We consider both a long-wave model and a first-order weighted-residual integral boundary layer (WIBL) model in the investigation of thin film flow down a topographical incline whilst under the effects of a normal electric field. The liquid is assumed to be a perfect dielectric, although is trivially extended to the case of a perfect conductor. The perfect dielectric case with no topography includes a simple modified electric Weber number which incorporates the relative electrical permittivity constant into itself. Linear stability analysis is carried out for both models, and critical Reynolds numbers which depend on the electric Weber number and the capillary number are produced. Regions of stability, convective instability and absolute instability are then determined for both models in terms of our electric Weber number and Reynolds number parameters in the case of no topography. Time-dependent simulations are produced to corroborate the aforementioned regions and investigate the effect of normal electric field strength in addition to sinusoidal and rectangular topographical amplitude on our system for various domain sizes. For the time-dependent simulations we find strong agreement with the linear stability analysis, and the results suggest that the inclusion of a normal electric field may have some stabilising properties in the long-wave model which are absent in the case of a flat wall, for which the electric field is always linearly destabilising. This stabilising effect is not observed for the same parameters in the WIBL model with a sinusoidal wall, although a similar effect is noticed in the WIBL model with a rectangular wall. We also investigate the simultaneous effect of domain size, wall amplitude and electric field strength on the critical Reynolds numbers for both models, and find that increasing the electric field strength can make large-amplitude sinusoidal topography stabilising rather than destabilising for the long-wave model. Continuation curves of steady solutions and bifurcation diagrams are also produced, and comparisons between the two models are made for various parameter values, which show excellent agreement with the literature. Subharmonic branches and time-periodic solutions are additionally included, similarly showing very good agreement with the literature.

La détection d'agents biologiques dans l'air ambiant est devenue un enjeu majeur notamment en environnement hospitalier et dans la protection contre le bioterrorisme. Dans ce contexte, la miniaturisation des dispositifs d'analyse permet d'envisager leur utilisation directement sur la zone d'étude. Afin d'obtenir un échantillon concentré et représentatif, la filtration de l'air reste cependant un point délicat. Parmi les différents principes exploitables pour la collecte de particules aéroportées, l'emploi des forces électriques semble être prometteur pour améliorer les performances des dispositifs qui se trouvent généralement fondés sur des forces inertielles. Dans cette étude, une modélisation fine des collecteurs électrostatiques a été conduite pour une géométrie fil / cylindre. Elle décrit tout d'abord les champs hydrodynamiques d'un écoulement charriant des inclusions dans lequel est imposée une décharge couronne. Une injection éventuelle de vapeur dans la chambre de collecte a nécessité ensuite la détermination des champs de température et concentration de la vapeur. Une analyse dimensionnelle inspectionnelle a montré que ces champs possèdent deux termes de couplage fort dont on a justifié l'omission dans cette étude ; les phénomènes physiques mis en jeu ont alors pu être classés selon une cascade d'influences non réciproques et la résolution numérique du modèle s'en est trouvée facilitée. Quatre configurations d'écoulement différentes, caractérisées par des recirculations d'origine électro hydrodynamiques, ont été identifiées et leurs impacts sur les rendements de collecte quantifiés. De plus, une procédure de dimensionnement des filtres électrostatiques fondée sur un nombre de Deutsch représentatif des rendements a été mise en place. Son exploitation a montré l'intérêt de la mise en parallèle de petits collecteurs pour filtrer des débits d'air importants. Cette étude s'est achevée par l'analyse des effets engendrés par l'injection de vapeur dans la chambre de collecte. Elle a jeté les bases d'une explication pour l'augmentation des rendements de collecte résultant de cette injection. / Detection of airborne biological agents has become a major challenge especially in hospitals and the protection against bioterrorism. In this context, the miniaturization of analytical devices allows to consider their direct use in the field. To obtain a representative and concentrated sample, air filtration remains a delicate point. Among the various principles used to collect airborne particles, the use of electrical forces seems to be promising to improve performance beyond these of devices that are based on inertial forces.In this study, a detailed model of electrostatic collectors was developed in the wire/cylinder geometry. It first describes the hydrodynamic flow fields carring inclusions in which a corona discharge is imposed. Afterwards, the possible injection of steam into the collection chamber required the determination of the temperature and vapor concentration fields. An inspectionnal dimensional analysis justified the omission of two strong coupling terms. Therefore, in this study, the involved physical phenomena could be classified according to a non-reciprocal influences cascade and the numerical model is become simpler. Four different flow patterns, characterized by their electrohydrodynamic secondary flows, were identified and their impact on the collection efficiencies was quantified. In addition a design procedure of electrostatic filters, based on a representative efficiency Deutsch number, has been developed. This procedure shows the interest of parallelizing small collectors to filter important airflows. This study was completed by the analysis of the effects of steam into the collection chamber. It provides the basis for an explanation of the collection efficiencies increase related to this injection.

