Boris Stoeber


Relevant Degree Programs


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
Paper drying - experimental studies on the influence of dryer fabric (2018)

This research is an experimental investigation on paper drying that primarily focuses on the effects of the dryer fabric on the drying process of paper. A novel method for moisture content measurement is presented. The working principle of this method is the strong correlation between the optical transparency of paper and its moisture content due to the refractive index matching role of water in wet paper. Spectrographic and microscopic measurement techniques were employed to characterize the relation of moisture content and relative transparency of paper. As optical access to the paper is restricted by the dryer fabric, the optical transparency of paper should be measured only with one-sided optical access. To achieve this goal, a novel technique of transmittance measurement is developed that is able to determine the transparency of thin film objects (i.e. paper) with only one-sided optical access. Employing a fluorescence imaging method, this optical configuration eliminates the spurious effect of reflection of the incident light by filtering the excitation wavelength before reaching the optical detector.To study the paper drying process in a multi-cylinder dryer, an experimental setup is designed to simulate realistic conditions of a typical paper dryer while providing optical access for the measurement system. Ten commercially available fabric types manufactured by weaving synthetic filaments are used in the investigations. It is shown that the fabric structure affects the drying progression and the drying time significantly. The contact area and three-dimensional arrangement of the filaments have the greatest impact on the drying process. To study through air drying (TAD), another experimental apparatus is designed to perform drying under controlled conditions of air temperature and mass-flowrate. Four commercially available TAD fabrics with different structural designs and characteristics are used in the investigations. It is shown that the geometry of the contact spots of the fabrics has a significant impact on the drying time at high drying intensities. Comparing the spatial maps of moisture content with the paper grammage distribution reveals that there is a correlation between the local grammage and the local moisture in a paper sheet during the drying process.

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Mitigating impacts of temperature-oxygen squeeze in a mesotrophic-eutrophic lake: Wood Lake, BC, Canada (2017)

No abstract available.

Skin mechanics, intradermal delivery and biosensing with hollow metallic microneedles (2017)

Microneedles (MNs) have gained significant attention over the past decade in drug delivery and biosensing due to their minimally-invasive and less painful nature of use compared to intramuscular/subcutaneous injections, and significant biological benefits. Several fundamental processes enabling MN functionality have not been completely understood, including mechanical interaction between MNs and skin for targeted depth penetration; and precise quantification of fluid delivery in the skin. This thesis presents novel materials, and methodologies for evaluating MN interactions with skin, and investigates the performance of hollow MNs in both intradermal fluid drug delivery and biosensing.A micromechanical comparison between human skin and porcine skin was performed using to determine their mechanical behavior affecting MN insertions. Stratum corneum (SC) of human skin was significantly stiffer (117 ± 42 MPa) than porcine skin (81 ± 32 MPa), requiring higher force of MN insertion to rupture the SC in human skin (107 ± 17 mN) than porcine skin (96 ± 23 mN). An artificial mechanical skin model was developed layer-by-layer to simulate tough human skin (MN insertion force 162 ± 11 mN) and to study the dynamics of MN insertion. Key factors that affected MN insertions into skin, including velocity of impact and total energy delivered to the skin, were identified. ID fluid delivery by hollow MNs was assessed using a novel method involving the low-activity radiotracer technetium-99m pertechnetate (⁹⁹mTcO₄₋). Its delivery allowed accurate quantification of fluid delivered into the skin, back-flowed to the skin surface, and total fluid ejected from the syringes via ID devices with sub-nanoliter resolution. Hollow MNs performed more accurate ID injections than conventional needles (93% vs. 69-87% of fluid per 0.1 mL injection volume).A MN-optofluidic biosensing platform capable of eliminating blood sampling was developed with MNs that can access dermal interstitial fluid that contains numerous drugs at concentrations comparable to blood. The MN lumen was functionalized to collect, trap and detect drugs in 0.6 nL of sample. The optofluidic components provided specific high-sensitivity absorbance measurements for drug binding using enzyme-linked assays. Streptavidin-horseradish peroxidase (LoD = 60.2 nM) and vancomycin (LoD = 84 nM) binding validated this point of care system.

