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). / 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. / by Joshua Paul Steimel. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Vita. / Includes bibliographical references (leaves 115-119). / by Donna Sue Chuang. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1995. / Vita. / Includes bibliographical references (p. 47-49). / by Jung Uk Yoon. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 107-112). / One-dimensional nanostructures with high aspect-ratios and nanometer cross-sectional dimensions have been the focus of recent studies in the persistent drive to miniaturize devices. Conventional bottom-up methods such as vapor-liquid-solid growth have been widely applied for the fabrication of uniform and high quality nanowires. Two challenges toward nanoelectronics and other applications remain: on the single-nanowire level, precisely manipulating an individual nanowire for the sophisticated functionalities, and on the multiple-nanowire level, integrating nanowires into designed architecture at large scale. Thus, an alternative approach with the capacity to achieve ordered and extended nanowires is highly desirable. In this thesis, we observe an intriguing phenomenon that a cylindrical shell upon reaching a characteristic thickness breaks up into filament arrays during optical-fiber thermal drawing. This structural evolution occurs exclusively in the cross-sectional plane, while the uniformity along the axial direction remains intact. We demonstrate crystalline semiconductor nanowires by post-drawing annealing procedure and characterize their electrical and optoelectric properties for the devices such as optical switch. This top-down thermal drawing approach provides new opportunities for nanostructure fabrication with high throughput and at low cost, and offers promising applications in renewable energy and data storage. In order to understand the stability (or instability) of thin shells and filaments, we explore a physical mechanism during the complicated thermal drawing. A perspective of capillary instability from fluid mechanics is focused. Axial stability of continuous filaments is consistent with capillary instability. Axial stability of a thicker cylindrical shell arises from large radius and high viscosity. These results provide theoretical guidance in the understanding of attainable feature sizes and in materials selection to expand the potential functionalities of devices in microstructured fibers. / by Daosheng Deng. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 147-162). / Desalination is one of the most promising approaches to supply new fresh water in the face of growing water issues. However, commercial reverse osmosis (RO) techniques still suffer from important drawbacks. In order for desalination to live up to the water challenges of this century, a step-change is needed in RO membrane technology. Thanks to significant advances in the field of computational materials science in the past decade, it is becoming possible to develop a new generation of RO membranes. In this thesis, we explore how computational approaches can be employed to understand, predict and ultimately design a future generation of RO membranes based on graphene. We show that graphene, an atom-thick layer of carbon with exceptional physical and mechanical properties, could allow for water passage while rejecting salt ions if it possessed nanometer-sized pores. Using computer simulations from the atomic scale to the engineering scale, we begin by investigating the relationship between the atomic structure of nanoporous graphene and its membrane properties in RO applications. We then investigate the thermodynamics, chemistry and mechanics of graphene and the water and salt surrounding it. Finally, we establish the system-level implications of graphene's promising membrane properties for desalination plants. Overall, this thesis reveals that graphene can act as an RO membrane with two orders of magnitude higher water permeability than commercial polymer membranes as long as the nanopores have diameters around 0.6nm, that graphene is strong enough to withstand RO pressures as long as it is supported by a substrate material with adequate porosity, and that a nanoporous graphene membrane could ultimately reduce either the energy footprint or the capital requirements of RO desalination. Ultimately, this thesis highlights a path for the development of next-generation membranes for clean water production in the 21st century. / by David Cohen-Tanugi. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Vita. / Includes bibliographical references (p. 175-188). / (cont.) The self consistent defect model established the defect chemistry of langasite, enabling important parameters describing reduction (Er = 5.70± -0.06eV and 6.57±-0.24eV for acceptor and donor doped langasite respectively) and oxidation (Eo = 2.18±0.08eV), intrinsic electron-hole generation (Eg [approx. equals] 4.0-4.4eV) and defect ionization (ED-ion = 52±0.06eV for Nb ionization), to be extracted. The predictive defect model was used to calculate the dependence of the partial ionic and electronic conductivities and mass change as functions of temperature, dopant level and pO₂. Given that the magnitudes of conductivity and mass change directly affect the resolution and sensitivity limits of langasite resonators, their predictions allowed for the definition of acceptable operating limits and/or the design of properties for optimum resolution and sensitivity. Two high temperature applications of resonant sensors were studied. Praseodymiumcerium oxide was selected for oxygen partial pressure monitoring and is representative of films which change mass upon absorption or desorption of gaseous species. Barium carbonate film was selected for NO₂ sensing and is representative of films which change mass upon reaction with the gas phase to form a new product phase. Both sensors showed sensitivity to their respective target chemicals and demonstrated the feasibility of high temperature sensor applications. The performance