L’ébullition est un mode de transfert de chaleur très efficace utilisé dans de nombreux systèmes technologiques comme les centrales nucléaires ou refroidissement de micro-électronique. La prédiction des échanges thermiques par ébullition reste actuellement très délicate, en raison de la complexité du phénomène, malgré des décennies de recherche sur le sujet. Le coefficient de transfert thermique est intimement lié à la dynamique de bulles (nucléation des bulles, croissance et détachement) ainsi qu’à des facteurs tels la densité de sites de nucléation ou les interactions entre bulles voisines et successives. La présente étude porte sur l’ébullition saturée sur un site de nucléation artificiel unique (ou deux sites voisins) sur une paroi en cuivre poli. La dynamique de croissance des bulles a été caractérisée pour différentes surchauffes de paroi et une loi expérimentale de croissance a été établie. Les interactions entre bulles successives issues du même site ont été étudiées, montrant qu’elles peuvent provoquer des oscillations de la bulle en croissance. Les forces agissant sur une bulle en croissance ont été clairement définies, et un bilan de quantité de mouvement a été réalisé à tous les stades de la croissance d’une bulle. La courbure le long de l’interface a été mesurée, ce qui a permis de mieux saisir le mécanisme de détachement de la bulle. L’ascension d’une bulle après son détachement a été analysée, et la vitesse maximale atteinte avant un changement de direction a été estimée et comparée aux modèles existants dans la littérature. L’interaction entre bulles croissant côte à côte a été étudiée. La génération et la propagation d’une onde lors de la coalescence a été mise en évidence. Dans le contexte de travaux de recherche sur des techniques d’intensification des échanges thermiques, cette étude se penche particulièrement sur l’intensification par électrohydrodynamique. Des expériences d’ébullition ont été réalisées en présence de champs électriques, et leurs effets sur les transferts thermiques et sur la dynamique des bulles ont été analysés. Bien que le volume au détachement des bulles et la relation entre la fréquence et la surchauffe reste inchangées, la courbe de croissance des bulles est modifiée. Les bulles sont allongées dans la direction du champ électrique, et cette élongation a été estimée et comparée à d’autres résultats de la littérature. La vitesse d’ascension des bulles est réduite en présence de champs électriques, et les interactions de bulles voisines sont modifiées: il s’avère qu’en présence de champs électriques les bulles ont tendance à se repousser. Ces résultats, obtenus dans un environnement parfaitement contrôlé apportent la preuve que la présence de champs électriques modifie la dynamique des bulles et par conséquents les transferts thermiques associés. / Since boiling heat transfer affords a very effective means to transfer heat, it is implemented in numerous technologies and industries ranging from large power generation plants to micro-electronic thermal management. Although having been a subject of research for several decades, an accurate prediction of boiling heat transfer is still challenging due to the complexity of the coupled mechanisms involved. It appears that the boiling heat transfer coefficient is intimately related to bubble dynamics (i.e. bubble nucleation, growth and detachment) as well as factors such as nucleation site density and interaction between neighbouring and successive bubbles. In order to contribute to the understanding of the boiling phenomenon, an experimental investigation of saturated pool boiling from a single or two neighbouring artificial nucleation sites on a polished copper surface has been performed. The bubble growth dynamics has been characterized for different wall superheats and a experimental growth law has been established. The interaction between successive bubbles from the same nucleation site has been studied, showing the bubble shape oscillations that can be caused by these interactions. The forces acting on a growing bubble has been reviewed, and a complete momentum balance has been made for all stages of bubble growth. The curvature along the interface has been measured, and indications concerning the mechanism of bubble detachment have been suggested. The rise of bubble after detachment has been investigated, and the maximum velocity reached before a change of direction has been estimated and compared to existing models from the literature. The interaction between bubbles growing side by side has been studied: the generation and propagation of a wave front during the coalescence of two bubbles has been highlighted. As boiling heat transfer enhancement techniques are being imagined and developed, this study also focuses on the electrohydrodynamic enhancement technique. Boiling experiments have been performed in the presence of electric fields, and their effects on heat transfer and bubble dynamics have been characterized. Although the volume of the bubbles at detachment and the relationship between the bubble frequency and the wall superheat were not affected, the bubble growth curve was modified. The bubbles were elongated in the direction of the electric field, and this elongation was estimated and compared to other studies from the literature. The rising velocity of the bubble was reduced in the presence of electric field, and the behaviour of bubbles growing side by side was modified, the electric field causing the bubbles to repeal each other. These results, obtained in a fully controlled environment, provide compelling evidence that electric fields can be implemented to alter the bubble dynamics and subsequently heat transfer rates during boiling of dielectric fluids.

