David S. Dandy

Studies on nucleation process in diamond CVD: an overview of recent developments

This paper presents an updated and systematic overview of the recent developments in studies on nucleation processes in diamond CVD. The nucleation mechanisms are discussed, and the nucleation enhancement methods developed to date are summarized. The effects of surface conditions and deposition parameters on the surface nucleation are described. Finally, a brief description of theoretical and modeling studies on the surface nucleation is given.

A sphere in shear flow at finite Reynolds number: effect of shear on particle lift, drag, and heat transfer

Three-dimensional numerical solutions have been obtained for steady, linear shear flow past a fixed, heated spherical particle over a wide range of Reynolds number (0.1 [les ] R [les ] 100) and dimensionless shear rates (0.005 [les ] α [les ] 0.4). The results indicate that at a fixed shear rate, the dimensionless lift coefficient is approximately constant over a wide range of intermediate Reynolds numbers, and the drag coefficient also remains constant when normalized by the known values of drag for a sphere in uniform flow. At lower values of the Reynolds number, the lift and drag coefficients increase sharply with decreasing R , with the lift coefficient being directly proportional to R −½ . For the range of shear rates studied here, the rate of heat transfer to the particle surface was found to depend only on the Reynolds number, that is, it was insensitive to the shear rate. The dimensionless rate of heat transfer, the Nussel number Nu , was seen to increase monotonically with R .

Buoyancy-driven motion of a deformable drop through a quiescent liquid at intermediate Reynolds numbers

Numerical solutions have been obtained for steady streaming flow past an axisymmetric drop over a wide range of Reynolds numbers (0.005 [les ] Re [les ] 250), Weber numbers (0.005 [les ] We [les ] 14), viscosity ratios (0.001 [les ] λ [les ] 1000), and density ratios (0.001 [les ] ζ [les ] 1000). Our results indicate that at lower Reynolds numbers the shape of the drop tends toward a spherical cap with increasing We , but at higher Re the body becomes more disk shaped with increasing We . Unlike the recirculating wake behind an inviscid bubble or solid particle, the eddy behind a drop is detached from the interface. The size of the eddy and the separation distance from the drop depend on the four dimensionless parameters of the problem. The motion of the fluid inside the drop appears to control the behaviour of the external flow near the body, and even for cases when λ and ζ [Lt ] 1 (a ‘real’ bubble), a recirculating wake remains unattached.

Analysis of diamond growth in subatmospheric dc plasma-gun reactors

The growth of diamond in a subatmospheric dc‐arc plasma‐jet reactor has been studied theoretically. Full transport equations for this geometry, including gas‐phase and surface chemistry, have been solved numerically. The surface‐reaction mechanism includes pathways for the incorporation of CH3, C2H2, and C from the gas phase, as well as growth of graphite. The surface mechanism includes full reversibility for all reactions, based on estimates of the thermochemistry. Results are presented for degrees of dissociation of H2 in the plasma gun ranging from 2.6% to 90%, and inlet levels of CH4 spanning 0.1–5.0 mol %. It is seen that CH3 is the predominant growth species when there is little H2 dissociation within the plasma gun, but C becomes the dominant species at higher dissociation levels. The third growth species, C2H2, does not play a role in diamond growth under these conditions when there is less than 1% CH4 in the feed; but, at higher CH4 levels both C and CH3 addition rates drop to 50 times greater th...

Numerical and Experimental Studies of Hydroxyl Radical Chemiluminescence in Methane-Air Flames

Abstract A numerical and experimental study of hydroxyl radical chemiluminescence from methane-air flames was performed. Measurements of the chemiluminescence per unit flame area for lean methane-air flames were obtained, and a model for use in predicting chemiluminescence was developed. The model was one-dimensional and unsteady, incorporating the equations of species continuity and energy, with temperature- and concentration-dependent transport and thermodynamic properties. The reaction mechanism included the chemiluminescent reaction, as well as reactions that both produced and quenched electronically excited OH. It was found, both experimentally and numerically, that the chemiluminescent intensity was highly dependent on the equivalence ratio. For !he range of equivalence ratios studied experimentally (0·65 < φ < 0·90) there was an exponential dependence of chemiluminescence on equivalence ratio. The predicted chemiluminescent intensity was most sensitive to the rate constant of the reaction, , in bot...

