Erol C. Harvey

Numerical investigation of mixing in microchannels with patterned grooves

Mixing in microchannels with patterned grooves was studied numerically by CFD simulations and particle tracking technique. Point location, velocity interpolation and a fourth-order adaptive Runge–Kutta integration scheme were applied in the particle tracking algorithms. Using these algorithms, Poincare maps were calculated from the 3D velocity field exported from a CFD package for microfluidics (MemCFD™). For small aspect ratio (α = 0.05) grooves, the results showed that there was no significant irregularity in the Poincare map, and indicated little chaotic effect. For high aspect ratio (α = 0.30) grooves, the flow pattern became more jumbled, but there was no apparent evidence that indicated the flow was chaotic, for Reynolds numbers up to five. However, Poincare maps could still be used to evaluate the performance of this type of micro-mixer. The particle trajectories recorded in the Poincare maps were circular-like patterns. By counting the number of dots to form one circle in the Poincare maps, the length of the channel to enable one recirculation could be calculated. The results indicated that this length had an exponential relation to the aspect ratio of grooves, and it was independent of the flow velocity. The CFD simulation showed that the transverse motion could fold and stretch fluids to increase their interfacial area. The results showed that micro-mixers with patterned grooves caused rotation of the fluid streams. This rotation can reorient the folding in the depth direction, and can enhance passive mixing in microfluidic devices with shallow channels.

Nanometer thickness laser ablation for spatial control of cell attachment

We demonstrate here a new method to control the location of cells on surfaces in two dimensions, which can be applied to a number of biomedical applications including diagnostic tests and tissue engineered medical devices. Two-dimensional control over cell attachment is achieved by generation of a spatially controlled surface chemistry that allows control over protein adsorption, a process which mediates cell attachment. Here, we describe the deposition of thin allylamine plasma polymer coatings on silicon wafer and perfluorinated poly(ethylene-co-propylene) substrates, followed by grafting of a protein resistant layer of poly(ethylene oxide). Spatially controlled patterning of the surface chemistry was achieved in a fast, one-step procedure by nanometer thickness controlled laser ablation using a 248 nm excimer laser. X-ray photoelectron spectroscopy and atomic force microscopy were used to confirm the production of surface chemistry patterns with a resolution of approximately 1 µm, which is significantly below the dimensions of a single mammalian cell. Subsequent adsorption of the extracellular matrix proteins collagen I and fibronectin followed by cell culture experiments using bovine corneal epithelial cells confirmed that cell attachment is controlled by the surface chemistry pattern. The method is an effective tool for use in a number of in vitro and in vivo applications.

Observation of an x-ray vortex

Phase singularities are a ubiquitous feature of waves of all forms and represent a fundamental aspect of wave topology. An optical vortex phase singularity occurs when there is a spiral phase ramp about a point phase singularity. We report an experimental observation of an optical vortex in a field consisting of 9-keV x-ray photons. The vortex is created with an x-ray optical structure that imparts a spiral phase distribution to the incident wave field and is observed by use of diffraction about a wire to create a division-of-wave-front interferometer.

Design, fabrication and testing of piezoelectric polymer PVDF microactuators

Piezoelectric polymers are increasingly considered as favorable materials for microactuator applications due to their fast response, low operating voltages and greater efficiencies of operation. However, the difficulty of forming structures and shapes has so far limited the range of mechanical design. In this work, the design and fabrication of a unimorph piezoelectric cantilever actuator using piezoelectric polymer polyvinylidene fluoride (PVDF) with an electroplated layer of nickel iron (permalloy) alloy is described. The modeling and simulation of the composite cantilever was performed using CoventorWare to optimize the design parameters in order to achieve large tip deflections. These simulation results indicated that the thicknesses of both the piezo and non-piezo layers of the composite cantilever affect the magnitude of deflection of the cantilever. It was shown that the tip deflection of such a cantilever with a length of 5?mm and a width of 1?mm can reach up to 70??m, when simulation was carried out using a 28??m thick PVDF layer at a non-piezo layer thickness of 5??m. A PVDF polymer cantilever is fabricated using a simple punching technique based on microembossing. The permalloy layer was electroplated on one side of the PVDF to form a composite cantilever. The tip deflection of the cantilever was observed and measured under an optical microscope. The experimental results showed deflection values which are 20% less than those predicted by the simulation and analytical results. The thickness non-uniformity, residual stresses and possible difference in Youngs modulus values of the bulk material to that of electroplated permalloy film are identified as some of the potential issues that might have caused this difference.

Microstructuring by excimer laser

Excimer laser ablation provides the micromachining engineer with a unique tool for patterning, cutting, and structuring a wide variety of materials, including ceramics, glasses, and polymers. The short pulse (20 ns) ultra violet laser beam is used for nonthermal ablative material removal producing structures with a depth resolution of the order of 0.1 micrometers and spatial resolutions of the order of 1 micrometers or better. Careful control of laser dose (usually done using CNC systems) enables multi-level machining to be performed producing 3D microstructures which may be used directly, or as mold tools for laser-LIGA replication. This talk aims to illustrate both the possibilities, and limitations, of micormachining by excimer laser ablation, and will highlight some practical examples of structures and devices manufactured using this tool, many of which are currently in or near commercial production.

