In-Vitro Wall Shear Stress Measurements for Microfluid Flows by Using Second-Order SPE Micro-PIV

Conventional single pixel evaluation (SPE) significantly improves the spatial resolution of PIV measurements to the physical limit of a CCD camera based on the forward difference interrogation (FDI). This paper further enhances the computational algorithm to second-order accuracy by simply modifying the numerical scheme with the central difference interrogation (CDI). The proposed central difference scheme basically superposes the forward-time and the backward-time correlation domains, thus resulting in reduced bias error as well as rapid background noise elimination. An assessment of the CDI SPE algorithm regarding the measurement errors was achieved via numerous synthetic images subject to a four-roll mill flow. In addition, preliminary wall shear stress (WSS) measurements regarding different algorithms are also evaluated with an analytical turbulent boundary flow. CDI scheme showed a 0.32% error deviated from the analytical solution and improved the same error in FFT-based correlation correlation (FFT-CC) by 32.35%. To demonstrate the performance in practice, in-vitro measurements were implemented in a serpentine microchannel made of polydimethyl siloxane (PDMS) for both CDI SPE and spatial cross-correlation. A series of steady-state flow images at five specified regions of interest were acquired using micro-PIV system. Final comparisons of the WSS regarding the Pearson correlation coefficient, R2, between the numerical schemes and the simulations showed that an overall result was improved by CDI SPE due to the fine resolution and the enhanced accuracy.

Optimization of Geometry for Continuous Flow PCR Devices in a Titer Plate-Based PCR Multi-Reactor Platform

A highly parallel, polymerase chain reaction (PCR) multireactor platform is in high demand to satisfy the high throughput requirements for exploiting the accumulated genetic information from the Human Genome Project. By incorporating continuous flow PCR (CFPCR) devices in a polymer 96-well titer plate format, DNA amplification can be performed with steady-state temperature control and faster reaction speed at lower cost. Prior to the realization of a PCR multi-reactor platform, consisting of a sample delivery chip, a PCR multireactor chip, and a thermal cycler, optimization of the geometry for CFPCR devices in a titer plate-based PCR multi-reactor chip based on manufacturing feasibility is necessary. A prototype PCR multi-reactor chip was designed in a 96-well titer plate format with twelve different CFPCR configurations. High quality metallic, large area mold inserts (LAMIs) were fabricated using an SU-8 based UV-LIGA technique by overplating nickel in SU-8 electroplating templates. Micro molding of polycarbonate (PC) was done using hot embossing, resulting in good replication fidelity over the large surface area. Thermal fusion bonding of the molded PC chips using a custom-made bonding jig yielded acceptable sealing results. The manufacturability investigation throughout the design and the process sequence suggested that the microchannel walls require a minimum width of at least 20 μm and an aspect ratio of 2 for structural rigidity. An optimal CFPCR device for use in a PCR multi-reactor chip can be selected with a series of amplification experiments with the development of a thermal cycler.

Liquid and Gas Phase Velocity Measurements for Two Phase Flow in a Branching Microchannel Network

This is a work in progress. The objective of the present work is to develop techniques for assessing velocity deficits in branching microchannel networks. Liquid velocity distributions were acquired using μPIV in gas-liquid flows through the initial branch in a fractal-like branching microchannel flow network. Gas interface velocities were determined along the centerline of the channel. The flow rate of air and water were 0.0016 g/min and 20 g/min, respectively. The primary observed flow regime was elongated bubbles. Experimental liquid velocities well matched the 0.20 m/s superficial liquid velocity. Experimental interface velocities were approximately an order of magnitude higher than the superficial gas velocity of 0.01 m/s. Velocity deficits based on measurements are on the order of 0.065 m/s. Using interfacial velocities at the channel centerline, the trailing edge velocity was observed to be 15% percent faster, on average, than the leading edge velocity. This could be attributed to bubbles expanding into the bifurcation. Twenty percent standard deviations in average interface velocities were attributed to insufficient samples as well as projected to be a consequence of changing shape of the interface between consecutive image pairs. Changes in bubble shape may also be responsible for the observed differences between leading and trailing edge velocities.

Droplet Formation Under the Effect of a Flexible Nozzle Plate

Droplet formation from a flexible nozzle plate driven by a prescribed-waveform excitation of a piezoelectric is numerically investigated using a computational fluid dynamics (CFD) model with the VOF method. The droplet generator with a flexible nozzle plate, which is free to vibrate due to the pressure acting on the plate, is modeled in a CFD computational domain. The CFD analysis includes the fluid-structure interaction between fluid and a flexible plate using large deflection theory. The problem is characterized by the nondimensional variables based on the capillary parameters of time, velocity, and pressure. The CFD model is validated with the experiment results. This study examines the characteristics of the applied waveforms and nozzle plate material properties to change the vibrational characteristics of the nozzle plate. The effect of fluid properties on the droplet formation process is also investigated focusing on surface tension and viscous forces. The mechanism of the droplet formation excited by a drop-on-demand piezoelectric waveform is investigated using a step-function and a pulse waveform. The piezoelectric displacement plays an important role in generating either forward-driven momentum or a suction pressure inside the chamber. For the step-function waveform, the nondimensional applied impulse is defined and used to characterize the post-breakoff droplet volume. Increasing the impulse of the piezoelectric can be used to cause a faster droplet velocity and it is shown that the vibration of the nozzle plate has a strong effect on the droplet velocity, shape, and volume. Surface tension has strong influence to the droplet formation characteristics which is contrast to a viscous force that makes no difference on the droplet formation for different viscosities. For the combination of a fluid with high surface tension and the most flexible nozzle plate, this system can not cause the droplet ejected out of the nozzle.

