Low-drift micro flow sensors

The emerging fields of micro total-analysis systems (micro-TAS), micro-reactors and bio-MEMS drives the need for further miniaturisation of sensors measuring quantities such as pressure, temperature and flow. The research described in this thesis concerns the development of low-drift micro flow sensors for accurate measurement of minute amount of liquid flow in the nl⋅min-1 range. Miniaturisation means that flow channel dimensions and flow rates become smaller. This requires thermally-isolated flow channels, where the complete fluid can be heated in order to obtain maximum sensitivity. A microchannel fabrication concept (Chap. 3) was developed, based on buried channel technology (BCT), allowing for easy fluidic interfacing and integration of transducer materials in close proximity to the fluid. This is achieved by the reliable fabrication of completely sealed microchannels directly below the substrate surface. The channel technology has found application in the fabrication of resonant flow sensors and low-drift micro flow sensors (Chap. 4-8) in the nl⋅min-1 range. Additionally, the technology has been extended by the possibility to integrate nanochannels using fluidic vias. This has been used in the fabrication of nano-nozzle electrospray emitters (Chap. 9). In current micromachined thermal flow sensors the elements for temperature sensing are made by thin films. The problem is that thin films reproduce poorly and that practically all materials properties are subject to drift. This translates directly into the accuracy of thermal micro flow sensors. In this thesis low-drift micro flow sensors were investigated, using two heaters and a thermopile in order to eliminate material drift (Chap. 5). The low offset drift of thermopiles has been exploited in a feedback loop controlling the dissipated powers in the heater resistors, minimising inevitable influences of resistance drift, mismatch of thin-film metal resistors and thermopile material drift. The control system cancels the flow-induced temperature difference across the thermopile by controlling a power difference between both heater resistors, thereby giving a measure of the flow rate. Alternatively, a sensor resistor and heat waves can be used to provide for a low offset-drift error signal (Chap. 8). It was demonstrated that material drift can largely be compensated. Sensitivity can be increased by designing flow sensors with a large number of integrated thermocouple junctions (Chap. 6), however it was observed that externally applied temperature gradients over the chip can still lead to drift of the sensor output signal. A special meandering microchannel layout was used to create a fully symmetrical flow sensor, where the arrangement of thermopile junctions has resulted in low-drift micro thermal flow sensors for liquids in the nl⋅min-1 range, with compensation for external temperature gradients (Chap. 7)

Mechanisms contributing to the deep chlorophyll maximum in Lake Superior

The seasonal appearance of a deep chlorophyll maximum (DCM) in Lake Superior is a striking phenomenon that is widely observed; however its mechanisms of formation and maintenance are not well understood. As this phenomenon may be the reflection of an ecological driver, or a driver itself, a lack of understanding its driving forces limits the ability to accurately predict and manage changes in this ecosystem. Key mechanisms generally associated with DCM dynamics (i.e. ecological, physiological and physical phenomena) are examined individually and in concert to establish their role. First the prevailing paradigm, “the DCM is a great place to live”, is analyzed through an integration of the results of laboratory experiments and field measurements. The analysis indicates that growth at this depth is severely restricted and thus not able to explain the full magnitude of this phenomenon. Additional contributing mechanisms like photoadaptation, settling and grazing are reviewed with a one-dimensional mathematical model of chlorophyll and particulate organic carbon. Settling has the strongest impact on the formation and maintenance of the DCM, transporting biomass to the metalimnion and resulting in the accumulation of algae, i.e. a peak in the particulate organic carbon profile. Subsequently, shade adaptation becomes manifest as a chlorophyll maximum deeper in the water column where light conditions particularly favor the process. Shade adaptation mediates the magnitude, shape and vertical position of the chlorophyll peak. Growth at DCM depth shows only a marginal contribution, while grazing has an adverse effect on the extent of the DCM. The observed separation of the carbon biomass and chlorophyll maximum should caution scientists to equate the DCM with a large nutrient pool that is available to higher trophic levels. The ecological significance of the DCM should not be separated from the underlying carbon dynamics. When evaluated in its entirety, the DCM becomes the projected image of a structure that remains elusive to measure but represents the foundation of all higher trophic levels. These results also offer guidance in examine ecosystem perturbations such as climate change. For example, warming would be expected to prolong the period of thermal stratification, extending the late summer period of suboptimal (phosphorus-limited) growth and attendant transport of phytoplankton to the metalimnion. This reduction in epilimnetic algal production would decrease the supply of algae to the metalimnion, possibly reducing the supply of prey to the grazer community. This work demonstrates the value of modeling to challenge and advance our understanding of ecosystem dynamics, steps vital to reliable testing of management alternatives