Semi-automatic calibration technique using six inertial frames of reference

A triaxial accelerometer calibration technique that evades the problems of the conventional calibration method of aligning with gravity is proposed in this paper. It is based on the principle that the vector sum of acceleration from three sensing axes should be equal to the gravity vector. The method requires the accelerometer to be oriented and stationary in 6 different ways to solve for the 3 scale factors and 3 offsets. The Newton-Raphson method was employed to solve the non-linear equations in order to obtain the scale factors and offsets. The iterative process was fast, with an average of 5 iterations required to solve the system of equations. The accuracy of the derived scale factors and offsets were determined by using them to calculate the gravity vector magnitude using the triaxial accelerometer to measure gravity. The triaxial accelerometer was used to measure gravity 264 times to determine the accuracy of the 44 acceptable sets of scale factors and offsets derived from the calibrations (gravity was assumed to equal 9.8000 ms-2 during the calibration). It was found that the best calibration calculated the gravity vector magnitude to 9.8156 ± 0.4294 ms-2. This equates to a maximum of 4.5% error in terms of a constant acceleration measurement. Because of the principle behind this method, it has the disadvantage that noise/error in only one axis will cause an inaccurate determination of all the scale factors and offsets.

Fabrication techniques and their application to produce novel micromachined structures and devices using excimer laser projection

New techniques for 3D micromachining by direct laser ablation of materials using excimer lasers have been developed. Basic to all of these techniques is the use of image projection in which the laser is used to illuminate an appropriate pattern on a chrome-on-quartz mask. The mask is then imaged by a high- resolution lens onto the sample. Non-repeating patterns with areas of up to 150 multiplied by 150 mm can be machined with sub-micron resolution and total accuracies of the order of a few microns by using synchronized scanning of the mask and workpiece. A combination of synchronized mask scanning and mask dragging techniques (in which the mask is held stationary and the workpiece moved during laser firing) enables patterns of up to 400 multiplied by 400 mm to be produced; the limiting feature being the travel and accuracy of the recision air- bearing stages used to support the workpiece. This talk describes the synchronized mask scanning and mask dragging techniques and illustrates their application by presenting details of novel micromachined structures and devices so produced. These include rapid prototyping of bioprocessor chips, fabrication of mechanical anti-reflection structures in CsI infra-red optical material, patterning films as frequency selective reflecting structures, laser-LIGA and high aspect ratio machining using lamination techniques to produce an optical methane detector.

Effects of polymer properties on laser ablation behaviour

An investigation aiming to seek a correlation between ablation rates and various polymer thermal properties, based on experimental ablation data generated for 14 polymers commonly used in microfluidics, is presented. A statistical analysis was carried out for laser fluence against various polymer descriptors and/or their combinations. The results of the analysis show a relatively high correlation coefficient of 0.82 for polymer ablation data when we compare fluence against the product of ablation rate and the difference between the glass transition temperature and room temperature. The effects of polymer properties are also illustrated by an investigation of ablation behaviour of DNQ/novolak thin films, which had been exposed to different levels of UV radiation prior to laser ablation, using atomic force microscopy. The surface characteristics of the thin films following laser irradiation are discussed in terms of differences in laser absorption and the glass transition temperature of the films. The results are consistent with the glass transition temperature being a critical factor affecting laser/polymer interaction.

Excimer laser patterning of thick and thin films for high-density packaging

Excimer laser projection methods have ben developed to directly create high resolution electrical circuits in both thin nd thick-film metallic layers in order to form robust, compact multi-chip module interconnection devices, miniature sensor elements, miniature flexible printed circuits, antennas etc at high sped and low cost. Patterning over small or large areas is possible at high speed using simple step and repeat or more complex synchronous mask and workpiece scanning methods. Ablation rates depend strongly upon the thickness of the metal layer varying from complete metal removal with 1 laser shot for thin films to multiple 10s of shots for films to a few J/cm2 for screen printed polymer thick films or thick sputtered films. Multiple layer interconnect circuits and complex advanced sensor devices have been successfully fabricated using these excimer laser metal film patterning methods together with laser via drilling and patterning of dielectric layers using a laser tool with appropriate level to level alignment and mask changing and scanning facilities.

AFM-measured surface roughness of SU-8 structures produced by deep x-ray lithography

Deep x-ray lithography is a well-known technique used to pattern ultra high aspect ratio microstructures. It relies on the fact that higher energy synchrotron x-rays have the ability to penetrate millimeters of resist layers. However, the spectral shape of the beam will vary as a function of penetration depth, sometimes by design, so as to distribute the dose differently for different thickness structures and always as a result of filtering of lower energies. Some studies have shown that in PMMA sidewall roughness can be affected by spectral issues. SU-8 is now the resist of choice for certain high aspect ratio structures due to its high sensitivity and contrast. As sidewall roughness is a key parameter in several potential applications of high aspect ratio structures, we therefore investigated the surface roughness of 500 µm thick SU-8 structures exposed using beam spectra with peak energies between 3 keV and 12 keV. Results indicate that as the x-ray energy increases so too does the surface roughness. The surface roughness also increases as a function of feature depth. We attribute this to the random secondary physical processes of photo and Auger electron scattering both of which are strongly energy dependent.