Drying contributes a significantly important role in processing of agricultural products in Vietnam, particularly for high-value agricultural exports. Conventionally thermal-based drying techniques have remained critical disadvantages in term of enhancing product quality and process efficiency. The typical disadvantages include deterioration of organoleptic and nutritional properties, highenergy consumption, expensive costs yet low efficiency and hazards to environment change due to the consumption of fossil fuel sources. Electrohydrodynamics (EHD) drying technology has been demonstrated as an innovative solution for drying enhancement in various applications. This paper aims at an overview of the state-of-the-art EHD drying technology to enhance heat and mass transfer in agricultural drying processes. A case study is then presented to demonstrate an even better process efficiency, compared to the state-of-the-art EHD drying technology, and to shorten the gap “research-to-market”. Finally, this study shows obviously potential applications of this innovative technology in sustainable development of food and post-harvesting agricultural processing for Vietnam in the future. / Sấy khô đóng một vai trò quan trọng trong việc chế biến và bảo quản nông sản sau thu hoạch tại Việt Nam, đặc biệt đối với các mặt hàng nông sản cao cấp phục vụ xuất khẩu. Các phương pháp sấy khô bằng nhiệt truyền thống tồn tại nhiều nhược điểm trong việc nâng cao hiệu quả sấy và bảo đảm chất lượng nông sản. Những nhược điểm nổi bật bao gồm: biến đổi thành phần hóa học và giảm giá trị dinh dưỡng của nông sản sau sấy do sử dụng nhiệt trong quá trình sấy, tiêu hao nhiên liệu lớn, chi phí nhiên liệu, lắp đặt và duy trì hệ thống sấy cao nhưng hiệu suất thấp và đặc biệt ảnh hưởng tới môi trường do sử dụng các nguồn nhiên liệu hóa thạch. Công nghệ sấy cải tiến khíđiện động lực học (EHD) hiện tại đã cho thấy tiềm năng thay thế các phương pháp sấy truyền thống. Bài báo tập trung giới thiệu cơ chế sấy các sản phẩm nông sản bằng công nghệ EHD; qua đó tác giả giới thiệu một mô hình sấy đã chế tạo thành công cho hiệu quả thậm chí còn cao hơn các mô hình hiện tại, đồng thời dễ dàng triển khai trong ngành công nghiệp sấy khô nông sản. Nghiên cứu này chỉ ra tiềm năng ứng dụng to lớn của công nghệ EHD trong phát triển bền vững các quá trình sấy khô nông sản cao cấp tại Việt Nam trong tương lai.

