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Theoretical and Experimental Study of Hydrodynamic Pair Interactions in Sheet Flow
Get PDFDepartment of PhysicsHydrodynamic interaction between two particles is symmetric at a low Reynolds number uniform flow, creating a hydrodynamic pair. When this pair is sufficiently far from other particles/pairs in the flow it moves with a constant velocity depending on pair length and tilt. In a high concentration of particles, this symmetry is frequently broken due to the presence of other nearby particles, leading to rapid formation/breaking of pairs and high-frequency disturbances in their velocities. Our results show that pair formation dominates the hydrodynamic interaction and controls the spreading of particles both in parallel and perpendicular directions to the background flow. In a continuous medium, pair formation leads to disturbances that propagate as a function of the mean speed of particles. We also study the pair formation dynamics in the regular periodic arrangement of particles in the one-dimensional chain and the two-dimensional square, and triangular geometries with periodic boundary conditions. In the disordered phase, measurements show a sub-population of long-lived particle pairs. Modeling and simulation of the ordered crystalline phase identify the pairs as effective quasiparticles, emerging at the Dirac cones of the spectrum and inducing the melting of the crystal. When the intrinsic threefold symmetry of the hydrodynamic interaction matches that of the crystal, the cones connect to a multicritical, monkey-saddle van Hove singularity, forming a flat band of slow low-frequency excitations whose divergent density drives a sharper melting transition. We compute structure factor, radial distribution, autocorrelation function, and power spectrum evolution in time for these arrangements and compared these with the experiment in which particles flow in microfluidic channels.ope
Theoretical and simulation studies of collective phenomena in beam-driven plasma wakefield accelerators
Get PDFDepartment of PhysicsThis thesis mainly discusses three topics in beam-driven plasma wakefield acceleration (PWFA), which have been intensively studied in the community over the last decade. The chapters are composed of the self-modulation of a long proton bunch for electron acceleration, which is the bunch shaping method of the high energy proton driver for the optimum wakefield generation, classification of long beam-plasma instabilities, which is the stability estimation of driver bunch during the wakefield generation in plasma, and low power laser ionization injection for PWFA, which is an advanced method for the ultra high brightness witness bunch, aiming the next generation free electron laser radiation source. These studies are based on analytical and numerical approaches. Analytical approaches are developed for the more detailed and systematic understanding of the systems and proved by particle-in-cell (PIC) simulations.ope
WearPut : Designing Dexterous Wearable Input based on the Characteristics of Human Finger Motions
Get PDFDepartment of Biomedical Engineering (Human Factors Engineering)Powerful microchips for computing and networking allow a wide range of wearable devices to be miniaturized with high fidelity and availability. In particular, the commercially successful smartwatches placed on the wrist drive market growth by sharing the role of smartphones and health management. The emerging Head Mounted Displays (HMDs) for Augmented Reality (AR) and Virtual Reality (VR) also impact various application areas in video games, education, simulation, and productivity tools. However, these powerful wearables have challenges in interaction with the inevitably limited space for input and output due to the specialized form factors for fitting the body parts. To complement the constrained interaction experience, many wearable devices still rely on other large form factor devices (e.g., smartphones or hand-held controllers). Despite their usefulness, the additional devices for interaction can constrain the viability of wearable devices in many usage scenarios by tethering users' hands to the physical devices. This thesis argues that developing novel Human-Computer interaction techniques for the specialized wearable form factors is vital for wearables to be reliable standalone products.
This thesis seeks to address the issue of constrained interaction experience with novel interaction techniques by exploring finger motions during input for the specialized form factors of wearable devices. The several characteristics of the finger input motions are promising to enable increases in the expressiveness of input on the physically limited input space of wearable devices. First, the input techniques with fingers are prevalent on many large form factor devices (e.g., touchscreen or physical keyboard) due to fast and accurate performance and high familiarity. Second, many commercial wearable products provide built-in sensors (e.g., touchscreen or hand tracking system) to detect finger motions. This enables the implementation of novel interaction systems without any additional sensors or devices. Third, the specialized form factors of wearable devices can create unique input contexts while the fingers approach their locations, shapes, and components. Finally, the dexterity of fingers with a distinctive appearance, high degrees of freedom, and high sensitivity of joint angle perception have the potential to widen the range of input available with various movement features on the surface and in the air. Accordingly, the general claim of this thesis is that understanding how users move their fingers during input will enable increases in the expressiveness of the interaction techniques we can create for resource-limited wearable devices.
