http://hdl.handle.net/1721.1/7678 2013-05-18T23:07:13Z http://hdl.handle.net/1721.1/78511 Synthetic creation of a chemotactic system via utilization of magnetically actuated microrobotic walkers Steimel, Joshua Paul Chemotaxis is a fundamental biological process that plays an important role in disease, reproduction, and most biological functions. Here, we present a radically novel method to create the first synthetic chemotactic system which utilized magnetically actuated microrobotic walkers. The system used a rotating magnetic field that once actuated induced the magnetic beads to self-assemble into microrobots and walk on surfaces. The velocity of these microrobotic walkers could be modulated by the frequency and the number of beads that composed the walkers. The receptor-ligand pair of biotin-streptavidin was utilized due to the extremely strong binding affinity of the pair. The presence of free biotin binding sites on the surface was required to obtain chemotactic motion as these binding sites modulated walker velocity. The walkers moved faster in areas with a high density of binding sites and slower in areas with a low density of binding sites. To achieve chemotaxis, gradients in the density of binding sites were required. Gradients were created by placing a droplet of concentrated streptavidin on a biotynlated slide and letting the droplet evaporate. The Gaussian evaporation process created differentials in the density of binding sites. A series of continuous velocity measurements were conducted across the sample to map the walker velocity profile. The velocity profile illustrated regions with a high density of binding sites as well as a local minimum in the density of binding sites. The discrete motion of the beads was analyzed to understand how chemotactic directed motion could be achieved by breaking the symmetry of the system. Walkers in an area with a high density of binding sites experienced a significant amount of "sticking" followed by hinge-like motion, while walkers in a low density area exhibited virtually no "sticking" and tended to slip much more frequently. Walkers were then placed on a random walk path and chemotactic directed motion was observed as the walkers drifted towards regions with a high density of binding sites. The drift velocities that were extracted from the random walk path illustrated the discrepancy between the chemical gradients present in this synthetic chemotactic system. Keywords: biomimetic, chemotaxis, superparamagnetic microrobotic walkers, biotin, streptavidin, PEG, drift velocity, random walk. Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 55-58). 2012-01-01T05:00:00Z http://hdl.handle.net/1721.1/77846 Stress relaxation of Zircaloy-4. Minar, Arthur Phillip Thesis. 1976. B.S.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.; Microfiche copy available in Archives and Science.; Includes bibliographical references. 1976-01-01T05:00:00Z http://hdl.handle.net/1721.1/76179 Variations in grain boundary segregation for nanocrystalline stability and strength Figueroa, Oscar, III In the last few decades, nanocrystalline metals have been of increasing interest. Their ability to show increased yield strength and uniform structure show them to be potentially useful in many applications. Additionally, nanocrystalline metals have become more easily manufactured in recent years, allowing for more testing and more use within industrial settings. However, nanocrystalline metals are still highly unstable, mainly due to temperature related growth. Grain boundary segregation is one way in which materials can keep nano length-scale grains. This process involves metal alloys that preferentially segregate the alloying material to the grain boundaries, potentially leading to Grain Boundary Embrittlement (GBE). Using an ideal work of fracture equation, [gamma] = 2[sigma]s - [sigma]g, the energy required to fracture nanocrystalline metal alloys was obtained, and predicted grain stability. Fracture toughness data is also calculated and compared. A contrast between bulk and nanocrystalline alloys is then made, showing benefits to the use of either set of materials for specific alloy functions. Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 39). 2012-01-01T05:00:00Z http://hdl.handle.net/1721.1/76173 Topological characterization of nanoporous gold during coarsening Rosario, Ryan (Ryan A.) Previous studies of nanoporous gold have found that, during the coarsening process, the genus per characteristic volume of nanoporous gold has remained constant. Using a rolling-ball type algorithm, in which a test probe rolls over the surface to identify atoms, several test structures and a small-scale nanoporous structure were meshed. The genus was then calculated for each of these meshed structures. It was found that an algorithm that accounts for periodic boundary conditions is required for an accurate genus calculation. Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 34-35). 2012-01-01T05:00:00Z