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ZnO-based thermoelectrics : modelling, electrochemical thick film growth, and characterization (2016)

The thermoelectric performance of electrodeposited, aluminum doped zinc oxide was assessed. In this work, wurtzite ZnO was first modelled using Mueller-Plathe to compare the effectiveness of different nanostructured configurations on reducing thermal conductivity. A new analysis technique, Local Vibrational Density of States Equilibrium Molecular Dynamics (LVDOS-EMD), was created to study localized lattice vibrations around nanostructural features of silicon and ZnO, and was used to predict thermal properties in materials of similar composition 17× faster than conventional thermal modelling methods. A 30% void density was determined to yield the best reduction in thermal conductivity by volume of voids in bulk Al:ZnO with a computed thermal conductivity of 0.77 W m-¹ K-¹ at room temperature, 3× below the threshold achieved through established experimental means with high electrical conductivity Al:ZnO. Thick film, electrodeposited Al:ZnO was grown using a nitrate system. Experiments on solution pH using various counter electrodes demonstrated that inert electrodes caused acidification of the growth solution, limiting film thickness. Chloride contamination from commonly used Ag/AgCl reference electrodes was also determined to affect thick film opacity, morphology, crystallinity, and electrical properties. Aluminum integration and activation was explored by adding Al(NO₃)₃ to the growth solution during film synthesis, yielding aluminum integration molar ratios of up to 1.72% (Al.₀₃₄Zn.₉₆₆₀). Partially doped films in excess of 95 µm thick, 4× the thickness reported elsewhere, were electrochemically grown and characterized. Sub-micron voids were integrated into the films using sacrificial material and annealing. A new electrochemical chromium etching methodology was developed and successfully used to free 20 films from their growth substrates for thermoelectric characterization. A new, reusable thermoelectric test apparatus for thin film thermoelectric testing was designed, implemented, calibrated, and successfully deployed to characterize ZnO and Al:ZnO thin films grown 79 – 95 µm in thickness. Extremely low thermal conductivity of 11 mW m-¹ K-¹ at room temperature was demonstrated concurrently with a Seebeck coefficient of -88 µV K-¹. Polycrystallinity and poor dopant activation yielded a low electrical conductivity of 0.75 mS/cm and corresponding low room temperature ZT of 1.3×10-⁵ for the Al:ZnO films.

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Flow physics during the drying of a thin polymer solution film near the contact line (2015)

The drying process of a thin polymer solution film has been studied inside a micro-liter cavity near the contact line. Confocal microscopy along with particle image velocimetry and laser induced fluorescence are used for the real time measurement of velocity and concentration fields during the drying process. In addition to the capillary flow and the Marangoni flow, the velocity field also reveals the possible existence of a single vortex and multiple vortices inside the creeping flow induced by evaporation. These vortices appear soon after the beginning of the evaporation process, their size shrinks over time, and they disappear before the end of the evaporation process. This thesis includes a study of the effect of rheological and geometrical parameters on the presence, size and endurance of these vortices. Significant concentration heterogeneity is observed across the film during the drying process, in particular near the contact line. The concentration at the solution-air interface is higher compared to the bulk, and it increases towards the contact line and also over time. A skin layer starts forming as soon as the surface concentration reaches the glass transition concentration after which the evaporation rate starts decreasing. The drying film undergoes a similar concentration evolution during the evaporation process, regardless of the cavity depth and the initial polymer concentration; although, minor differences can be recognized that are associated with the flow recirculations that delay the concentration increase inside the vortex. Finally, a theory is developed based on experimental data which explains the existence and behavior of viscous vortices near the bottom wall of the cavity. The competition between the capillary flow and the Marangoni flow results in flow separation on the bottom wall which leads to such vortices. This study provides better understanding of the drying process of thin polymer solution films near the contact line. Furthermore, viscous flow separation adds to the current understanding of flow physics during the drying process, in addition to the well-known evaporation induced capillary transport and the Marangoni effect.

