A numerical simulation of electrowetting on a dielectric was performed in COMSOL to grant insight on various parameters that play a critical role in system performance. The specific system being simulated was the Open Drop experiment and the parameters being investigated were the applied voltage, contact angle at the advancing triple point, and droplet overlap onto neighboring actuated electrodes. These parameters were investigated with respect to their effect on droplet locomotion performance. This performance was quantified by the droplets velocity and the dielectrophortic (DEP) force’s magnitude; the DEP force was calculated from integration of the Maxwell Stress Tensor, however, the force was not integrated into the simulation to assist with droplet movement. It was found that as the droplet overlap onto the neighboring electrode, or droplet radius to electrode size ratio, decreased, the droplet velocity increased. As the applied potential increased, and induced contact angle at the advancing triple point decreased, droplet velocity also increased. Both the decreasing overlap and increasing voltage had a linear effect on droplet velocity. As the droplet overlap increased, the rate of change of droplet velocity decreased as increasing voltages were considered. A 2D DEP calculation illustrated that an increase in voltage induced a tenfold increase in the corresponding DEP force; a linear relationship was found between droplet overlap and DEP force for the Open Drop size regime.

Multi-phase fluid systems are an important concept in fluid mechanics, seen every day in how fluids interact with solids, gases, and other fluids in many industrial, medical, agricultural, and other regimes. In this thesis, the development of a two-dimensional digital microfluidic device is presented, followed by the development of a two-phase microfluidic diagnostic tool designed to simulate sandstone geometries in oil reservoirs. In both instances, it is possible to take advantage of the physics involved in multiphase flows to affect positive outcomes in both. In order to make an effective droplet-based digital microfluidic device, one must be able to precisely control a number of key processes including droplet positioning, motion, coalescence, mixing, and sorting. For planar or open microfluidic devices, many of these processes have yet to be demonstrated. A suitable platform for an open system is a superhydrophobic surface, as suface characteristics are critical. Great efforts have been spent over the last decade developing hydrophobic surfaces exhibiting very large contact angles with water, and which allow for high droplet mobility. We demonstrate that sanding Teflon can produce superhydrophobic surfaces with advancing contact angles of up to 151° and contact angle hysteresis of less than 4°. We use these surfaces to characterize droplet coalescence, mixing, motion, deflection, positioning, and sorting. This research culminates with the presentation of two digital microfluidic devices: a droplet reactor/analyzer and a droplet sorter. As global energy usage increases, maximizing oil recovery from known reserves becomes a crucial multiphase challenge in order to meet the rising demand. This thesis presents the development of a microfluidic sandstone platform capable of quickly and inexpensively testing the performance of fluids with different rheological properties on the recovery of oil. Specifically, these microfluidic devices are utilized to examine how shear-thinning, shear-thickening, and viscoelastic fluids affect oil recovery. This work begins by looking at oil displacement from a microfluidic sandstone device, then investigates small-scale oil recovery from a single pore, and finally investigates oil displacement from larger scale, more complex microfluidic sandstone devices of varying permeability. The results demonstrate that with careful fluid design, it is possible to outperform current commercial additives using the patent-pending fluid we developed. Furthermore, the resulting microfluidic sandstone devices can reduce the time and cost of developing and testing of current and new enhanced oil recovery fluids.

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Precise manipulation of heterogeneous droplets on an open droplet microfluidic platform could have numerous practical advantages in a broad range of applications, from proton exchange membrane (PEM) fuel cells and microreactors, to medical diagnostic platforms capable of assaying complex biological analytes. Toward the aim of developing electrically controllable micropost arrays for use in open droplet manipulation, custom-designed titanium dioxide (TiO2)- loaded poly(dimethylsiloxane) (PDMS) micropost arrays were developed in this work and indirectly mechanically actuated by applying an electric field. Initial experiments explored the bulk properties of TiO2-loaded PDMS films, with scanning electron microscopy (SEM) confirming a uniform TiO2 particle distribution in the PDMS, and tensile testing of bulk films showing an inverse relationship between TiO2 % (w/w) and Young’s Modulus with the Young’s Moduli quantified as 4.22 ± 0.51 MPa for unloaded PDMS, 2.27 ± 0.18 MPa for 10 % (w/w) TiO2, and 1.39 ± 0.20 MPa for 20 % (w/w) TiO2. Following bulk material evaluation, soft lithography methods were developed to fabricate TiO2- loaded PDMS micropost arrays. Mathematical predictions were applied to design microposts of varying shape, length, and gap spacing to yield super-hydrophobic surfaces actuatable by an electric field. Visual inspection and optical microscopy of the resulting arrays confirmed a non- collapsed micropost geometry. Overall, round microposts that were 100, 200, and 300 μm in length, 15 μm in diameter, and spaced 50 μm apart were produced largely free of defects, and used in contact angle measurements and micropost deflection experiments. Droplet contact angles measured on the arrays remained above 120° indicating the arrays successfully provided super- hydrophobic surfaces. Individual microposts deflected most notably above an electric field strength of 520 kV/m (12.5 kV nominal voltage). The ability to mechanically deflect customized microposts using an electric field demonstrated by this work is promising for translating this technology to precise droplet manipulation applications. Indirect actuation of droplets could enable the manipulation of liquids with varying electrical properties, which is a limitation of current micropumping technologies. Once optimized, electrically actuated micropost arrays could significantly contribute to the micro- handling of heterogeneous, highly ionic, and/or deionized fluids. / Thesis (Master, Chemical Engineering) -- Queen's University, 2013-03-03 17:25:49.785

