David R. Mott

Two simple and rugged designs for creating microfluidic sheath flow

A simple design capable of 2-dimensional hydrodynamic focusing is proposed and successfully demonstrated. In the past, most microfluidic sheath flow systems have often only confined the sample solution on the sides, leaving the top and bottom of the sample stream in contact with the floor and ceiling of the channel. While relatively simple to build, these designs increase the risk of adsorption of sample components to the top and bottom of the channel. A few designs have been successful in completely sheathing the sample stream, but these typically require multiple sheath inputs and several alignment steps. In the designs presented here, full sheathing is accomplished using as few as one sheath input, which eliminates the need to carefully balance the flow of two or more sheath inlets. The design is easily manufactured using current microfabrication techniques. Furthermore, the sample and sheath fluid can be subsequently separated for recapture of the sample fluid or re-use of the sheath fluid. Designs were demonstrated in poly(dimethylsiloxane) (PDMS) using soft lithography and poly(methyl methacrylate) (PMMA) using micromilling and laser ablation.

Design and evaluation of a Dean vortex-based micromixer

A mixer, based on the Dean vortex, is fabricated and tested in an on-chip format. When fluid is directed around a curve under pressure driven flow, the high velocity streams in the center of the channel experience a greater centripetal force and so are deflected outward. This creates a pair of counter-rotating vortices moving fluid toward the inner wall at the top and bottom of the channel and toward the outer wall in the center. For the geometries studied, the vortices were first seen at Reynolds numbers between 1 and 10 and became stronger as the flow velocity is increased. Vortex formation was monitored in channels with depth/width ratios of 0.5, 1.0, and 2.0. The lowest aspect ratio strongly suppressed vortex formation. Increasing the aspect ratio above 1 appeared to provide improved mixing. This design has the advantages of easy fabrication and low surface area.

3D hydrodynamic focusing microfluidics for emerging sensing technologies.

While the physics behind laminar flows has been studied for 200 years, understanding of how to use parallel flows to augment the capabilities of microfluidic systems has been a subject of study primarily over the last decade. The use of one flow to focus another within a microfluidic channel has graduated from a two-dimensional to a three-dimensional process and the design principles are only now becoming established. This review explores the underlying principles for hydrodynamic focusing in three dimensions (3D) using miscible fluids and the application of these principles for creation of biosensors, separation of cells and particles for sample manipulation, and fabrication of materials that could be used for biosensors. Where sufficient information is available, the practicality of devices implementing fluid flows directed in 3D is evaluated and the advantages and limitations of 3D hydrodynamic focusing for the particular application are highlighted.

A combinatorial approach to microfluidic mixing

A new computational approach to the modeling and design of efficient microfluidic mixers is demonstrated. The mixers created provide far more rapid mixing than previous designs. A set of mixer components is created and mapped using a traditional Navier–Stokes fluid solver. The maps are used to quickly model the effect each component has on the lateral distribution of species in the channel. For a mixer of a given length, all the possible combinations of components can be evaluated, and the best mixer for a given metric can be found. Although the mixers presented in this study are short (length-to-width ratios below 8), they show degrees of mixing comparable to much longer mixers found in the literature.

Microfilter simulations and scaling laws

A series of direct simulation Monte Carlo calculations of flows through microfilters were performed to evaluate the range of validity of a previously derived scaling law. This scaling law, which describes how the pressure drop across a filter depends on the Reynolds number and filter geometry, is based on Navier-Stokes calculations and experiments in the continuum regime. The simulations show that this scaling law predicts the correct Reynolds number dependence for a fixed Knudsen number, but the magnitude of the pressure drop is overpredicted as effects due to collisional nonequilibrium become important. The results of the simulations were used to derive a correction term to the scaling law that includes the Knudsen number and, thus, accounts for nonequilibrium effects.

