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• 1. [Article] On the optimization of performance in fractal-like branching microchannel heat transfer devices

  Fractal-like branching microchannel flow networks have been found to improve wall temperature uniformity and decrease both pressure drop and flow power compared to arrays of straight microchannels. The ...

  Citation × Citation Citation Abstract Copy Title: On the optimization of performance in fractal-like branching microchannel heat transfer devices Author: Heymann, Douglas Benjamin Fractal-like branching microchannel flow networks have been found to improve wall temperature uniformity and decrease both pressure drop and flow power compared to arrays of straight microchannels. The present study seeks to maximize the benefits of fractal-like branching channels by means of a gradient-based optimization algorithm. The algorithm identifies the geometric parameters that yield the highest ratio of benefit to cost; the benefit being the heat transfer with cost being flow power. The stream-wise pressure and wall temperature distributions were determined numerically using one-dimensional models validated using experimental diagnostics and computational flow analyses. Pressure distributions were used to assess flow power, and wall temperature was used as an optimization constraint. Several geometric constraints were imposed during optimization to ensure sufficient bonding area for fabrication, to maximize convective surface area and to prevent channel overlap. Three fractal-like devices were studied and optimized: a single-phase heat sink, a two-phase heat sink and a single-phase oil-to-water heat exchanger. The flow rate of the three devices was constrained using a maximum wall temperature constraint for the single-phase heat sink, the critical heat flux for the two-phase heat sink, and a fixed cold side mass flow rate and temperature rise in the case of the heat exchanger. The optimized solutions were found to depend highly on both the geometric and flow constraints imposed. As the terminal channel width was reduced, flow power was reduced in both single-phase and two-phase heat sinks. However, the flow power was increased in the heat exchanger due to the constrained volumetric flow rate. In addition, results show that if a maximum wall temperature constraint is employed, single-phase flow outperforms two-phase flow in terms of reduction in flow power. However when constrained by a critical heat flux, two-phase networks outperform single-phase networks by up to 150% Title: On the optimization of performance in fractal-like branching microchannel heat transfer devices Author: Heymann, Douglas Benjamin Fractal-like branching microchannel flow networks have been found to improve wall temperature uniformity and decrease both pressure drop and flow power compared to arrays of straight microchannels. The present study seeks to maximize the benefits of fractal-like branching channels by means of a gradient-based optimization algorithm. The algorithm identifies the geometric parameters that yield the highest ratio of benefit to cost; the benefit being the heat transfer with cost being flow power. The stream-wise pressure and wall temperature distributions were determined numerically using one-dimensional models validated using experimental diagnostics and computational flow analyses. Pressure distributions were used to assess flow power, and wall temperature was used as an optimization constraint. Several geometric constraints were imposed during optimization to ensure sufficient bonding area for fabrication, to maximize convective surface area and to prevent channel overlap. Three fractal-like devices were studied and optimized: a single-phase heat sink, a two-phase heat sink and a single-phase oil-to-water heat exchanger. The flow rate of the three devices was constrained using a maximum wall temperature constraint for the single-phase heat sink, the critical heat flux for the two-phase heat sink, and a fixed cold side mass flow rate and temperature rise in the case of the heat exchanger. The optimized solutions were found to depend highly on both the geometric and flow constraints imposed. As the terminal channel width was reduced, flow power was reduced in both single-phase and two-phase heat sinks. However, the flow power was increased in the heat exchanger due to the constrained volumetric flow rate. In addition, results show that if a maximum wall temperature constraint is employed, single-phase flow outperforms two-phase flow in terms of reduction in flow power. However when constrained by a critical heat flux, two-phase networks outperform single-phase networks by up to 150% Close

• 2. [Article] Fractal Electrodes for Interfacing Neurons to Retinal Implants

  With life expectancy on the rise, age-related ailments are a significant strain on the welfare of individuals and the economy. Progress is being made towards combating the leading cause of unavoidable ...