La recherche développée durant cette thèse constitue la première étape de développement d’une nouvelle méthodologie de détection sans marquage à l’échelle d’une goutte : un nouveau mécanisme de transduction est développé, basé sur l’actuation par électromouillage bi-fréquence d’un réseau d’ondes capillaires à la surface d’une goutte. La résonance capillaire de la goutte est étudiée par détection interférométrique de son apex en mouvement.La faisabilité de cette méthodologie est prouvée en caractérisant la cinétique d’évaporation de la goutte par glissement spectral des ondes capillaires.La deuxième partie de la thèse porte sur la modélisation du transport de biomolécules tensioactives dans une goutte : le transport diffusif, les processus d’ad/désorption aux surfaces, la condition cinématique d’un rayon variable(évaporation) ainsi qu’une quantité molaire finie de biomolécules sont pris en compte. Un nouveau nombre adimensionnel est proposé pour rendre compte de la taille finie d’une goutte. Puis le transport chimique associé au courant de dérive induit par les ondes capillaires (excitées par électromouillage oscillatoire)est pris en compte à l’aide de simulations numériques. Finalement,l’électromouillage haute et basse fréquence est appliqué au cas d’une goutte enrichie par des molécules d’albumine de sérum bovin. En utilisant conjointement l’interférométrie optique et l’imagerie, et en considérant la loi de Lippmann-Young, le vieillissement biochimique de la surface solide mouillée par la goutte est détecté. / This PhD research is thought of as a first step towards label freedigital biosensing. A novel transduction mechanism is developed, based on thegeneration of capillary waves along a drop surface induced by dual-frequencyelectrowetting. Acapillary resonance is measured by interferometry at the movingdrop apex. Aproof of concept of this electro-optical methodology for furtherintegration in a digital microsystem is given with characterization of drop evaporationkinetics. A second part focuses on modeling and numerical calculationsof the transport of surface active biomolecules in the drop. The model includesdiffusion, ad-/desorption phenomena together with a kinematic condition of avariable drop radius (evaporation) as well as a finite molecular amount of surfaceactive biomolecules. A dimensionless number is suggested to take into accountfinite size effects. Drop steady streaming due to oscillating electrowetting isfinally taken into account. Low- and high-frequency electrowetting is appliedto a drop laden with bovine serum albumin. By jointly using interferometry andimaging, enhancement of surface ageing is made evident under electrowetting.

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La détection sensible et sélective des métaux lourds, en particulier les métaux detransition, est d’une grande importance pour la santé publique ainsi que pour la surveillancede l’environnement. Les méthodes actuelles de référence, de par leur non portabilité, limitentla possibilité de disposer de mesures à haute résolution spatiale et temporelle. Lesmicrocapteurs optiques offrent un moyen attrayant et pratique pour surmonter ces limitationsde coût global et de temps d’analyse, en permettant la mesure en temps réel sur site.Pour démontrer ce potentiel, ces travaux de thèse sont orientés sur la détermination duchrome hexavalent Cr(VI) en solution à l’aide d’une réaction colorimétrique avec le 1,5-diphénylcarbazide (DPC), permettant de créer un complexe présentant un maximumd’absorption dans le domaine du visible. Ces travaux s’inscrivent dans la volonté dedévelopper un véritable laboratoire sur puce, intégrant la fonction fluidique parélectromouillage sur diélectrique pour créer la réaction colorimétrique, ainsi que la fonctionde mesure par intégration d’un capteur optique dédié à la mesure d’absorption dans desmicrovolumes (< μL). Pour la mesure d'absorption sur de si faibles volumes, l'utilisation demicrorésonateurs vise à augmenter de façon importante le chemin optique effectif et ainsi lasensibilité du capteur.Nous décrivons nos travaux sur la conception, la fabrication de la plateformemicrofluidique digitale ainsi que du résonateur optique en anneaux à des dimensionssubmicroniques par photolithographie par projection. Les matériaux polymères sontprivilégiés pour une intégration totale bas coût à terme, ainsi qu’un substrat verre, dont lespropriétés sont particulièrement adaptées aux applications optiques dans le domaine duvisible. / The selective and sensitive detection of heavy metals, such as transition metals, is ofparamount importance for health and safety an environmental monitoring. Current referencemethods, due to their lack of portability, are limiting factors to obtain high-resolution spatialand temporal data. Optical sensors offer an attractive and convenient way to overcome theselimitations of cost and time per analysis by offering real time, on-site measurementcapabilities.In order to demonstrate this potential, this thesis is focused on the detection and quantificationof hexavalent chromium Cr(VI) in water samples by a colorimetric reaction based on areaction with the 1,5-diphenylcarbazide (DPC), that produces a complex possessing anabsorption maximum in the visible range. This works endorse the goal of creating a true labon-chip, integrating both the fluidic function based on ElectroWetting on Dielectric (EWOD)to create the colorimetric reaction, and the sensing function based on the integration of anoptical sensor able to measure absorption variations in micro-volumes (< μL). In order toobtain sufficient sensitivity on such small volumes, optical microring resonators are used inthis work, due to their ability to enhance the effective optical path length by constructiveinterferences.This thesis describes the conception and fabrication of the EWOD microfluidic platform, aswell as the conception, simulation and fabrication of submicronic microring resonators usingstepper lithography. Polymer materials and glass substrates are selected, due to their greatoptical properties in the visible range, their compatibility with the EWOD platform, and theirintegrability at a reasonable cost.