Non-Reactive and Reactive Micromixing at Low Peclet Number Conditions

Micro-scale mixing coupled with chemical and biological reactions is a largely overlooked phenomenon that underpins numerous seemingly different industrial, biomedical, energy, and environmental applications. It is of particular importance to current efforts aiming at precise predictions of subsurface contaminant transport; existing models lack adequate integration of non-reactive and reactive micromixing of the species involved. Our work aims at bridging this gap by providing fundamental understanding of non-reactive and reactive micromixing under different physical and chemical heterogeneities and providing quantitative data suitable for integration into large scale models. In the present work, we present results of non-reactive and reactive micromixing experiments at low Peclet number (Pe) conditions and comparisons with Lattice-Boltzmann simulations. The experiments were performed by parallel injection of two nonreactive or reactive species into a porous silicon micromodel etched using standard photolithographic techniques and having dimensions of 1-cm x 2-cm x 40-microns. Advanced fluorescent microscopy and imaging techniques were used to visualize and quantify the flow fields and reactions. To complement the experiments, 2-D Lattice-Boltzmann (LBM) simulations were performed using the exact geometry of the micromodel used in the experiments. The LBM is a relatively new and powerful numerical tool for simulating single-phase, multi-component flow and reactive transport in porous media at the pore scale. It has the advantage of describing non-equilibrium dynamics, especially in fluid-flow applications involving interfacial dynamics and complex boundaries and is well suited for capturing the details of the evolving geometry of a porous medium, and the relevant physical and chemical processes. Results of both experiments and LBM simulations showed strong qualitative agreement, but quantitatively the simulation results indicated mixing evolution rates that are about 4 times higher than those in the experiments. This discrepancy is attributed to difference in the dimensionality (2-D vs. 3-D) between the experimental and simulation geometries. Both experiments and LBM simulations captured the pre-equilibrium (transient) processes underpinning reactive and non-reactive micro-mixing at Low-Pe conditions and elucidated the relative roles of transverse and longitudinal dispersion. This work is funded by Los Alamos National Laboratory's LDRD Program, project No. 20070267ER