<p>A short, 30cm, test section was used to study the effect of electrohydrodynamic (EHD) forces on flow redistribution in a horizontal, shell and tube heat exchanger subject to both boiling and condensation. The use of a short test section allows for a consistent flow pattern across the test section length which provides further insight into the true effect of EHD.</p> <p>It was found that the voltage polarity of the applied voltages influences the flow distribution. For the current geometry studied, it was found that positive polarity voltages tend to pull liquid away from heat transfer surface and that negative voltages tended to repel more liquid toward the heat transfer surface. Using this knowledge we were able to show that positive voltages were more effective for convective condensation heat transfer enhancement, whereas negative voltages were more effective for convective boiling heat transfer enhancement. A twofold enhancement of convective boiling heat transfer was achieved for positive voltages and a 4fold enhancement was achieved for negative voltages. Similar pressure drop penalties were seen for both cases, approximately twice that of the no EHD case.</p> <p>Furthermore, the effect of DC level, peak to peak voltage, frequency and duty cycle waveform parameters on convective boiling enhancement were studied to explore the range of controllability for the current set of flow parameters. It was found that these various waveform parameters can induce different flow patterns and consequently different heat transfer and pressure drop configurations. In general the heat transfer is enhanced by EHD, but different pressure drop penalties can be achieved for a given enhancement ratio using different waveforms. High heat transfer for relatively low pressure drop was achieved using either negative DC signals or 50%duty cycle pulse waveforms. In some cases the enhancement is quite little compared to the pressure drop, for example the zero DC level, varying peak to peak voltage data. It is suggested that in a system where the heat exchanger pressure drop due to EHD is more dominant than the system pressure drop, it may be possible to use EHD as a method of retarding the system rather than enhancing it thereby broadening the scope of controllability.</p> <p>Finally we showed the proof of concept of using DC EHD as a rapid control mechanism for the load conditions. Using -8kVDC the water side heat flux could be varied by approximately ±3.2 kW/m² within 5 seconds. As a comparison, the same experiment was repeated using the refrigerant flow rate to control the load. Response times were similar for both experiments and although the power required for the flow rate control was less, the minimal variability in flow parameters for the EHD control make it a more attractive method of load control.</p> / Master of Applied Science (MASc)

<p>Electrohydrodynamics (EHD) was used to enhance the thermal performance of a latent heat thermal storage cell by reducing the charging time for a given amount of latent heat stored. Paraffin wax, which is an organic dielectric commercially available material was selected as the phase change material (PCM).</p> <p>Electric field was applied into the cell by using 9 electrodes kept at -8 kV in an effort to establish EHD forces inside the PCM. The EHD effect was studied in an originally conduction dominated melting environment. That was achieved by the cell design which promoted unidirectional melting downwards to prevent natural convection from occurring by assuring a thermally stratified molten phase. The target was to study the EHD mechanisms of enhancement with less interfering physics.</p> <p>Melting was studied under constant heat flux boundary condition. The temporal thermal profile of the surface heater and the melt front location were used to assess the EHD effect by comparing it to a 0 kV (no EHD) case.</p> <p>It was found that by using EHD (-8 kV), the time required to melt 7 mm thickness of the PCM can be reduced by 40 % when compared to 0 kV case. Through a four hour experiment time, the amount of molten PCM can be increased by 29 % by using EHD compared to 0 kV. The EHD power consumption was less than 0.17 W which is equivalent to 2.4 % of the thermal energy stored in the PCM.</p> <p>A new phenomena was discovered when applying EHD in the tested cell, which is Solid Extraction, where the solid dendrites within the mushy zone were extracted from the mushy zone into the liquid bulk towards regions of higher electric field.</p> <p>A new criteria was developed to quantify the EHD enhancement level and was called EHD enhancement factor. An enhancement factor up to 13 could be reached by using EHD. The effect of changing the heat flux on the enhancement factor was investigated, and it was found that the enhancement factor decreased by increasing the heat flux.</p> <p>Numerical simulations were performed in an effort to understand the EHD mechanisms of enhancement. The static electric field distribution, the interfacial extraction forces and the body forces acting on suspended dendrites were evaluated. The results of numerical simulations were supported by the high speed imaging and the experimental data to explain the EHD mechanisms of enhancement and the regions where solid dendrites extraction happened.</p> <p>Finally an analytical model was developed to estimate the energy stored in the different components of the tested latent heat storage cell and to estimate the amount of energy lost to the surroundings in order to quantify the accuracy of the experiment and a maximum of 18 % heat loss was estimated.</p> / Master of Applied Science (MASc)

