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published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The upper atmosphere of the sun, the solar corona, is approximately 1,000,000K hotter than the surface of the Sun, a property which cannot be explained by the normal processes of heat conduction and radiation. It is now commonly believed that the magnetic fields which fill the solar atmosphere, and propagate down into the interior of the Sun, are important for transferring and transforming energy from the strong plasma flows inside the Sun into the corona as heat. I have investigated an elementary flux interaction which forms a fundamental building block of the coronal heating process. This interaction involves two opposite polarity sources on the Sun's surface in the presence of an overlying magnetic field. To fully understand how this interaction transfers heat into the solar corona, the magnetic skeleton is required, which shows possible sites of heating that are due to magnetic reconnection. A magnetic field is best described by its magnetic skeleton. The most important parts of the magnetic skeleton to find are the null points, from which separatrix surfaces extend that divide magnetic flux of different topology. Part of this thesis proposes a new method of finding null points, for which the accuracy is shown and then compared with another commonly used method (which gave false results). Using these techniques for finding the magnetic skeleton in the magnetic interaction above, the evolution of the skeleton was found to head through seven distinct states, some of which were far more complicated than expected. This included a high number of separators (the intersection of two separatrix surfaces), which are a known location of magnetic reconnection. This separator reconnection was shown to be the main heating mechanism in this interaction, from which the total amount and rates of reconnection in the experiment was calculated. This led to the discovery of recursive reconnection, a process where magnetic flux is reconnected before reconnecting back to its original state, to allow for the process to repeat again. This recursive reconnection was shown to allow far more reconnection than would have been previously expected, all of which releases heat into the neighbouring areas of the atmosphere. Finally, the interaction was modelled with sources of different magnetic radii but of equal flux. This showed that when the antisymmetric nature of the previous interactions was removed, there was little change in the reconnection rates, but when the strength of the overlying magnetic field was increased, the reconnection rates were found to increase. This increase in the overlying magnetic field strength also produced a new magnetic feature called a bald-edge, which was found to replace some of the null points. These bald-edges were found to be associated with surfaces similar to separatrix surfaces that divide flux of different topology but do not extend from a null point. Also features similar to separators extend from these bald-edges.

In this thesis, the collective motion of Magnetospirillum magneticum strain AMB-1 is studied. Bacteria have developed various types of mechanism in order to search for the optimum environment, which are called taxis. There are two types of taxis, aerotaxis and magnetotaxis, that are involved in AMB-1. It is commonly believed that the vertical component of the Earth magnetic field helps to align the bacteria and confine them in one dimension, whereas the oxygen gradient determines their swimming velocity. In this way, magnetotaxis is conceived as a passive response, while aerotaxis is active and they work independently. Previous works done by our group may suggest an alternative picture. In this thesis, two experiments were carried out to gain a better understanding of the taxis of AMB-1. In the first experiment, bacterial suspension of density around 10⁸ cells cm⁻³ was injected into a chamber of dimensions 5×5×0.025 mm³. The motion of the bacteria was captured by a video camera mounted in the microscope under darkfield condition. Tracking of the quasi-2D motions under different applied magnetic field generated by two pairs of Helmholtz coils was done. In this way, we could determine the effect of magnetic field on basic physical properties of the bacteria, such as the diffusive motion, the rate of change of the motion direction. In the second experiment, a one dimensional oxygen gradient was achieved inside a microfluidic chamber. An aerotactic band of highly concentrated bacteria was observed under darkfield condition when bacterial suspension of density around 10⁹ cells cm⁻³. It is found that the time for the formation of the band varies under different applied magnetic fields, which suggests that the magnetotaxis in AMB-1 may not be completely passive. / 這篇論文報告關於趨磁螺菌AMB-1在集體運動上的研究。為了在野外尋找最好的生存環境，細菌發展出不同的趨向性機制。AMB-1擁有趨磁性和趨氧性兩個機制。在普遍的認知裡，地球磁場的垂直分量會把牠們的運動局限為一維，氧氣濃度梯度則影響牠們游泳的速度。趨磁性一直被認為是被動反應，而趨氧是主動的，兩者獨立運作。我課題組最近的實驗結果提出了存在另一種機制的可能性。為更了解AMB-1的趨向性機制，我們進行了兩個實驗。 / 在首個實驗中，濃度約為每立方厘米10⁸個的細菌溶液被注入到5×5×0.025立方毫米的空間。我們用攝像機把顯微鏡暗場觀測下的細菌運動記錄下來，在兩對亥姆霍茲線圈產生的不同磁場下，追蹤細菌的準二維運動。這樣，我們可以測定細菌的基本物理性質在不同磁場下的影響，例如其擴散運動、運動方向的轉換頻率等等。在第二個實驗中，我們在一個微流控空間裡建立了一維的氧氣濃度梯度。當細菌懸液濃度達到每立方厘米10⁹個時，在暗場條件下可以一條由高濃度細菌組成的趨氧性帶。我們發現帶形成所需的時間隨不同的外加磁場而改變，顯示AMB-1的趨磁性可能不是一個完全被動的反應。 / Lau, Tsz Ki = 趨磁螺菌AMB-1的集體運動觀測 / 劉子麒. / Thesis M.Phil. Chinese University of Hong Kong 2013. / Includes bibliographical references (leaves 53-58). / Abstracts also in Chinese. / Title from PDF title page (viewed on 12, October, 2016). / Lau, Tsz Ki = Qu ci luo jun AMB-1 de ji ti yun dong guan ce / Liu Ziqi. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