CLIMATE ANOMALIES AND PRIMARY PRODUCTION IN LAKE SUPERIOR

This dissertation supports the modeling of primary production in Lake Superior by offering site specific kinetics and algorithms developed from lab experiments performed on the natural phytoplankton assemblage of Lake Superior. Functions, developed for temperature, light and nutrient conditions and the maximum specific rate of primary production, were incorporated in a 1D specific primary production model and confirmed to published in-situ measured rates of primary production. An extensive data set (supporting model calibration and confirmation), with a fine spatiotemporal resolution, was developed from field measurements taken bi-weekly during the sampling seasons of 2011, 2012 and 2014; considered to be meteorologically average, extremely warm and cold years, respectively. Samplings were taken at 11 stations along a 26 km transect extending lakeward from Michigan’s Keweenaw Peninsula covering the nearshore to offshore gradient. Measurements included: temperature, solar radiation, transparency, beam attenuation, chlorophyll-a fluorescence, colored dissolved organic matter, suspended solids and phosphorus and carbon constituents. Based on these measurements and application of the developed primary production model, patterns in primary production and driving forces (i.e. temperature, light and nutrients) are described in a seasonal, spatial, and interannual fashion. The signal feature in 2011 was the development of a mid-summer “desert” in the offshore surface waters (a period of suboptimal temperatures coincident with a high degree of phosphorus limitation). The manifestation of the “summer desert”, however, was most extreme during the warm year and nonexistent during the cold year. Offshore primary production in all years manifested a subsurface maximum in the upper area of the metalimnion, distinctly above the deep chlorophyll maximum, with rates of production being highest In 2011 (~20 mg C m-3 d-1) followed by 2012 (~17 mg Cm-3 d-1) and lowest in 2014 (~12 mg Cm-3 d-1). Driven by variances in biomass and forcing conditions, offshore areal primary production manifested differences in seasonal patterns between years as well. In 2011 and 2014 a negatively skewed bell-shape pattern was observed, differing in magnitude and timing. The pattern in 2012 differed from these years in magnitude and timing, manifesting elevated production in April and decreased production in September. Greatest areal production in 2012 occurred in July and August (~320 mg Cm-2 d-1), in 2014 in August (~265 mg Cm-2 d-1) and in 2011 production was greatest in July (253 mg C m-2 d-1). Areal production in the summer of 1998, calculated for EPA’s 19 offshore stations in Lake Superior, manifested comparable rates and averaged 224 ± 90 mg C m-2·d-1. Although in all years the development of the thermal bar (TB) occurred after the spring runoff event, an increase in chlorophyll-a concentration during the presence of the TB was observed in 2012. Rates of primary production during this period, however, decreased while the opposite occurred in 2014, signifying that changes in chlorophyll-a concentration should be interpreted carefully (especially if used to identify spring blooms). The information presented in this work not only offers site specific kinetics, appropriate algorithms in support of primary production modeling and an extensive dataset supporting model calibration and confirmation, it also offers new insights into the dynamics of the Lake Superior ecosystem and the forces driving its function

Modeling and design of a spiral-shaped Mach-Zehnder interferometric sensor for refractive index sensing of watery solutions

The modeling and design of a spiral-shaped Mach-Zehnder Interferometric sensor (sMZI sensor) for refractive index sensing of watery solutions is presented. The goal of the running project is to realise a multi-sensing array by placing multiple sMZIs in series to form a sensing branch, and to place several sensing branches in parallel. In such an arrangment it is possible to use a single light source for several sensors. Each sensor will contain an electro-optical modulator, which makes it possible to separately interrogate and accurately read-out each sensor in the same sensing branch. One of the novelties in this project is the spiral-shaped layout of the MZI, which has several advantages: a long sensor window length can be placed in a compact sensor chip: within an area of 1 × 1 cm2 lengths of several tens of cm are feasible. Another advantage of the spiral shape is that if both MZI branches are identical (except for the sensor layer) the sensor should be very insensitive to fluctuations in temperature and even to temperature gradients across the chip. Beside robustness, the spiral shape also allows cascading of several sensors. A parametrised sMZI has been designed such that the position, slope, and curvature are continuous. The sensors can each be coated with e.g. a specific immunolayer to be able to detect changes in concentration of viri, bacteria or enzymes. In this project, technology is being developed for the immobilisation and photolithographical patterning of such immunolayers, which should result in a demonstrator to monitor the ripening process of cheese by measuring changes in the concentration of several different enzymes involved in this process

Social Dilemmas as Exchange Dilemmas

We develop a new paradigm to study social dilemmas, called exchange dilemmas. Exchange dilemmas arise from externalities of exchanges with third parties, and many real-life social dilemmas are more accurately modeled as exchange dilemmas rather than prisoner's dilemmas. Bulding on focusing and framing research we predict that defection is omnipresent in exchange dilemmas, which is corroborated in to very different experiments. Our results suggest that the fundamental problem of cooperation in many real-life social dilemmas may be more severe and harder to solve than suggested by traditional prisoner's dilemma research, due to the presence of third parties. Directions for future research are suggested, focusing on relations with third parties

Micromachined two dimensional resistor arrays for determination of gas parameters

A resistive sensor array is presented for two dimensional temperature distribution measurements in a micromachined flow channel. This allows simultaneous measurement of flow velocity and fluid parameters, like thermal conductivity, diffusion coefficient and viscosity. More general advantages of measuring temperature distributions are the inherent compensation of heat losses to the support and the insensitivity to variations in the temperature coefficient of resistance

Fabrication of microcantilever-based IO grated waveguide sensors for detection of nano-displacements

We propose a novel and highly sensitive integrated read-out scheme, capable of detecting sub-nanometre deflections of a cantilever in close proximity to a grated waveguide structure. A very compact and stable sensor element can be realized by monolithically integrating a microcantilever structure with the grated waveguide (GWG), using conventional layer deposition and sacrificial layer etching techniques. The platform integrating a high quality GWG and a low initial bending cantilever has been fabricated and characterized

Highly sensitive micro coriolis mass flow sensor

We have realized a micromachined micro Coriolis mass flow sensor consisting of a silicon nitride resonant tube of 40 ?m diameter and 1.2 μm wall thickness. Actuation of the sensor in resonance mode is achieved by Lorentz forces. First measurements with both gas and liquid flow have demonstrated a resolution in the order of 10 milligram per hour. The sensor can simultaneously be used as a density sensor