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Particle Transport in Microfluidic Environments: Particle Adsorption at the Polydimethylsiloxane-Water Interface and the Effects of Flow Field and Image Processing on the Measurement Depth in Micro Particle Image Velocimetry (2015)

This thesis investigates several research questions associated with the transport of microscopic particles in microfluidic environments. This includes an investigation of the motion and deposition of micron sized polystyrene particles at the planar polydimethylsiloxane–water interface. Particle tracking shows that particles near the substrate can be immobilized to different degrees. Careful analysis of the more weakly immobilized particles reveals that there is a buildup of a particle accumulation layer near the substrate in which particle motion parallel to the substrate is hindered by non-hydrodynamic effects. The presence of lateral surface interaction forces resulting from charge heterogeneity of the PDMS substrate is found to be the most plausible explanation for the hindered particle transport across the substrate. The two other problems are concerned with micro particle image velocimetry (μPIV), which is a particle transport-based optical method for the characterization of flow fields in microfluidic devices. One limitation μPIV is the finite measurement depth associated with the optical setup, which can lead to bias errors in the measured flow-velocity. Analytical and numerical models are developed that describe the effect of common image pre-processing filters on the measurement depth in μPIV. Further, previous models are revisited that describe the effect of flow velocity gradients on the measurement depth in μPIV. Supplementary video material is available at:

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Effect of Substrate Cooling and Droplet Shape and Composition on the Droplet Evaporation and the Deposition of Particles (2014)

Sessile droplets are liquid droplets resting on a flat substrate. During the evaporation of small sessile droplets, the contact line of the droplet undergoes two different stages: pinned stage with fixed contact area and de-pinned stage with fixed contact angle. An evaporation with a pinned contact line produces a flow inside the droplet toward the contact line. This flow carries particles and deposits them near the contact line. This causes the commonly observed “coffee-ring” phenomenon. This thesis provides a study of the evaporation process and the evaporation-induced flow of sessile droplet and brings insights into the deposition of particles from colloidal suspensions. Here we first study the evaporation of small sessile droplets and discuss the importance of the thermal conductivity of the substrate on the evaporation process. We show how current evaporation models produce a significant error for droplet sizes below 500 μm. Furthermore, we study the evaporation of line droplets with finite sizes and discuss the complex behavior of the contact lines during evaporation. We apply an energy formulation and show that the contact line starts receding from the two ends of line droplets with a contact angle above the receding contact angle of spherical droplets. And then we show the evaporation-induced flow inside the line droplets. Finally, we discuss the behavior of the contact line under the presence of surfactant and discuss the Marangoni flow effects on the deposition of the particles. We show that the thermal Marangoni effect affects the amount of the particles deposited near the contact line, where a lower substrate temperature corresponds to a larger amount of particles depositing near the contact line.

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Fabrication of out-of-plane microneedles for drug delivery and biosensing (2014)

Hollow microneedles can be used to painlessly inject drugs or extract dermal interstitial fluid for biosensing. However, their fabrication so far has been associated with costly and time-consuming steps restricting their batch production as a viable option. This thesis presents novel methods for fabricating inexpensive hollow microneedles, and investigates new methods of characterizing the drug delivery and interstitial fluid sampling using microneedles. First, a method is presented for fabrication of hollow polymer microneedle arrays. Microneedles are formed during a solvent casting process, which leaves a polymer layer around pillars in a pre-fabricated mold. Arrays of microneedles with lengths up to 250 µm have been fabricated. The strength of the microneedles was evaluated to ensure reliable skin penetration and their suitability for drug delivery was demonstrated by injection of fluorescent beads into a skin sample. A second fabrication method is presented for making metallic microneedles with high aspect ratios. Solvent casting was used to coat a mold with a conductive polymer composite layer, which was then used as a seed layer in a metal electrodeposition process to form 500 µm tall microneedles. Some fabrication process steps were characterized and the strength of the microneedles was evaluated. Their usefulness for drug delivery was also demonstrated by injection of fluorescent microspheres into animal skin. Designing effective microneedles requires understanding the drug diffusion process in skin. Here, a novel method is used to characterize diffusion of a chemotherapeutic drug injected with microneedles into skin. Using confocal microscopy, the concentration distribution of the drug was measured over time and then compared to an analytical diffusion model to obtain the drug’s diffusion coefficient. Using this method, different skin storage conditions were evaluated. It was concluded that using previously frozen skin should be avoided for transdermal drug delivery studies. Finally, using the proposed processes, hollow and solid microneedles were fabricated for sampling interstitial fluid for biosensing applications. Minimal removal of the interstitial fluid was achieved with a solid microneedle design as well as a hollow metallic microneedle array attached to a vacuum probe, while no trace of the fluid was observed when using hollow polymer microneedles.