This thesis demonstrates the general claim by providing evidence in various wearable scenarios with smartwatches and HMDs. First, this thesis explored the comfort range of static and dynamic touch input with angles on the touchscreen of smartwatches. The results showed the specific comfort ranges on variations in fingers, finger regions, and poses due to the unique input context that the touching hand approaches a small and fixed touchscreen with a limited range of angles. Then, finger region-aware systems that recognize the flat and side of the finger were constructed based on the contact areas on the touchscreen to enhance the expressiveness of angle-based touch input. In the second scenario, this thesis revealed distinctive touch profiles of different fingers caused by the unique input context for the touchscreen of smartwatches. The results led to the implementation of finger identification systems for distinguishing two or three fingers. Two virtual keyboards with 12 and 16 keys showed the feasibility of touch-based finger identification that enables increases in the expressiveness of touch input techniques. In addition, this thesis supports the general claim with a range of wearable scenarios by exploring the finger input motions in the air. In the third scenario, this thesis investigated the motions of in-air finger stroking during unconstrained in-air typing for HMDs. The results of the observation study revealed details of in-air finger motions during fast sequential input, such as strategies, kinematics, correlated movements, inter-fingerstroke relationship, and individual in-air keys. The in-depth analysis led to a practical guideline for developing robust in-air typing systems with finger stroking. Lastly, this thesis examined the viable locations of in-air thumb touch input to the virtual targets above the palm. It was confirmed that fast and accurate sequential thumb touch can be achieved at a total of 8 key locations with the built-in hand tracking system in a commercial HMD. Final typing studies with a novel in-air thumb typing system verified increases in the expressiveness of virtual target selection on HMDs.
This thesis argues that the objective and subjective results and novel interaction techniques in various wearable scenarios support the general claim that understanding how users move their fingers during input will enable increases in the expressiveness of the interaction techniques we can create for resource-limited wearable devices. Finally, this thesis concludes with thesis contributions, design considerations, and the scope of future research works, for future researchers and developers to implement robust finger-based interaction systems on various types of wearable devices.ope
Combining dithieno[3,2-f:2 ',3 '-h]quinoxaline-based terpolymer and ternary strategies enabling high-efficiency organic solar cells
No full textBy incorporating a dithieno[3,2-f:2 ',3 '-h]quinoxaline unit into a PM6 polymer backbone, we developed a novel terpolymer family, demonstrating composition-dependent optical, electrochemical, and morphological characteristics. Organic solar cells based on the combination of a terpolymer and ternary strategy achieved a high power conversion efficiency of 17.60%, demonstrating the validity of our combination strategy
First principles study on Li metallic phase nucleation at grain boundaries in a lithium lanthanum titanium oxide (LLTO) solid electrolyte
No full textSolid electrolytes (SEs) are critical for next-generation all solid-state batteries with high energy density and fire safety. However, recent studies observed that the Li metallic phase nucleates at the electrode interfaces as well as the interfaces between crystalline grains of SEs. Many studies have revealed the origins and control methods for Li metallic phase formation at the anode interface, but a thorough understanding of metallic Li formation at intergranular regions in SEs has not been developed yet. Through systematic DFT simulations, we present a thorough atomistic study that reveals the impact of intergranular regions on Li-metallic phase formation in SEs using the perovskite Li3xLa(2/3)-x(1/3)-2xTiO3 (0 < x < 0.167) (LLTO) as a model SE. We investigated the three representative model structures for intergranular regions, which are experimentally observed with various microstructure configurations: (i) stoichiometric grain boundary (GB), (ii) A-site deficient GB, and (iii) intergranular pore space. In the stoichiometric GB, the GB region has an electron insulating feature regardless of A-site compositions (0 < x < 0.167). In the A-site deficient GB, however, the GB region has electronic conductivity, but it has a high repulsive force against Li-ions moving into the GB region. However, in the intergranular pore structure, Li-ions prefer to move with a neutral charge state into the pore space which shows a p-type conductive property. Accordingly, Li metallic phase nucleation starts in the intergranular pore space of the SE. These results elucidate the critical role of pore space in SEs for Li metallic phase nucleation and provide an insight into the design of Li metallic phase-free SEs and further studies on SE materials