Paper-Based Microfluidic Devices: Emerging Themes and Applications

Applications Yuanyuan Yang,† Eka Noviana,† Michael P. Nguyen,† Brian J. Geiss,‡ David S. Dandy, and Charles S. Henry*,†,§ †Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States ‡Department of Microbiology, Immunology, and Pathology, Colorado State University, Fort Collins, Colorado 80523, United States Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado 80523, United States ■ CONTENTS Fabrication 71 Hydrophobic/Solvent Barrier 72 Deposition 73 Flow and Injection Control 74 Three-Dimensional Devices 75 Incorporating Nonsensing Electrodes 75 Colorimetric Detection 75 Detectors and Readout 75 Reflectance-Based Measurement 75 Transmittance-Based Measurement 77 Instrument-Free Measurement 77 Biomedical Applications 77 Enzymatic Methods 77 Immunoassays 78 Other 79 Environmental Applications 79 Other Applications 80 Electrochemical Detection 80 Electrodes 80 Carbon Electrodes 81 Metallic Electrodes 81 Biological Applications 82 Glucose Sensors 82 Immunosensors 84 Other Examples 84 Environmental Applications 84 Other Technologies 85 Chemiluminescence and Electrochemiluminescence 85 Fluorescence 85 Surface-Enhanced Raman Spectroscopy 85 Separation 86 Preconcentration 86 Conclusions and Future Directions 87 Author Information 87 Corresponding Author 87 ORCID 87 Notes 87 Biographies 87 Acknowledgments 88 References 88

Computational simulation of diamond chemical vapor deposition in premixed C2H2/O2/H2 and CH4O2-strained flames

We have modeled combustion synthesis of CVD diamond in a stagnation-flow reactor under atmospheric conditions. In this configuration a premixed flat flame flows over a flat deposition substrate that lies perpendicular to the flow and parallel to the burner face. Optimal growth conditions occur when the flame is lifted from the burner surface and stabilized at the deposition surface. A similarity transformation for the stagnation flow field reduces the governing equations to a one-dimensional boundary value problem, significantly simplifying the computational task. The simulations include elementary gas-phase and surface chemistry as well as multicomponent molecular transport in the flame gas. Our model shows good qualitative agreement with observed growth parameters for the experimental conditions of Murayama et al. [1], who employed a premixed C2H2/H2/O2 gas mixture. Modeling CH4O2 flame synthesis demonstrates that methane is less effective for diamond growth due to the decreased flame temperature and stability compared with C2H2 combustion.

Passive microfluidic pumping using coupled capillary/evaporation effects

Controlled pumping of fluids through microfluidic networks is a critical unit operation ubiquitous to lab-on-a-chip applications. Although there have been a number of studies involving the creation of passive flows within lab-on-a-chip devices, none has shown the ability to create temporally stable flows for periods longer than several minutes. Here a passive pumping approach is presented in which a large pressure differential arising from a small, curved meniscus situated along the bottom corners of an outlet reservoir serves to drive fluid through a microfluidic network. The system quickly reaches steady-state and is able to provide precise volumetric flow rates for periods lasting over an hour. A two-step mathematical model provides accurate predictions of fluid and mass transport dynamics in these devices, as validated by particle tracking in laboratory systems. Precise flow rates spanning an order of magnitude are accomplished via control of the microchannel and outlet reservoir dimensions. This flow mechanism has the potential to be applied to many micro-total analytical system devices that utilize pressure-driven flow; as an illustrative example, the pumping technique is applied for the passive generation of temporally stable chemical gradients.

Array feature size influences nucleic acid surface capture in DNA microarrays

Analyte affinity capture by surface-immobilized diagnostic agents is a routinely used assay format for profiling numerous medically and technologically important target analytes. These assays suffer from numerous performance limitations, including sensitivity and rapidity. Assay miniaturization is advocated to improve surface-capture performance, specifically exploiting the inverse relationship between analyte flux and capture feature size under mass transfer-limiting capture conditions that characterize many such assay formats. Reduced capture feature sizes, e.g., microarrays, are proposed to overcome mass transfer limitations, yet this is difficult to achieve across several size scales. This study validates certain advantages advocated for capture spot miniaturization using a rationale to understand surface capture miniaturization strategies. Experimentally derived immobilized ligand and target capture densities as a function of microspot size for DNA oligomers immobilized on model gold substrates are compared directly with theoretical analysis, validating the hypothesis that miniaturization yields many practical assay advantages. Specifically, results show that transitions from assay mass transfer limiting to kinetically limiting conditions as feature size decreases identify an optimal microspot size range for a specific bioassay system. Analytical advantages realized from such assay miniaturization are more uniform target-spot coverage and substantially increased rate of capture (hybridization), increasing assay signal and rapidity.

Effects of temperature and filament poisoning on diamond growth in hot-filament reactors

The growth of diamond in a hot‐filament reactor has been modeled, and compared with existing experimental data. Studies have been carried out on non‐growth systems containing only hydrogen, as well as on systems where the methane concentration at the inlet was varied between 0.4% and 7.2%. The one‐dimensional stagnation flow model used here includes detailed gas‐phase and surface kinetics. A simple model of filament poisoning has been implemented. The effect of the gas/filament temperature discontinuity on species distributions has also been examined. Gross errors between theory and experiment are obtained when filament poisoning is neglected, but good agreement is found using a simple linear poisoning model. A nonzero temperature discontinuity at the filament produces good overall agreement with experiment.