  Citation × Citation Citation Abstract Copy Title: Fractal Electrodes for Interfacing Neurons to Retinal Implants Author: Montgomery, Rick Year: 2015 With life expectancy on the rise, age-related ailments are a significant strain on the welfare of individuals and the economy. Progress is being made towards combating the leading cause of unavoidable blindness, age-related macular degeneration (AMD). AMD affects ten million Americans and costs the world economy $343 billion annually. Retinal implants promise to restore sight by replacing the eye's damaged photoreceptors with electronic photodiodes. Clinical trials succeed at restoring some vision, but are limited by the stimulating electrodes. We study the electrode-neuron interface with a focus on the geometrical dependence of the electrode. The functionality of neurons is intimately connected to their branching and curving shape, described by fractal geometry. We examine the morphology of neurons using fractal analysis. The results inform our electrode designs, which are fabricated using top-down lithographic and bottom-up self-assembly techniques. A novel technique for fabricating a fractal electrode is presented. Heating and cooling a film of poly(methyl methacrylate) on a SiO2 substrate causes fractal structures to form on the surface. The geometry of the structures is temperature dependent, producing crystalline branches at lower temperatures and diffusion-limited aggregates at higher temperatures. Subsequent deposition of antimony nanoclusters shows preferred diffusion to the fractal surface features. The dependence of a photodiode's performance on its top contact geometry is explored using modified nodal analysis. The results reinforce the need to balance a low mean semiconductor-metal separation distance with an adequate contact width for low resistance, all while maximizing light input. Future designs will benefit from the spatial voltage maps produced by the simulation. The electric field emanating from an electrode is also dependent on the geometry of the electrode. The Faraday cage effect is exploited to achieve similar electric field responses to traditional electrode shapes. A preliminary study of neural adhesion to SU-8 fractal electrodes is promising. The neuron grows along the electrode even at 90° turns. The role the fractal geometry plays in neuron and electrode functionality is shown to be significant. Continued study of, and experimentation with, new electrode designs is sure to produce exciting possibilities in the future. This dissertation includes previously unpublished co-authored material. Title: Fractal Electrodes for Interfacing Neurons to Retinal Implants Author: Montgomery, Rick Year: 2015 With life expectancy on the rise, age-related ailments are a significant strain on the welfare of individuals and the economy. Progress is being made towards combating the leading cause of unavoidable blindness, age-related macular degeneration (AMD). AMD affects ten million Americans and costs the world economy $343 billion annually. Retinal implants promise to restore sight by replacing the eye's damaged photoreceptors with electronic photodiodes. Clinical trials succeed at restoring some vision, but are limited by the stimulating electrodes. We study the electrode-neuron interface with a focus on the geometrical dependence of the electrode. The functionality of neurons is intimately connected to their branching and curving shape, described by fractal geometry. We examine the morphology of neurons using fractal analysis. The results inform our electrode designs, which are fabricated using top-down lithographic and bottom-up self-assembly techniques. A novel technique for fabricating a fractal electrode is presented. Heating and cooling a film of poly(methyl methacrylate) on a SiO2 substrate causes fractal structures to form on the surface. The geometry of the structures is temperature dependent, producing crystalline branches at lower temperatures and diffusion-limited aggregates at higher temperatures. Subsequent deposition of antimony nanoclusters shows preferred diffusion to the fractal surface features. The dependence of a photodiode's performance on its top contact geometry is explored using modified nodal analysis. The results reinforce the need to balance a low mean semiconductor-metal separation distance with an adequate contact width for low resistance, all while maximizing light input. Future designs will benefit from the spatial voltage maps produced by the simulation. The electric field emanating from an electrode is also dependent on the geometry of the electrode. The Faraday cage effect is exploited to achieve similar electric field responses to traditional electrode shapes. A preliminary study of neural adhesion to SU-8 fractal electrodes is promising. The neuron grows along the electrode even at 90° turns. The role the fractal geometry plays in neuron and electrode functionality is shown to be significant. Continued study of, and experimentation with, new electrode designs is sure to produce exciting possibilities in the future. This dissertation includes previously unpublished co-authored material. Close

• 3. [Article] Electron Transport Dynamics in Semiconductor Heterostructure Devices

  Modern semiconductor fabrication techniques allow for the fabrication of semiconductor heterostructures which host electron transport with a minimum of scattering sites. In such devices, electrons populate ...