The present thesis deals with two-phase electrohydrodynamic simulations of bubble and droplet dynamics under externally applied electric fields. We used the Coupled Level-Set and Volume-of-fluid method (CLSVOF) and two different electrohydrody-namic formulations to study the process of bubble and drop formation from orifices and needles, the interactions of two conducting drops immersed in a dielectric medium, and the oscillations of sessile drops under two different ways of applying external elec-tric field. For the process of bubble formation in dielectric liquids due to the injection of air from submerged orifices and needles, we show that a non-uniform electric field pro-duces smaller bubbles while a uniform electric field changes only the bubble shape. We further explain the reason behind the bubble volume reduction under a non-uniform electric field. We show that the distribution of the electric stresses on the bubble inter-face is such that very high electric stresses act on the bubble base due to a non-uniform electric field. This causes a premature neck formation and bubble detachment lead-ing to the formation of smaller bubbles. We also observe that the non-uniform elec-tric stresses pull the bubble interface contact line inside the needle. With oscillatory electric fields, we show that a further reduction in bubble sizes is possible, but only at certain electric field oscillation frequencies. At other frequencies, bubbles bigger than those under a constant electric field of strength equal to the amplitude of the AC electric field, are produced. We further study the bubble oscillation modes under an oscillatory electric field. We implemented a Volume-of-fluid method based charge advection scheme which is charge conservative and non-diffusive. With the help of this scheme, we were able to simulate the electrohydrodynamic interactions of conducting-dielectric fluid pairs. For two conducting drops inside a dielectric fluid, we observe that they fail to coalesce when the strength of the applied electric field is beyond a critical value. We observe that the non-coalescence between the two drops occur due to the charge transfer upon drop-drop contact. The electric forces which initially bring the two drops closer, switch direction upon charge transfer and pull the drops away from each other. The factors governing the non-coalescence are the electric conductivity of the drop’s liquid which governs the time scale of charge transfer relative to the capillary time scale and the magnitude of the electric forces relative to the capillary and the viscous forces. Similar observations are recorded for the interactions of a charged conducting drop with an interface between a dielectric fluid and a conducting fluid which is the same as the drop’s liquid. For the case of a pendant conducting drop attached to a capillary and without any influx of liquid from the capillary, we observed that the drop undergoes oscillations at lower values of electric potential when subjected to a step change in the applied electric potential. At higher values of electric potential, we observed the phenomenon of cone-jet formation which results due to the accumulation of the electric charges and thus the electric forces at the drop tip. For the formation of a pendant conducting drops from a charged capillary due to liquid injection, we observed that the drops are elongated in presence of an electric field. This happens because the free charge which appears at the drop tip is attracted towards the grounded electrode. This also leads to the formation of elongated liquid threads which connect the drop to the capillary during drop detachment. We plotted the variation of total electric charge inside the drops with respect to time and found the charge increases steeply as the drop becomes elongated and moves towards the grounded electrode. For sessile drop oscillations under an alternating electric field, two different modes of operations are studied. In the so called ‘Contact mode’ case, the droplet is placed on a dielectric coated grounded electrode and the charged needle electrode remains in direct contact with the drop as it oscillates. In the ‘Non-contact mode’ case, the drop is placed directly on the grounded electrode and electric potential is applied to a needle electrode which now remains far from the drop. We show that the drop oscillations in the contact mode are caused by concentration of electric forces near the three phase contact line where the electric charge accumulates because of the repulsion from the needle. For the non-contact mode, we observe that the electric charge is attracted by the needle towards the drop apex resulting in a concentration of the electric forces in that region. So the drop oscillates due to the electric forces acting on a region near the drop tip. We also present the variation of the total electric charge inside the drop with respect to time for the two cases studied.