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Vibration-based Electromagnetic Energy Harvesters for MEMS Application (2011)

This thesis investigates vibration-based electromagnetic energy harvesters (EMEHs) for application in low power autonomous sensors. It makes contributions pertaining to the development of a low cost fabrication technology, analytical modeling, simulations and characterization of EMEHs under harmonic and random vibrations. A novel, low cost, one mask fabrication technology devised in this thesis is used to develop a copper foil-type linear EMEH, and a polydimethylsiloxane (PDMS) membrane type nonlinear EMEH. The voltage and power generated by these harvesters are comparable to existing EMEHs which use more involved fabrication processes. In the membrane type EMEH the inclusion of a more flexible PDMS membrane design reduces the harvester resonant frequency and makes it suitable for extracting energy from low level vibration environments. For acceleration levels greater than 0.1 g, this harvester exhibits a nonlinear behaviour. At higher levels of narrow band random excitations, the device therefore exhibits broadening of the load voltage spectrum in comparison to the response under relatively low levels of narrow band random excitations.Analytical models for linear EMEHs with non-uniform magnetic field for harmonic vibrations are developed. A simple analytical model based on Faraday’s law and uniform gradient of the normal component of the magnetic flux density is developed for EMEHs where the entire coil experiences approximately the same gradient of the normal component of the magnetic flux density. However, for EMEHs where the entire coil is not exposed to the same magnetic flux gradient a more robust model, based on the off-center analytical solution of the magnetic flux density is devised. The simulation results of the developed models are in good agreement with the experimental observations.Analytical models for linear and nonlinear EMEHs under random vibrations are derived. The models are parameterized such that they are applicable to all architectures of EMEHs and can be utilized for designing and performance estimation of EMEHs. Nonlinear harvesters with spring nonlinearity and with combined spring and damping nonlinearity are modeled using the statistical linearization method. The developed models are useful in investigating the effects of the mechanical nonlinearity on the performance and bandwidth of the harvesters under random vibrations.

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High-dynamic Range Projection Using a Steerable MEMS Mirror Array (2010)

This thesis describes a novel way to improve the contrast and peak brightness of conventional projectors by directing the light from the lamp away from the dark parts of the image towards the light parts before it reaches the projector's primary image modulator. AMicroelectromechanical Systems (MEMS) micromirror array is inserted into the optical path between the lamp and the image-forming element. Each element of the array can be tip/tilted to divert portions of the light from the lamp. By directing these mirrors on an image-dependent basis, we can make the dark parts of the image darker and the bright parts brighter. In effect, this method will produce a low resolution approximation of the image on the image-forming element. The micromirror array will allow the projector to adapt its light source to the image being projected in order to maximize peakbrightness, contrast and efficiency.Employing such an mechanism within a projector's display chain requires contributions to a number of different fields related to displays. Tradeoffs between the distance on the screen that a light spot from a mirror (mobile light, or ML) could be moved, and its spatial extent were established. Micromirrors suitable for this application were designed, simulated and fabricated. A novel way of optimizing the tradeoffs between tilt angle, mirror size, and mirror resonance frequency by splitting the mirrors into smaller functionalsubsections was employed. We developed several algorithms that determine favourable placement of the mobile lights from each of themicromirrors in the array, in order to best improve the image. From simulations, the projector average brightness could be increased by a factor of 1.4 if micromirrors were available that could be tilted to 3.5 degrees with the addition of this technology, without changing the projector lamp. If the requirement for perfect image reconstruction is relaxed, the improvement factor increases to 2.5. A prototype was system was developed that allows for fast control of mirror elements, and the positive effect of employing adaptive lightdistribution in this manner was demonstrated.

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Master's Student Supervision (2010 - 2018)
Intradermal injections through hollow microneedles (2018)