  Citation × Citation Citation Abstract Copy Title: Electron Transport Dynamics in Semiconductor Heterostructure Devices Author: Pilgrim, Ian Year: 2014 Modern semiconductor fabrication techniques allow for the fabrication of semiconductor heterostructures which host electron transport with a minimum of scattering sites. In such devices, electrons populate a two-dimensional electron gas (2DEG) in which electrons propagate in exactly two dimensions, and may be further confined by potential barriers to form electron billiards. At sub-Kelvin temperatures, electron trajectories are determined largely by reflections from the billiard walls, while net conduction through the device depends on quantum mechanical wave interference. Measurements of magnetoconductance fluctuations (MCF) serve as a probe of dynamics within the electron billiard. Many prior studies have utilized heterostructures employing the modulation doping architecture, in which the 2DEG is spatially removed from the donor atoms to minimize electron scattering. Theoretical studies have claimed that MCF will be fractal when the confinement potential defining the billiard is soft-walled, regardless of the presence of smooth potentials within the billiard such as those introduced by remote ionized donors. The small-angle scattering sites resulting from these potentials are often disregarded as negligible; we use MCF measurements to investigate such claims. To probe the effect of remote ionized donor scattering on the phase space in electron billiards, we compare MCF measured on billiards in a modulation-doped heterostructure to those measured on billiards in an undoped heterostructure, in which this potential landscape is believed to be absent. Fractal studies are performed on these MCF traces, and we find that MCF measured on the undoped billiards do not exhibit measurably different fractal characteristics than those measured on the modulation-doped billiards. Having confirmed that the potential landscapes in modulation-doped heterostructures do not affect the electron phase space, we then investigate the effect of these impurities on the distribution of electron trajectories through the billiards. By employing thermal cycling experiments, we demonstrate that this distribution is highly sensitive to the precise potential landscape within the billiard, suggesting that modulation-doped heterostructures do not support fully ballistic electron transport. We compare our MCF correlation data with the dynamics of charge transfer within heterostructure systems to make qualitative conclusions regarding these dynamics. Title: Electron Transport Dynamics in Semiconductor Heterostructure Devices Author: Pilgrim, Ian Year: 2014 Modern semiconductor fabrication techniques allow for the fabrication of semiconductor heterostructures which host electron transport with a minimum of scattering sites. In such devices, electrons populate a two-dimensional electron gas (2DEG) in which electrons propagate in exactly two dimensions, and may be further confined by potential barriers to form electron billiards. At sub-Kelvin temperatures, electron trajectories are determined largely by reflections from the billiard walls, while net conduction through the device depends on quantum mechanical wave interference. Measurements of magnetoconductance fluctuations (MCF) serve as a probe of dynamics within the electron billiard. Many prior studies have utilized heterostructures employing the modulation doping architecture, in which the 2DEG is spatially removed from the donor atoms to minimize electron scattering. Theoretical studies have claimed that MCF will be fractal when the confinement potential defining the billiard is soft-walled, regardless of the presence of smooth potentials within the billiard such as those introduced by remote ionized donors. The small-angle scattering sites resulting from these potentials are often disregarded as negligible; we use MCF measurements to investigate such claims. To probe the effect of remote ionized donor scattering on the phase space in electron billiards, we compare MCF measured on billiards in a modulation-doped heterostructure to those measured on billiards in an undoped heterostructure, in which this potential landscape is believed to be absent. Fractal studies are performed on these MCF traces, and we find that MCF measured on the undoped billiards do not exhibit measurably different fractal characteristics than those measured on the modulation-doped billiards. Having confirmed that the potential landscapes in modulation-doped heterostructures do not affect the electron phase space, we then investigate the effect of these impurities on the distribution of electron trajectories through the billiards. By employing thermal cycling experiments, we demonstrate that this distribution is highly sensitive to the precise potential landscape within the billiard, suggesting that modulation-doped heterostructures do not support fully ballistic electron transport. We compare our MCF correlation data with the dynamics of charge transfer within heterostructure systems to make qualitative conclusions regarding these dynamics. Close