular viscoelastic. The nonlinearities associated with viscoelasticity lead to secondary flows, thus constituting a convenient way to passively promote stirring by chaotic advection and the more so because often these secondary flows lead to flow instabilities which further enhance mixing. This combination of effects for viscoelastic fluids remains to be investigated at microscales and is the motivation for the present research proposal. Here, we wish to investigate in detail, both experimentally and numerically, the flow of viscoelastic fluids in a number of fundamental geometries in order to assess the role of rheology upon mixing mechanisms, in order to propose and possibly patent new efficient micromixers for non non-Newtonian fluids.
The simplest mixer is the 90° T mixer and this research project is based on this simple geometry. Two colored streams of the same fluid will be mixed in the mixer, using fluorescent dyes and microspheres, and from the probability density function of the intensity images the concentration of dye, and hence the mixing efficiency, can be quantified. The following typified geometries will be investigated, as well as some combined geometries: (1) the base case is the 90° mixer with a square outer cross section, where pure diffusion (Newtonian fluid) and the effect of a weak secondary flow due to fluid elasticity are investigated initially; (2) to enhance mixing asymmetric baffles are put along the outer straight square duct in order to create a meandering flow; (3) to promote elastic instabilities, the square cross cross-section duct will no longer be straight but made from successive counter rotating 180° turns of constant curvature, but all contained in the same plane; (4) finally, a fourth geometry will have irregular obstacles to further enhance mixing. The numerical simulations will attempt to predict the measured flows and the results will be validated by experimental data. At the end, numerical simulations will also be carried out in a fifth geometry alike geometry (3), but with the duct twists taking place in the third dimension. This should enhance chaotic advection, but at an extra manufacturing cost. |