John C. Wells

Design, simulation and fabrication of a total internal reflection (TIR)-based chip for highly sensitive fluorescent imaging

This paper presents a total internal reflection based chip which generates evanescent waves for highly sensitive fluorescent imaging. The chip is monolithically, massively cast in polydimethylsiloxane (PDMS) at a very low cost using a Si mold fabricated by Si anisotropic wet etching and deep reactive ion etching (DRIE). Our method integrates all miniaturized optical components, namely cylindrical microlens, prism and waveguide, into one monolithic PDMS chip; thus assembly is unnecessary, and misalignment is eliminated. The slide-format and monolithic chip can be used with both upright and inverted fluorescent microscopes with flexible sample delivery platforms. The flexibility of sample delivery platforms facilitates various surface treatment/immobilization techniques required in fluorescent imaging. Moreover, the fiberoptics coupling into the chip allows a broad choice of wavelengths and types of laser sources ranging from UV to IR. We have successfully demonstrated the capability of the chip in highly sensitive imaging of tetramethylrhodamine (TMR) fluorescent dye and immobilized fluorescent nanobeads. Our monolithic, miniaturized TIR-based chip could potentially serve as an evanescent excitation-based platform integrated into a micro-total analysis system (μ-TAS).

Six-degree of freedom micro force-moment sensor for application in geophysics

This paper presents the design concept, fabrication and calibration of a six-degree of freedom (6-DOF) turbulent flow micro sensor utilizing the piezoresistive effects in silicon. The proposed sensor can independently detect six components of force and moment on a test particle in a turbulent flow. By combining p-type conventional and shear piezoresistors in Si [111], and arranging them suitably on the sensing area, the total number of piezoresistors used in this sensing chip is only eighteen. Calibration for six components of force versus output voltages was carried out. The sensitivities are linear, close to the design values, and high enough to measure the forces and moments expected to act on the particles in turbulent flow in geophysics.

Numerical Simulation of Granular Materials Based on Smoothed Particle Hydrodynamics (SPH)

Granular flows play an important role in many industrial processes. To optimize these processes, it is necessary to simulate the flow accurately. However, due to the complexity of the granular flows, there is no generally accepted theory or computational method at the present. This paper presents a new numerical approach based on the smoothed particle hydrodynamics method (SPH) to simulate granular materials, which are assumed to be elasto‐plastic. Herein, the Drucker‐Prager model with non‐associated plastic flow rule was employed to describe elasto‐plastic behavior of granular flows, and the accuracy of SPH approximation of governing equation is improved by adopting a procedure to renormalize the kernel derivative. Application of SPH to simulate granular flow in a silo was presented after a validation of SPH with experiment. It is shown that the current SPH model can capture overall behavior of granular flows.

Micro force-moment sensor with six-degree of freedom

This paper describes the design concept, theoretical investigation and fabrication process of a micro multi-axis force-moment sensor utilizing the piezoresistive effect in silicon. The purpose of the sensor development is to measure the force and moment acting on boundary particles in a turbulent liquid flow. The sensor was designed to independently detect 3 components of force and 3 components of moment in three orthogonal directions. Conventional type and four-terminal piezoresistors have been combined in a single sensing chip.

Explicit vs. Implicit Particle-Liquid Coupling in Fixed-Grid Computations at Moderate Particle Reynolds Number

In an attempt to develop a reliable numerical method that can deal economically with a large number of rigid particles moving in an incompressible Newtonian fluid at a reasonable cost, we consider two fictitious-domain methods: a Constant-density Explicit Volumetric forcing method (CEV) and a Variable-density Implicit Volumetric forcing method (VIV). In both methods, the mutual interaction between the solid and the fluid phase is taken into account by an additional body force term to the Navier-Stokes equations, but the physical meaning of the forcing is different for the two methods. In the CEV method, which is built on a constant-density Navier-Stokes solver, the net forcing added to the fluid is generally not zero, and must be cancelled by applying Newton’s first law to a rigid particle template which has the same shape as the rigid particle and carries the “excess mass” of the rigid particle, i.e. the excess over the mass of the displaced fluid. The “target velocity” to which one forces the velocity within the particle is evaluated through the equation of motion for the rigid particle template. In the VIV method, built on a variable-density incompressible flow solver, the rigid particle (angular) velocity is determined by averaging the (angular) momentum, within the particle domain, of the fractional-step velocity field, and the net forcing is zero. By design, this method does not require any rigid particle template equations, so it can be applied for both neutral and non-neutral density ratios without any difficulties. We consider two test problems with single freely moving circular disks: a disk falling in quiescent fluid, and a disk in Poiseuille channel flow. At near-neutral density ratios, the CEV method is found to perform better, while the VIV method yields more accurate results at higher relative density ratios.Copyright

A MEMS-based microsensor to measure all six components of force and moment on a near-wall particle in turbulent flow

This paper presents the development of a six-degree of freedom (6-DOF) force moment sensor utilizing the piezoresistive effects in silicon. The proposed sensor can independently detect six components of force and moment on a test particle in a turbulent flow. By combining p-type conventional and shear piezoresistors in Si (111), and arranging them suitably on the sensing area, the total number of piezoresistors used in this sensing chip is only eighteen, much fewer than that of the prior art piezoresistive 6-DOF force sensors. Calibration for six components of force versus output voltages was completed. The sensitivities are linear, close to the design values. Preliminary results of measurement of forces and moments acting on a test particle in turbulent flow will be presented.