Optimizing mixing in lid-driven flow designs through predictions from Eulerian indicators

In this paper, we further develop the notion of Eulerian indicators (EIs) for predicting Lagrangian mixing behavior. We employ a two-dimensional “blinking” Stokes flow as a model for mixing in a three-dimensional, spatially periodic channel flow. Each blinking flow alternates two distinct velocity fields that were calculated using a lid-driven cavity model. A new EI termed mobility is introduced to measure how effectively the blinking velocity fields transport fluid throughout the domain. We also calculate the transversality for these flows, which is an EI measuring how much the velocity direction at each point in the domain changes when the velocity fields blink. For the studied flows, we show that although individually the mobility and transversality do not correlate well with mixing as measured by the decay of the variance of concentration, the product of mobility and transversality does correlate well with the decay of the variance of concentration and predicts which combinations of velocity fields wi...

Designing Microfluidic Components for Enhanced Surface Delivery Using a Genetic Algorithm Search

*† ‡ § The Toolbox approach to the automated design of microfluidic components is extended to include a genetic algorithm search of candidate designs. Performance metrics for characterizing surface delivery are described, and the software is applied to choose sequences of grooves to add to a rectangular microchannel in order to optimize surface delivery for pressure-driven flow. Initial searches using five groove shapes identify designs that perform much better than standard mixers found in the literature. These initial searches produced two sets of competing designs, and each set was dominated by a different subset of the allowable groove shapes. Additional targeted searches that limited the groove choices to each of these two subsets produced additional significant improvements in the designs.

Viscous Normal Shock Solutions Including Chemical, Thermal and Radiative Nonequilibrium

An existing axisymmetric body viscous shock layer code including thermochemical and thermodynamic nonequilibrium and nonequilibrium radiative gasdynamic coupling was adapted to simulate the one-dimensional flow within a shock tube. A suitable solution scheme for this case and additional radiation modeling were developed in order to compare the current computational results with experimental radiation measurements. Spectrally integrated intensity traces, time to peak radiation, and ionization distance data were generated for shocks in air with speeds between 9.5-12.6 km/s. Using the current model, the dual peak characteristics of Wilsons experimental results are reproduced without the introduction of contaminant radiation. Overall, good agreement is seen between the current calculations and the available experimental data, justifying the use of the current nonequilibrium models for engineering applications.

Microfabricated Gas Chromatograph for Trace Analysis

Microfabricated portable gas analyzers with high sensitivity and selectivity offer utility in a variety of critical applications including aviation security, food safety and toxic industrial waste monitoring. An integral component of such analyzers is the gas chromatographic (GC) column which is used for separations of an injected mixture based on the relative sorption of the various analytes in the carrier gas by the stationary phase. In this interim report, we describe our efforts in the design and development of a microfabricated GC column for the trace detection of hazardous chemicals. Specifically, in this work an optimized serpentine layout with a circular cross- sectional profile has been microfabricated. In this work, computational fluid dynamic (CFD) modeling has been employed as a method to aid in the GC column design optimization. Selectivity to hazardous hydrogen bond basic (HBB) analytes (e.g. TNT, GB, VX) was achieved by using an NRL developed hydrogen bond acid (HBA) sorbent polymer HCSFA2 as the stationary phase. HCSFA2 offers a higher partition coefficient than the more commonly used polysiloxanes.

A Lagrangian Advection Routine Applied to Microfluidic Component Design

An advection routine for determining the distributions of passive scalars flowing through microfluidic components is presented. This approach is then used to analyze and design passive, in-channel microfluidic mixers. The Lagrangian advection routine takes a steadystate velocity field through the component and calculates the distribution of conserved scalars exiting the component by backtracking upstream from points in the outflow plane. Each velocity component is assumed to vary linearly within a computational cell, and this assumption leads to an analytical solution for particle paths passing through the cell. The advection routine then constructs piecewise-exact solutions for the particle path through the component and need not numerically integrate the trajectory. This approach prevents particles from being “trapped” in slow regions near walls, and in contrast to placing tracer particles in the inflow and calculating where they exit the component, backtracking provides a uniform distribution of information across the entire outflow plane.