In this thesis we have studied systems that driven by mechanisms broadly known as phoresis. More specifically, in the second chapter we calculate the excess noise in electrophoresis of a colloid due to microion fluctuations. In the next three chapters we study in detail a system of self-phoretic colloids, propelled by the energy released when an ambient fuel molecule makes contact with a catalytic region on the particle’s surface. We start with the behaviour of a single particle in a linear substrate gradient, then go on to study interactions between two particles due to their diffusion clouds, and finally obtain the collective equations of motion by a systematic coarse-graining of the microscopic Langevin dynamics. To understand the role of nonequilibrium fluctuations in an electrophoretic system we have theoretically analyzed the dynamics of a single colloidal particle in an externally applied electric field. We have studied the colloidal dynamics in two scenarios: a particle free to move in an unbounded fluid and a colloid near a wall which is stationary due to a balance between gravity and the electric field. The thermal motions of microions lead to an anisotropic, nonequilibrium noise, proportional to the field, in the effective Langevin equation for the colloid. The fluctuation-dissipation ratio depends strongly on frequency, in contrast to an equilibrium system, and the colloid if displaced from its steady-state position relaxes with a velocity not proportional to the gradient of the logarithm of the steady-state probability. Other measurable effects of this noise are a superdiffusive peak at short times and an enhanced diffusity at long times. We have then studied the effective potential and obtained a non-dimensional measure of the size of the excess noise. Possible extensions of this study to include the behaviour of the mean and fluctuation properties in the case of an applied alternating potential, and the effect of the excess noise on electrohydrodynamic aggregation of colloids. We next turn to a phoretic system that has been much studied in the recent years – active Janus colloids . On one hand these colloids are an important contribution to the general class of problems on self-propulsion at low Reynolds number. On the other hand since their behaviour can be tuned at the level of single particle we can ask how their collective behaviour depends on the swimmer design. This makes it a very rich field with lots of challenging questions. We first study the single particle behaviour of an active Janus colloid in an imposed substrate gradient, then build the two-particle interactions and ultimately the collective equations of motion by a generalisation of these results. Our work presents a new approach to active matter. We show theoretically how to design particles that are not only motile but can reorient in response to gradients, thus mimicking chemotaxis. We outline the collective behaviour emerging from these single-particle properties, including colloidal realisations of gravitational collapse, plasma oscillations and spontaneously ringing states, and present a phase diagram, in terms of single particle parameters, that can be tested in experiments. This provides a template to design collective behaviours of interest by tuning the surface properties of the colloids. We can also control the range of the interaction by varying the concentration of reactant. Our coarse-grained equations of motion for the polar orientation and number density fields for a collection of colloids propelled by and interacting through long-ranged dif-fusion fields are novel in a number of ways. This is the first example in active matter literature of a microscopic derivation of collective dynamics for particles interacting via long-ranged diffusion fields. The instabilities and possible phases that we predict are different from those in traditional flocking models, which consider only short-ranged aligning interactions. The long-ranged interactions of interest here cannot produce a globally polar ordered state, and we work in a concentration regime where steric and collisional interactions are not important. Instabilities towards flocking, and the advective nonlinearities of the Toner-Tu model, although not ruled out by the symmetries of our model, do not play a significant role in our system. The collective behaviour we predict will not be seen in purely locally interacting active-particle systems. The mechanisms at work in the “saturated” case where reactant is abundant cannot be viewed as totally generic features of collections of self-driven particles; they require interactions mediated by the production or consumption of long-ranged diffusing solute fields. Earlier work on saturated systems resolved neither interactions mediated by the polarity of the objects nor chemotactic effects. Their treatment truncated the equations at the level of the concentration [1]. In the “unsaturated” case more than one mechanism operates. One is related to the motility-induced phase separation discussed phenomenologically in refs. [2,3] (for which our system provides an important microscopic realisation). The other is due to chemo-taxis and phoresis which we report for the first time. Our expression of the various coefficients in the equaions of motion in terms of the single particle properties can also be used to design systems in which one or the other of these mechanisms dominate. We are now planning to study a collection of these particles in a fluid and examine the diffusion of a tracer particle as was done by Yeomans et al. [4] for hydrodynamic interactions. The Levy flights obtained in [4] is due to the long-ranged nature of the hydrodynamic fields, which cause