Hollow microneedles are a promising alternative to conventional drug delivery techniques such as oral drug administration and hypodermic injections, and are used for delivering drugs and therapeutics into the skin. Although the benefits of intradermal drug delivery have been known for decades, our understanding of fluid absorption by skin tissue has been limited due to the difficulties in imaging a highly scattering biological material such as skin. In this thesis, we report the results from ex-vivo injection experiments into excised porcine skin tissue using hollow microneedles. We introduce the use of optical coherence tomography (OCT) for real-time imaging of skin tissue at the micro-scale during intradermal injections through hollow microneedles. We identify two modes of flow into the skin – microinjection, a region of high transient flow-rate, and microinfusion, a region of lower steady-state flow-rate. We relate the two modes of flow to tissue deformation. Using images from the OCT, we find that the skin tissue behaves like a deformable porous medium and absorbs fluid by locally expanding rather than rupturing to form a fluid filled cavity. We measure the strain distribution in a cross section of the tissue to quantify local tissue deformation using digital image correlation (DIC), and find that the amount of volumetric expansion of the tissue corresponds closely to the volume of fluid injected. Mechanically restricting the tissue expansion limits fluid absorption into the tissue, and allowing the tissue to expand leads to increased fluid absorption. Our experimental findings can provide physical insights for optimizing the delivery of drugs into the skin for different therapeutic applications, and for better modelling fluid flow into biological tissue.

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A Magnetic Sensor to Measure Wear in Centrifugal Pumps (2016)

On average, centrifugal pumps consume between 25% and 60% of the total consumed electrical energy inside process plants. Erosion inside open-impeller centrifugal pumps leads to a reduction in pump efficiency and occasional plant downtime. This work demonstrates a new concept for an online instrument capable of monitoring wear with the objective of improving the maintenance scheduling of centrifugal pumps and the prevention of unexpected failure through a predictive maintenance system. A magnetic wear sensor is designed and fabricated that allows for wear measurement while the pump is in operation. This sensor can be installed on existing centrifugal pumps and does not require any pump modifications. Wear mostly occurs on the tip of the impeller blades reducing the thickness of the impeller which in turn increases the gap width between the impeller and the side plate inside the pump housing, from 0.65 mm (no wear) to 2.50 mm for maximum allowable wear on the pump used for prototyping. By using a magnetic circuit with the pump and its components, wear is estimated by measuring the change in the width of the varying gap between the impeller and the side plate. To assemble the magnetic circuit, a high-permeability clamping mechanism with a relative permeability of 10,000 is designed and fabricated along with a magnetic coil excited using a 1.0 V AC voltage signal at 70 Hz to drive flux through the circuit. As wear occurs, the total reluctance of the magnetic circuit increases causing the inductance of the coil to drop. The coil's inductance is also a function of the impeller's angular position. To estimate wear, data is collected at a sampling frequency of 500 kHz and then assessed in the frequency domain after fast Fourier transform (FFT). The amplitude of the FFT signal at the frequency correlated with the pump's rotational speed is then considered to estimate wear. For a data sampling time of one second the sensor has a signal to noise ratio of 17.8 dB with an average sensitivity of 0.022 mV/mm and a resolution of 0.38 mm.

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Droplet formation on demand at a microfluidic T-junction (2015)

Microfluidic aliquot dispensing technologies have been utilized in many chemical or biological high throughput assays to provide high-speed deposition of high-resolution droplets. Particularly, these aliquots can be used to dispense a controlled volume of liquid into multi-well plates in drug screening applications. However, existing platforms are often designed to print a limited set of materials where each material is handled by an independently actuated nozzle. A more convenient and scalable technology that could allow digital output of multiple materials on demand must be explored. Monodispered droplets can be formed continuously in a microfluidic T-junction when two input flows of immiscible fluids are maintained. However, it is difficult to form stable air separated aliquots in a simple T-junction configuration. Electrowetting-on-dielectric (EWOD) is commonly used to control individual droplets in a planar configuration. But it is not usually used in microfluidic channels. In this device, a new method for creating air separated aliquots on demand using EWOD at a microfluidic T-junction was demonstrated. Since air is the carrier fluid, these aliquots can be dispensed without further liquid processing. A device that consists of SU-8 channels with electrodes in alignment with the channels was designed and fabricated. A double wedged junction was incorporated at the dispersed channel to create a barrier pressure that allows the meniscus location to be controlled by the applied pressures. Droplets of approximately 15 nL can be generated by applying a voltage at the electrode at the junction. Liquid flow is stopped when the voltage is removed. This mechanism can be used as a digital valve to generate a sequence of aliquots of different materials for a multi-material printing platform.