Auxeticity of monolayer, few-layer, vdW heterostructure and ribbon penta-graphene
No full textUsing molecular statics simulations, we specifically focus on investigating the negative Poisson's ratio of the monolayer, few-layer, van der Waals, and ribbon penta-graphene. As a result, we provide evidence to show that the Poisson's ratio is the combination of bond stretching and angle rotating mechanism. The auxeticity of monolayer penta-graphene is due to the dominance of bond stretching. However, the significant effect of the angle rotating mechanism causes the enhancement of the in-plane Poisson's ratio of few-layer penta-graphene. Furthermore, the elongation of interlayer bonds results in a negative out-of-plane Poisson's ratio in few-layer penta-graphene. The strong dependence of Poisson's ratio on stacking configuration and number of layers was found. We also show that the van der Waals interaction slightly enhances the auxeticity of heterostructure penta-graphene. Finally, we discuss the significant effects of warped edges on the auxeticity of penta-graphene ribbons
Unveiling the Role of Electrode-Level Heterogeneity Alleviated in Silicon???Graphite Electrode Under Operando Microscopy
No full textReaction heterogeneity is a crucial factor that influences the design of composite electrodes. Silicon???graphite composites exhibit practical use as anodes, but the complex mechanisms in blended electrodes have not been investigated. Considering mechanisms at an electrode level, intra-/interparticle heterogeneity depending on state-of-charge (SOC) becomes problematic due to their complex kinetic properties. We investigate the complex dynamics of a silicon???graphite blended electrode using side-view operando optical microscopy, highlighting the proper mechanisms of SOC heterogeneity. Graphite and silicon simultaneously lithiated during lithiation by competition between redox potentials and interparticle diffusion. During delithiation, the driving force of electrochemical potentials increases, resulting in sequential reactions from graphite to silicon. Different kinetics induces interplay during the competitive reaction, affecting depth heterogeneity. Silicon mitigates the depth heterogeneity of graphite during lithiation due to its rapid surface diffusion and kinetic-derived overpotential. These findings pave new avenues for a more sophisticated design of high-energy Si-graphite anodes
Application of poly (vinyl alcohol)-cryogels to immobilizing nitrifiers: Enhanced tolerance to shear stress-induced destruction and viability control
No full textThe hardness of poly (vinyl alcohol)-cryogels (PVA-CGs) was improved under three parameter conditions: 7.5 %- 12.5 % PVA, 1-5 freezing-thawing cycles (FTCs), and the addition of 0 %-10 % glycerol as a cryoprotectant. This study investigated the effects of shear stress-induced destruction (SSID) on mechanical strength by inducing rapid ero-sion with a high frictional force. Tolerance to SSID (Tol-SSID) exhibited different sensitivities and trends depending on the above three fabrication parameters. The measured Tol-SSID exhibited consistent and inconsistent trends with ten -sile strength and swelling, respectively. Tol-SSID evaluation provides new insights into the practically meaningful me-chanical strength of PVA-CGs against strong friction, which simulates extreme shear stress in a bioreactor. A PVA-CG with a PVA concentration of 10 % and in two FTCs resulted in Tol-SSID and tensile strength of 88.3 % and 0.59 kPa, respectively. Here, 5 % glycerol was added to maintain the bacterial respiration activity of immobilized nitrifiers of 0.097 mg-O2/g-VSS center dot min and survival of 88.6 %. The continuous mode of nitrification using the optimized PVA-CG for 10 days resulted in an ammonia removal rate of 0.2173 kg-N/m3 center dot d, which is an improvement over cases without glycerol addition: 0.1426 and 0.1472 kg-N/m3 center dot d for PVA-CGs in two and three FTCs, respectively