effects like entrainment leading to interesting tracer dynamics. In this thesis we have considered colloids in which the symmetry axis of the colloid and the catalytic coat coincide. It might be of interest to consider cases when the axes are at an angle making the swimmer biaxial, or more complicated arrangements leading to chirality and thus rotation. Collective dynamics and two particle interaction between such swimmers can also be interesting. The formalism developed for the study of interaction between two active colloids through their diffusion fields and hydrodynamics can be extended to study their interaction with extended passive surfaces like walls or spheres. The collective dynamics of this class of active systems when it is confined between parallel walls is also of interest. Work in progress includes studies of the motion of the swimmer in a periodic array of passive colloids. In this study of collective dynamics, we have ignored the role of hydrodynamics, as the slowest decay of the field is 1/r3, which is subdominant to the decay of the chemical fields and in the dilute limit is expected to change things only qualitatively. However their role would be more important when we consider the stability of ordered structures like an aster in the saturated case. Another effect of hydrodynamics is to stir the fluid. It might be interesting to study the finite-P´eclet number regime [5, 6] of our system particularly in the unscreened region when advection of the scalar fields s and p by the velocity can affect clustering. We have derived the form of the nonlinear equations of motion in both the saturated and the unsaturated regimes. It will be interesting to investigate their relevance in the dynamics and phases that this extremely rich system can form. Even in the overdamped limit where we obtain an effective density equation it is not clear that the dynamics will resemble that of the Keller-Segel model due to the presence of the interesting nonlinear terms. Also, in this thesis, we have only looked at the fluid-like state of the system. We have just started exploring the high concentration regime where we can check the propensity of the system to develop crystalline order. In the screened limit where we obtain a condensation due a negative squared sound speed, it is posssible to study the condensation phenomenon in greater detail. In future we also plan to examine whether the tendency to condense at nonzero wavenumber (See Fig 5.1), i.e., microphase separation, can lead to liquid-crystalline phases like smectics. The systems described in this thesis are extremely rich and the few ideas mentioned above form just a small subset of the plethora of exciting theoretical and experimental explorations that can be performed with them. Since they can be “designed”, unlike biological substances, they can also become a test-bed for testing theoretical predictions of the nonequilibrium statistical mechanics of self-propelled systems.

This work studies the influence of field effect on large area 2 dimensional ribbons of fluids. A fluid of choice is confined in the channel of a metal-insulator-channel-insulator-metal architecture and is subjected to constant (d.c) or alternating (a.c) fields (de-pending on the application) along with a pressure drive flow. A general fluid would be composed of molecules having certain polarizability and be a dispersion of non-ionic and ionic particulates. The field effect response under pressure driven flow for this fluid would result in electrophoresis, electro osmosis, dielectrophoresis, dipole-dipole interaction and inverse electro osmosis phenomena. Using some of these phenomena we study applications related to desalination and energy harvesting with saline water as the ex-ample fluid for the former case, and solution processed poly vinyldene fluoride (PVDF) for the latter case. The geometrical features of \large area" and the \ribbon shape" can be taken advantage of to influence the design and performance for both applications. With regards to desalination, it is shown via experiments and theoretical models that the presence of alternating electric fields aid in ion separation along the flow when the saline water is subjected to laminar flow. Moreover, the power consumption is low due to the presence of the insulator. An average of 30% ion removal efficiency and 15% throughput is observed in the systems fabricated. Both performance parameters are discussion can be improved upon with larger channel lengths. The \2-D ribbon" and alternating field effect aid in achieving this by patterning the randomly distributed ions in the bulk into a smooth sheet charge and then repelling this sheet charge back into the bulk. The electric field exhibited by this sheet charge helps trap more ion sheets near the interface, thereby converting a surface ion trapping phenomena (when d.c is used) to a bulk phenomena and thereby improving efficiency. With regards to energy harvesting, a solution of PVDF in methyl ethyl ketone and 1-methyl-2-pyrollidone is confined to the \2-D ribbon" geometry and subject to high d.c fields. This aids in combining the fabrication, patterning and poling process for PVDF into one setup. Since the shape of the ribbon is defined by the shape of the channel, the ribbons (straight or serrated) can be used to sense forces of various magnitudes. More importantly experiments and theoretical models are studied for energy harvesting. Since the ribbon geometry defines the resonant frequency, large PVDF ribbon can be used to harvest energy from low frequency vibrations. Experiments show that up to 60 microwatt power can be harvested at 200 Hz and is sufficient to supplement the power for ICs.

No description available.