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Electro-Hydrodynamic Micro Prism with Applications to Automultiscopic Displays (2015)

The recent trend in display technology is to provide the viewer with an artificial three- dimensional (3D) experience using lenses or aides, however the number of viewers and resolution of the display is limited. To remedy these problems, an array of prisms can be placed over the display redirecting the projected light at specific angles in a time-multiplexed fashion and at full resolution. The difficulty in this approach is that the angle of the prism needs to be adjustable with accurate and fast control.This thesis presents the theory, development, and analysis of a novel adjustable prism coined an electro-hydrodynamic micro prism (EHMP). An EHMP consists of an elongated conducive water droplet with pinned contact lines using hydrophobic surface patterning. By applying a voltage between the droplet and an offset electrode above it, the shape of the droplet is deformed into a triangular prism where the angle of the prism is dictated by the strength of the applied voltage.A numerical model of an EHMP was developed using finite element analysis and smoothed particle hydrodynamics to model the electro-hydrodynamics of the system. The numerical model was qualitatively verified using the collapsing square and oscillating droplet tests, and then used to predict an operating voltage range of 400 – 550 V for a 200 μm droplet, and that the leading edge of the electrode dictates the final deformation of the drop.To fabricate a prototype EHMP, a microcontact printing technique was developed to pattern polytetrafluoroethylene nanoparticles onto an indium tin oxide coated glass slide creating the hydrophobic patterning. A prototype 1 mm diameter prototype EHMP was fabricated and tested in the 1.5 – 2 kV range. It was found that there was minimal droplet deformation before failure due to electrospray formation. Though not useful for 3D displays, the results from these large-scale experiments experimentally validate the numerical model. Model simulations showed ideal EHMP deformations can occur under the right conditions, however its performance under current conditions is limited due to dielectric breakdown failure and a fill factor of only 0.66 thus proving not to be a practical solution to automultiscopic displays.

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Use of Compound Microlens Arrays as a Magnifier in Near-Eye Head-Up Displays (2015)

This thesis reports a new approach for making a very compact near-eye display (NED) using two microlens array (MLA) layers. The two MLAs will work in conjunction as a magnifying lens (MLA magnifier). The purpose of the MLA magnifier is to aid the eye accommodate on a display that is positioned within several centimeters from the eye, by generating a virtual image of the display at optical infinity. While there are recently developed techniques for similar purposes such as waveguides [17, 18], and retinal scanning methods [21], using a magnifying lens has been the most exploited avenue for generating a virtual image due to its rather simple, tried-and-true optical properties; near-eye display systems that incorporate a magnifying lens, whether it is a single piece or a compound, has been well-studied since the dawn of head-up displays. However, magnifying lens-based optics is inherently hard to make compact, because as the focal length becomes smaller, the thickness of the lens becomes larger. This thesis presents in detail the method for making a MLA magnifier that retains a thin profile of about 2 mm in thickness with a system focal length of about 6 mm. Thus the total thickness of the MLA magnifier system is around 8 mm (excluding the thickness of the display) in non-folded optics configuration, which is much more compact in comparison to other popular near-eye displays such as Google Glass or Recon Instrument’s Snow HUD goggles having folded optics.

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All-polymer flexural plate wave devices for sensing and actuation (2014)

Polymer based flexural plate wave (FPW) chemical sensors are unique among guided acoustic devices inthat they are comprised of components with similar elastic moduli. As a result, they can be very sensitiveto stiffness and stress variations in an applied sensing layer. This property may be leveraged to detect thepresence of an analyte or to interrogate the mechanical properties of the applied polymer. In this work,polyvinylidene fluoride (PVDF) based, polyethylene dioxythiophene polystyrene sulfonate (PEDOT:PSS)coated FPW devices are fabricated and tested with the purpose of developing an all-polymer VOC sensor.The sensors are coated in polyvinyl acetate (PVAc) and polystyrene (PS) sensing layers and exposed tovarying concentrations of toluene during testing. The PS coated sensors show a sensitivity of -80 cm²/g to -200 cm²/g while the PVAc coated devices demonstrate a sensitivity of -240 cm²/g to -490 cm²/g. A performance model is proposed which seeks to describe the sensing layer mechanical properties as a function of analyte vapour concentration in order to predict the sensor resonant frequency. The predictions of this model are compared with experimental results and design modifications are proposed. Along with this, soft material mechanical characterisation is investigated with the purpose of developing a tool for measuring composite resin properties during cure. Finally, it is proposed that these devices may be used to drive acoustic streaming in microfluidic systems. To test the concept, droplets of fluid are applied to thedevice substrates and acoustically driven flow rates are measured.