A large amount of research has been done in which the magnetocrystalline anisotropy energy for fcc Ni and bcc Fe was calculated based on the electronic structure of these elements. Unfortunately; the results of these studies don't agree with each other and also differ from the experimental observation. In a previous thesis the effects of numerical errors in the Brillouin zone integrations were investigated. The results of that work explain why different calculations give different results, but do not explain the difference with experiment. The conclusion was that the underlying bandstructure, which was calculated using standard approximations, was not correct. The bandstructure of these elements will be different when improved prescriptions for the exchange-correlation energy are used. There is, however, no clear indication along which lines this approximation should be improved. Here we have taken a different approach to change the bandstructure. We suspected that some important interactions between different atomic orbitals are either ignored or miscounted. In this work, we examined the sensitivity of the energy on the interaction between those orbitals and studied in detail the consequences of changes in some interaction parameters which gave rise to a large energy change. The main result of this work is a better understanding of the relation between changes in the electronic structure in k-space and the resultant change in the magnetocrystalline anisotropy energy. In addition, this work takes another step in trying to find a better understanding how the magnetocrystalline anisotropy energy relates to interactions between neighboring atoms. / Graduation date: 2000

Graduation date: 1995

Controlling the orientation and spatial distribution of discontinuous fibers in composite materials enables product properties to be tailored to anticipated use. Electric fields are already (albeit rarely) used to affect alignment in lignocellulosic (LC) fiber composites. The use of magnetic fields has not, however, been suggested or explored; this is apparently because LC fibers are essentially non-magnetic. The approach may offer, however, some considerable advantages, as long as ferromagnetism may be imparted to the fibers. In the present research several fiber modification processes were considered and two, electroless nickel plating and spray application of a coating containing nickel in suspension, were investigated in more depth. The latter was chosen to render highly engineered, elongated wood particles responsive to magnetic fields. Individual treated particles were suspended in viscous, newtonian silicone fluids and their rotation under the influence of a controlled magnetic field was video recorded. The magnetic torque on the particle was, under the above conditions, directly proportional to the fluid viscosity, to the particle's angular velocity and to a characteristic shape constant. The maximum of the specific magnetic torque (magnetic torque divided by the shape constant) was found to reflect the influence of field strength and particle Ni-treatment on rotation. Results were scaled to an arbitrarily chosen viscosity for comparison. The dependencies of the magnetic torque found in the present research compare with those theoretically predicted for ellipsoidal and cylindrical bodies. For field strengths ranging from 0.07T to 0.15T (below magnetic saturation) the magnetic torque increased almost linearly with increasing field strength. Magnetic torque was also found to increase nearly linearly with increasing bulk Ni-concentration (5g/kg - 50g/kg). Rotational motion was sometimes impeded at low field strengths and this was attributed to a permanent magnetic moment obtained by the particle. A coercive field strength of 7600A/m supported this hypothesis. Judiciously switched field polarity increased magnetic torque at small alignment angles. The present research indicates that orienting LC fibers with magnetic fields is possible and promising. To study dynamics of fiber motion in low viscosity fluids, such as air, a different experimental method is necessary; however, dependencies of the magnetic torque found in the present study still hold true. / Graduation date: 1993

Thesis (Ph. D.)--Worcester Polytechnic Institute. / Keywords: diffusion; tendon; NMR; fMRI. Includes bibliographical references (p. 172-179).

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