Development of PIV/Interface Gradiometry to Handle Low Tracer Density and Curved Walls

Despite the importance of near-wall flow, the well-known difficulties of applying PIV adjacent to walls have attracted little attention. In recent work, the authors have proposed and validated an extension of PIV, called “Interface Gradiometry” (PIV/IG), designed to directly measure the velocity gradient at a fixed wall. For a suitable choice of template height, combined with a sufficiently high density of flow tracers, the method was found to yield substantially more accurate results for wall velocity gradient than PIV/PID. This notwithstanding, the following restrictions and issues demand attention. For accurate measurements the template height must be sufficiently thin that the velocity increases only linearly therein; this is often a restrictive condition at practical Reynolds numbers. On the other hand, tracer concentration is often low adjacent to a wall, and it is often difficult to obtain enough tracers within templates satisfying the linearity condition. Finally, the method requires precise knowledge of the position of the wall. Accordingly, we present a technique that relies less critically on the choice of template height and on presumed wall position, while still exploiting most constraints that the wall imposes on the adjacent flow. Assuming the wall to be horizontal on the image, the basic method is, very simply, to perform 1D PIV on each horizontal line of pixels within the template. The principal “deliverable” at each point on the boundary is the wall-normal profile of horizontal velocity. In addition, our new work handles curved walls by transforming image segments into rectangles; this proposed enhancement should be significant in applications to real boundaries found industrially, or in biomedical imaging. The method is tested successfully on experimental images from a turbulent, locally recirculating flow over a sinusoidal wall.Copyright

Dye Visualization and P.I.V. in the Cross-Stream Plane of a Turbulent Channel Flow

Illuminating in a quasi-cross-stream plane of an open channel flow of water at low Re and injecting fluorescene dye from a bed slit, mushroom patterns reliably inferred to result from counter-rotating streamwise vortices are found to be typically oriented at some 30° from the cross-stream vertical. PIV in the cross-stream plane shows that the averaged field, and two-point correlation, of the quantity \({Q_x} = \left( {\omega _x^2 - S_x^2} \right)/2,\left( {{S_x} = \frac{{\partial v}}{{\partial z}} + \frac{{\partial w}}{{\partial y}}} \right)\) exhibit preferred directions in the cross-stream plane at about 45 degrees to the vertical. Spatio-temporal measurements of streamwise vorticity provide direct experimental evidence, possibly the first, for the staggered arrangement of streamwise vortices previously educed from DNS data.

A dual-color Total Internal Reflection (TIR)-based chip for simultaneous detection of two fluorophores

We report a novel, dual-color total internal reflection (TIR)-based chip which can generate two overlapping evanescent fields for simultaneous detection of two fluorophores. The chip was monolithically fabricated using Si bulk micromachining and PDMS casting. Our proposed method integrated all miniaturized components, including two cylindrical microlenses, one prism, two fiber alignment grooves and two fiber stoppers, into one monolithic PDMS chip; thus assembly is unnecessary, and misalignment is avoided. We first demonstrated the capabilities of the chip by detecting simultaneously two fluorescent dyes, namely Tetramethylrhodamine (TMR) and Fluorescein. We then employed the chip to image mixture of Nile-red (NR) and Dragon Green (DG) fluorescent beads. Our miniaturized, integrated device could be an alternative to the conventional dual-color Total Internal Reflection Fluorescent Microscopy (dual-color TIRFM) systems. It could also be a useful component of a micro-Total Analysis System (mu-TAS) for highly-sensitive dual-color fluorescent detection.

A Monolithic Dual-Color Total-Internal-Reflection-Based Chip for Highly Sensitive and High-Resolution Dual-Fluorescence Imaging

We report a dual-color total-internal-reflection (TIR)-based chip that can generate two overlapping evanescent fields with different wavelengths for simultaneous imaging of two types of fluorophores. We derived a general relationship among the dimensions of the components of the chip to guarantee the overlap of two evanescent fields. Optical simulation results also confirm the generation and overlap of two evanescent fields. Using Si bulk micromachining and poly(dimethylsiloxane) (PDMS) casting, our fabrication method integrates all miniaturized optical components into one monolithic PDMS chip. Thus, assembly is unnecessary, and misalignment is avoided. Our PDMS chip can be employed with various sample delivery platforms, such as glass slide, flow cell, microchannel, etc. We first demonstrated the capability of the chip by imaging TIR fluorescent spots of a mixture of two fluorophores, namely, fluorescein and tetramethylrhodamine. We then employed the chip to observe the Brownian motion of a mixture of nile-red and dragon-green 500-nm microbeads. Our chip could potentially be integrated into a micro-total analysis system for highly sensitive and high-resolution dual-fluorescence imaging applications.