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Design and Performance of All-Polymer Acoustic Sensors (2012)

All-polymer flexural plate wave (FPW) sensors based on piezoelectric polyvinylidene fluoride (PVDF) thin-film with interdigital transducer (IDT) electrodes composed of poly(3,4-ethylenedioxythiophene) poly-(styrenesulfonate) (PEDOT:PSS) are studied, optimized, and assessed for their potential in various sensing applications. PVDF offers unique opportunities as a substrate material due to its low stiffness, low cost, low density, and ease of preparation compared with many other piezoelectric materials commonly used in acoustic sensing applications. Substrates are prepared using a variety of material thicknesses of PVDF through a stretching and poling process, followed by conductive IDT patterning by inkjet printing using a PEDOT:PSS-based ink. Sensor behaviour is studied using electrical and optical measurement techniques. Material and gas loading tests are performed to demonstrate gas sensing and polymer characterization applications. The devices demonstrate good adherence to analytical and FEA models, and although the high attenuation and low coupling coefficients of the substrate material reduce signal to noise ratio and quality factor, vapour sensing and polymer/absorbent material characterization applications are realized experimentally. Other factors such as environmental influences are also considered, demonstrating a very high sensitivity to temperature and humidity changes. The sensors also demonstrate high sensitivity to variations in substrate and sensing layer stiffness, reducing their effective mass sensitivity, but also increasing their potential for simultaneous mass and stiffness measurements. Parameter sensitivity studies are generated to better optimize the design and improve performance of the sensor for specific applications, suggesting benefits from thinner substrates, lower in-plane stress, and more IDT fingers.

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All-polymer flexural plate wave sensors (2011)

A novel all-polymer flexural plate wave (FPW) sensor from a piezoelectric polyvinylidene fluoride (PVDF) thin-film with poly-(3,4-ethylenedioxythiophene) poly- (styrenesulfonate) (PEDOT:PSS) interdigital transducer (IDT) electrodes has been fabricated and characterized. PVDF films are made piezoelectric by stretching and poling. X-ray diffraction measurements confirm the transition of the PVDF into its piezoelectric β-phase. Inks of PEDOT:PSS, dimethyl sulfoxide and Triton X-100 are deposited on the PVDF films by inkjet printing to produce the IDT patterns. The sensor operates using fundamental frequency, f0, detection of Lamb waves propagating through the PVDF film. Upon the application of a time-varying voltage signal to the input IDT, acoustic waves are generated and measured using a laser Doppler vibrometer, as well as through an electric signal at the output IDT. The output signal is amplified, filtered and processed using an analog to digital converter, digital signal processor and a computer program. The measured fundamental frequencies range from 330 to 1600 kHz for devices with 18 and 125 micron thick PVDF substrates and 800 and 400 micron acoustic wavelengths. These values for fundamental frequency are well predicted by the device geometry using Lamb wave theory. The effect of mass-loading was characterized by inkjet printing layers of polyvinyl alcohol on the sensors and measuring the resulting frequency shift. The devices demonstrate a frequency shift, Δf , to mass loading, m, with a measured resonance frequency mass sensitivity of Δf/(mf0) = -55.9 cm²/g. Temperature sensitivity was measured to be 1870 Hz/ C. Sensors were also coated with a sensing layer of polyvinyl acetate and its response to toluene and acetone vapour concentrations was characterized.

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Fabrication and testing of a disposable all-polymer micronozzle (2011)

