Session U10: Microscale Flows: Mixing and Chemical Reactions (8:45am - 9:30am CST)

Chemical kinetics and spectroscopy enabled by 3D hydrodynamic focusing and mixing in a 3D-printed microfluidic device

We have developed a three-dimensional (3D) hydrodynamic focusing and 3D-printed microfluidic mixer for chemical kinetics studied by X-ray absorption and emission spectroscopy (XAS/XES). To trigger reactions, our device co-flows a 30 \textmu L min-1 sample stream into a 1 mL min-1 sheath stream. This sheath focuses the sample from a 75 to 10 \textmu m hydraulic diameter within 500 \textmu s. After mixing sheath species into the sample stream, the sample stream is expanded to 50 \textmu m where XAS/XES measurements are performed. The residence times between mixing initiation and measurement are within 2.5 to 350 ms. The fused silica component of our device is a clear monolithic chip fabricated using a femtosecond laser exposure and chemical etching technique. This chip interfaces with a polyimide capillary which provides a low X-ray adsorption region. The system enables X-ray studies of order millisecond reactions, toxic chemicals, and/or anaerobic conditions. We will present the device design and fabrication and the development and experimental validation of convection-diffusion-reaction models. The models are quantitatively validated by (widefield and confocal) microscopy and by XAS/XES experiments of reacting ferricyanide and ascorbic acid. Our combined system and models are applicable to studies of the electronic structure of reaction intermediates.   

Highly diffusive fluid threads in microchannels

The behavior of viscous fluid threads concurrently flowing with fully and partially miscible solvents is experimentally investigated in square microchannels. Diffusive fluid threads significantly swell at low flow velocities due to large specific interfacial area and hydrodynamic lubrication. An approach based on bounded function analysis of confined thread diameter is employed to model diffusive behavior of viscosity-differing fluids at the small scale. This works shows the determination of a critical flow rate associated with each fluid pair and the use of dynamic similarity to calculate diffusion coefficients between oils and organic solvents.   

Understanding flow focusing during the dissolution of porous media

Dissolution by subsurface flows is the critical component in the development of karst systems which transport much of the water we consume. Modeling subsurface flow and reactant transport on large (km) scales involves statistical descriptions of the underlying pore space. However, the presence of heterogeneity, particularly fractures, complicates any averaging or homogenization method. More importantly, heterogeneity typically increases with time due to feedback between dissolution and flow, which amplifies and localizes the flow along preferred paths. We are using finite-volume simulations, based on the OpenFOAM toolkit, to investigate the development of these flow paths in simple well-controlled models of a porous material. I will first present a validation of the physical model, by comparing simulations of a dissolving cylinder with the results of a microfluidic experiment. The accuracy of the numerical method, and in particular the evolution of the boundary, was confirmed by comparison with solutions of closely related problems amenable to conformal mapping. Finally, I will present recent results for the dissolution of arrays of disks, which are a simple model of a porous matrix.   

Modeling the Mixing Efficiency of a low-Re Passive Microfluidic Mixer

Microfluidic mixing is a well-established problem. Multiple microfluidic solutions have been designed to enhance mixing by increasing the interfacial area between initially distinct streams by generating cross-stream mixing. Previous work has modeled the mixing within these devices for flows with characteristic Reynolds numbers (Re) of approximately two orders of magnitude below unity. However, some applications, such as bioprinting, operate in regimes beyond this range. In this work we evaluate a passive microfluidic mixer in this extended Re range. Flows within these micromixers were modeled using the finite volume method to solve the Navier-Stokes equations. The results characterize the mixing efficiency and associated mixing costs for these low-Re flows. Flows with a range of Reynolds numbers were achieved by varying the viscosity and mass flow rates to better describe their differential impacts on mixing. In addition to modeling the mixing of fluids with identical properties, flows involving fluids of unique viscosities and flow rates were also characterized. Modeling the mixing of these different flows enhances understanding of mixing within low-Re flows.