Browsing by Subject "Oscillations"
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Item Electrical control and enhancement of dropwise condensation(2020-06-22) Wikramanayake, Enakshi Devinka; Bahadur, Vaibhav; Ezekoye, Ofodike; Shi , Li; Bonnecaze , RogerCondensation of vapor typically occurs via the formation of condensate films on condensing surfaces; however, the liquid film imposes a substantial thermal resistance to heat transfer. Filmwise condensation heat transfer can be enhanced by 5-7X by condensing vapor as droplets, which roll-off the surface, thereby preventing buildup of a liquid film. Dropwise condensation heat transfer can be enhanced by the use of electrowetting (EW) to enhance coalescence, growth and shedding of condensed droplets. This dissertation includes several fundamental studies on EW-enhanced dropwise condensation. Experiments, analytical modeling and statistical modeling are used to gain a deeper understanding of droplet growth, coalescence and shedding under EW. Chapter 1 details the motivation for this study and the objectives of this dissertation. Chapter 2 includes a literature review of condensation, electrowetting and data science- based statistical methods. Chapter 3 presents a detailed experimental study of dropwise condensation of humid air under the influence of electrowetting fields. An analytical heat transfer model, which accounts for the presence of non-condensable gases, is used to predict the heat transfer benefits associated with electrowetting-assisted condensation. Chapter 4 presents a detailed analysis of electrowetting-induced coalescence dynamics of a distribution of water droplets. Statistical modeling-based algorithms are used to identify key electrowetting-related parameters that influence droplet coalescence; the influence of these parameters on coalescence is quantified. Chapter 5 studies droplet shedding dynamics under electrowetting and shows that an intermittent electric field can significantly increase condensation rates (as compared to a continuous electric field). A key finding is the almost complete removal of water from surfaces in very short durations (< 1 sec) is observed. It is also found that the extent and rate of water removal depends on the applied voltage and frequency of the AC EW waveform, respectively. Chapter 6 presents a novel approach and an experimentally validated model to analyze the oscillations of water droplets under the influence of AC electrowetting. Chapter 7 summarizes key conclusions and outlines suggestions for future work. Overall, the research reported in this dissertation has led to fundamental contributions in the areas of condensation and microfluidics. This multidisciplinary work has involved experiments, analytical modeling and statistical modeling. Results show that electrowetting fields influence all the phenomena important in dropwise condensation (growth, coalescence, shedding of droplets). Electrowetting is therefore a powerful tool to control and enhance condensation heat transfer. This research impacts applications in energy (steam condensation, refrigeration), water (atmospheric water harvesting, desalination) and infrastructure (self-cleaning).Item Theta-frequency oscillatory synchrony in the dendrites of hippocampal CA1 pyramdial neurons(2013-05) Vaidya, Sachin Prashant; Johnston, Daniel, 1947-A CA1 pyramidal neuron in the rodent hippocampus integrates inputs from as many as 30,000 synapses distributed over hundreds of microns, making synaptic integration an intricate spatio-temporal computation. Crucial to this computation, is the timing of synaptic inputs at the axo-somatic integration site. Consequently, it would be beneficial if co-incident proximal and distal inputs arrive simultaneously at the axo-somatic integration site. This, however, is a challenge considering that spatially dispersed inputs have to propagate varying distances, leading to location-dependent temporal differences at the soma. Here we show that CA1 pyramidal neurons have an intrinsic biophysical mechanism in the form of a gradient of HCN channels that actively counteracts location-dependent temporal differences of dendritic inputs at the soma. HCN channels, due to their slow kinetics and unusual gating properties, impart an inductive reactance to the neuronal membrane properties. Using multi-site whole cell recordings, we show that this gradient of inductive reactance actively compensates for the location-dependent capacitive delay of dendritic inputs. This leads to a response synchrony of spatially dispersed inputs at the soma. This response synchrony is optimum for oscillatory signals in the theta frequency range (4-12 Hz). Using computational modeling we show that the characteristic sigmoidal distribution of HCN channels in CA1 neurons is crucial for the efficient and exclusive transfer of these synchronous theta frequencies from dendrite to the soma. To understand the significance of this oscillatory synchrony during synaptic integration, we used the dynamic clamp technique to simulate different temporal patterns of synaptic input in the dendrites of CA1 neurons. Our results reveal that this oscillatory synchrony is best harnessed by theta and gamma (40-140 Hz) frequency synaptic input patterns in CA1 neurons. Gamma and theta oscillations are associated with synchronizing activity across space in the hippocampal network. Our results thus identify a novel mechanism by which this synchrony extends to activity within single pyramidal neurons with complex dendritic arbors.