Theoretical Investigation of On-Chip Multi-Species Transport in Micro-Channels for Analysis Applications

A complete mathematical model is developed for application to simulate the unsteady two-step on-chip sample injection and separation processes in microfluidic devices. The origin and applicability of the slip-wall velocity boundary condition is discussed. Due to electrophoresis effect, migration influence of every species is considered in the model and then solved for separation analysis. The model is non-dimensionalized in a unique manner to reveal effects of some key fundamental parameters: the Reynolds-Schmidt number, electrophoretic mobility of sample species, applied potentials, etc. In particular, the influence of ReSci is examined over the commonly encountered range and the effect of electrophoretic mobilities on separation is investigated for three different types of samples. Results include center-line concentration profiles as well as concentration contour plots over a range of nondimensional time (less than 400). Resolution is defined and employed to evaluate the separation results. The magnitude of calculated separation resolution (around 2.0) is comparable to experimental results. Through parametric studies, the characteristics of both injection and separation are revealed numerically and well understood for future effective control and innovative chip design.

Flow Field Analysis of Dielectrophoretically-Suspended Particles

Understanding the fluid dynamics around a particle in suspension is important for a complete investigation of many hydrodynamic phenomena, including microfluidic models. A novel tool that has been used to analyze fluid velocity fields in microfluidics is micro-resolution particle image velocimetry (μPIV) [1]. Dielectrophoresis (DEP) is a technique that can translate and trap particles by induced polarization in the presence of nonuniform electric fields. In this paper, DEP has been used to capture and suspend a single 10.1μm diameter spherical particle in a microfluidic channel. μPIV is then used with smaller tracer particles (0.5μm) to investigate the hydrodynamics of fluid flow past the trapped particle.

Atomistic Mechanism of Ion Beam Deposition Induced Curvature Formation in Thin-Films

Molecular dynamics (MD) simulations are performed to study the stress generation mechanisms in cantilever graphene sheets impacted by energetic carbon neutrals. The carbon-carbon interactions are described by the Tersoff-Brenner potential [1]. The MD simulations show that the free-end deflection of the graphene sheets is strongly dependent on the kinetic energy of the incident ions. At low incident energy (<<10eV), the free end bends towards to the side on which ions are deposited (upward deflection); at high incident energy, the free end bends away from the side on which the ions are deposited (downward deflection). The downward deflection reaches its maximum at around 50 eV, beyond which the downward deflection decreases with increasing incident energies. In addition, the evolution of the free-end deflection in terms of the number of deposited atoms is also dependent on the kinetic energy of the incident ions. These numerical observations suggest that intrinsic stress of different levels in the graphene sheets is generated. A close examination of the microstructures of the grown films indicates that the generated stress can be attributed to a competing mechanism of the production and annihilation of vacancy-like and interstitial-like defects in the films.

A Micromachined Silicon Flexure for Tunable External Cavity Diode Laser

We present a novel external cavity diode laser design developed for atomic physics which employs a micromachined silicon flexure to sweep the laser frequency and a volume holographic reflection grating (VHG) to provide the optical feedback. The advantages of using a silicon flexure are its simple microfabrication process and reduction of the overall size of the laser system. The results demonstrate an optimized threshold current reduction from 59mA to 43mA, a frequency sweeping range of 2.069 GHz, and 87Rb, 85Rb (Rubidium) D2 line absorption at 780 nm.

Manipulation of Organic and Inorganic Colloidal Particles by Dielectrophoretic Forces

Recently, manipulation of the micro and nanoscale objects has been of great interest in verity of engineering and scientific areas. Dielectrophoretic force (DEP) induced technique is predominantly used in the manipulation process in a liquid medium. The phenomenon behind DEP involves the creation of electric forces on particles to generate momentum in nonuniform electric fields, usually coming from AC electric fields. In the present study, we will discuss about the effects of DEP for the manipulation of organic and inorganic particles at micro and nanoscale in detail.

Simulation of a Rectangular Cylinder in Cross Flow in a Microchannel Using Information Preservation (IP) Method

A numerical simulation is performed to study the characteristics of the gas flow over a constant temperature rectangular cylinder in a cross flow in a micro channel. The non-isothermal Information Preservation (IP) method is employed to eliminate the statistical scatter of Direct Simulation Monte Carlo (DSMC) at low Reynolds numbers. Pressure boundary conditions based on the characteristic theory are implemented in the algorithm. The simulation results are compared with the references available in the literature. This study will form a base for our future particle-atomistic hybrid computations.