A disposable all-polymer micronozzle was designed and fabricated by merging the two different technologies of microfluidics and microneedles together. Polymer micronozzles (polyimide and SU-8) were fabricated using different steps of spin casting and one step of photolithography. Microfluidic devices consisting of one input channel and one output channel each with a 500µm diameter, and connected with a channel 100µm in width, were fabricated using the PDMS polydimethylsiloxane (PDMS) casting. To achieve a thin PDMS membrane, spin casting of PDMS over the mold is required. The fabricated thin PDMS microfluidic layers were bonded to polymer nozzles using oxygen plasma treatment and precisely aligning the two layers together.The resulting polymer nozzles were connected to the pressure system of a custom made inkjet printer, by the means of a plastic holder device. The holder device was designed in SolidWorks and printed using a 3D printer. Finally a solenoid actuator was attached to the setup. Different solenoid plunger tips were designed to maximize the deformation of the PDMS membrane which is used to attempt liquid ejection and printing. First the internal pressure was tuned. The effect of frequency, duty cycle and input voltage of the solenoids input pulse on the created pending droplet’s volume was characterized experimentally. The maximum displaced volume was found with actuation for a 12V input pulse with 10% duty cycle. For a 50µm nozzle diameter this volume is 4.78×10⁻¹¹ L and for a 200µm it is 3.83×10⁻¹ºL. Reducing the surface tension of water using surfactant resulted in flow of ink onto the hydrophilic plasma-treated SU-8 surface, and total surface wetting.

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Transport and size-separation of airborne particles in a microchannel for continuous particle monitoring (2011)

Microfluidics research and development has emerged as a novel and promising tool for the development of sensors and actuators. However, one area in which microfluidics has been only minimally employed is in the handling of airborne particles, or aerosols. The real-time monitoring of aerosols is important for protecting human health and earth’s environment. The small size of microchannels, coupled with the opportunity to integrate sensing technologies, suggests them as a promising tool for the next generation of aerosol sensors. To that end, this thesis presents a microfluidics-based system for the size-separation of aerosols. Specifically, centrifugal force is exerted on each particle as it travels around a curved microchannel, resulting in the particle occupying a size-dependent lateral position in the channel. The behaviours of aerosols in a microchannel are examined, including the effects of flow focusing, the diffusion of airborne particles in a channel, and the centrifugal and viscous forces exerted on particles in a curved microchannel. Mathematical descriptions and computer simulations of these effects are developed to model these effects. Straight and curved microchannels were fabricated and each of these effects was measured experimentally, and compared to the models. Various combinations of airborne particles between 0.2 µm and 3.2 µm were successfully separated by size. A prototype optical particle detector was built and tested for its suitability as a candidate for integration with the microchannel particle separator. This represents the first approach in which aerosols have been separated by centrifugal forces in a microchannel, and one of very few approaches that have been used for any kind of size-based separation of airborne particles in microchannels. The small footprint and potential for integration offered by microsystem fabrication technology make it a desirable avenue of pursuit for the development of small, portable particulate monitors. The results presented here confirm that this approach to size-separation is a feasible option for a future microsystem based size-selective particulate monitor.

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Towards All-polymer Surface Acoustic Wave Chemical Sensors for Air Quality Monitoring (2010)

Volatile organic compounds (VOCs) are a precursor to the formation of ground-level ozone and airborne particulate matter, both of which are hazardous to human health. Currently in Canada, other air pollutants such as ozone and nitrous oxides are measured by an air quality monitoring network in real-time, while VOCs are collected in canisters and sent to a central laboratory for analysis. This is a time-consuming and non real-time method, and due to the spatial variability of air pollution, many points of measurement are needed. A distributed point sensor network could address the resolution and real-time challenges, but would impose an added operating expenditure burden on air quality monitoring agencies. Low-cost, yet sensitive chemical sensors could contribute to lowering operating expenditures of a network’s sensing units over the installed lifetime of the units. The objective of this work was to lay the groundwork for a sensing platform from which low-cost yet sensitive chemical sensors can be developed. The sensing platform is an all-polymer surface acoustic wave (SAW) device, and the materials selected for its fabrication are Polyvinylidene Fluoride (PVDF) for the sensor substrate and Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) for the interdigital transducer electrodes. In this work, an apparatus and a process for preparing piezoelectric PVDF film was developed. PVDF-based resonators were successfully demonstrated. In addition, repeatable processes for inkjet micropatterning highly electrically conductive PEDOT:PSS electrode tracks on PVDF were developed for three inkjet nozzle orifice sizes (20, 30, 40 µm). For tracks micropatterned using the same process, the electrical resistances have a standard deviation of 8.5% of the average. The electrical conductivity of micropatterned tracks is approximately 150 S/cm, or one-sixth of the manufacturer’s claimed bulk film conductivity. Using the 30 µm nozzle, the smallest electrode track width that can be micropatterned repeatably is 75 µm. A track width of 55 µm was achieved using the 20 µm nozzle.

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