of each sensor was discussed and suggestions for improving sensor performance were presented. / The high temperature transport properties of langasite, La₃Ga₅SiO₁₄, were investigated with special attention focused on their potential impact on the utilization of langasite as a mass sensitive resonant platform for high temperature sensor applications. The electrical properties of acceptor and donor doped langasite were examined at temperatures ranging from 700 to 1000 ⁰C, and pO₂ of 1 to 10-25atm. Acceptor doped langasite was shown to exhibit mixed ionic-electronic conductivity behavior, with predominant ionic conduction due to mobile oxygen vacancies at high pO₂, and n-type electronic conduction due to electrons at low pO₂. Increasing acceptor level resulted in the appearance of p-type hole conduction at high pO₂ and increased ionic conductivity, while the n-type electron conduction was depressed. Donor doped langasite was shown to be electronic at all temperatures and pO₂. The electron mobility of langasite was found to be activated (polaron hopping) with an activation energy of 0.15(±0.01)eV, whereas the holes were assumed to be quasi free carriers. The activation energy for oxygen vacancy migration was estimated to be 0.91(±0.01)eV under dilute solution conditions and 1.27(±0.02)eV for 1% Sr level under concentrated solution conditions. Both values of activation energy of ionic conductivity-temperature product are consistent with activation energy of oxygen self-diffusivity in the respective materials. The electrical properties were related to the underlying defect and transport processes using defect modeling. / by Huankiat Seh. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. 142-147). / Shape memory alloys exhibit interesting and useful properties, such as the shape memory effect and superelasticity. Among the many alloy families that have been shown to exhibit shape memory properties the ones based on copper are interesting because they are relatively inexpensive and show excellent properties when made as single crystals. However, the performance of these alloys is severely compromised by the introduction of grain boundaries, to the point where they are too poor for commercial applications. This thesis studies the mechanical properties of fine Cubased wires with a bamboo microstructure, i.e., where triple junctions are absent and grain boundaries run perpendicular to the wire axis. These microwires are not single crystals, but their microstructure is not as complex as that of polycrystals either: we call this new class of shape memory alloys oligocrystals. This thesis seeks to better understand the relationship between microstructure and properties in these alloys through a combination of mechanical testing, in situ experiments and modeling. First, in situ scanning electron microscopy, together with finite element modeling, is used to understand the role of grain constraint on the martensitic transformation. Grain constraints are observed to be much less severe in oligocrystalline wires as compared to polycrystals. Oligocrystalline microwires are then thermomechanically tested and shown to exhibit excellent properties that approach those of single crystals. Next, property evolution during cycling is investigated, revealing training effects as well as fatigue life and fracture. Finally, size effects in damping and transformation morphology are studied and it is shown that a transition from a many-domain to a single domain martensite morphology takes place when the wire diameter is decreased. / by Stian Melhus Ueland. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999. / Vita. / Includes bibliographical references (p. 129-134). / by Clifford Takashi Tanaka. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1990. / Vita. / Includes bibliographical references (leaves 135-139). / by Carol Lynn Schutte. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references. / Amphiphilic comb-type graft copolymers comprising a poly(methyl methacrylate) (PMMA) backbone and short, polyethylene oxide (PEO) side chains, PMMA-g-PEO, are proposed to self-organize at the polymer/water interface, resulting in quasi-2D confinement of the backbone at the immediate surface. The branched architecture and amphiphilic chemistry of these polymers results in a dense PEO brush that resists cell adhesion. To facilitate specific cell-surface interactions, small biological molecules such as adhesion peptides can be selectively tethered to PEO chain ends. Quasi-2D confinement of the polymer backbone results in clustering of tethered epitopes on a length scale dictated by the backbone. The present work investigates two aspects of this polymer architecture on organization of tethered ligands: nanometer length-scale clustering through backbone 2D confinement, and tether length effects on the availability of tethered peptides for cell adhesion. / (cont.) To directly probe 2D confined polymer conformations, combs at the film/water interface were labeled with gold nanoparticles and observed by transmission electron microscopy. A 2D radius of gyration (Rg) was calculated by reconstructing nanoparticle-decorated chain trajectories, and compared with Monte Carlo simulations of a 2D melt of similarly broad length distribution. The 2D Rg calculated from observed conformations scaled with the number of backbone segments (N) as Rg - N.69-0.02 Monte Carlo simulations yielded a scaling exponent v = 0.67 + 0.03, suggesting that the deviation from classical 2D melt behavior (v= 0.5) arose from polydispersity. Tether length effects on cell adhesion to comb copolymer films functionalized with the adhesion peptide PHSRNGGGK(GGC)GGRGDSPY were further investigated by observing cell attachment and spreading on combs with long (22 EO unit) and short (10 EO unit) tethers. Lofiger tethers increased the rate of spreading and reduced the time required to form focal adhesions. Fluorescence resonance energy transfer (FRET) measurements suggest that the added mobility afforded by longer tethers allowed cells to reorganize tethered peptides. / (cont.) In addition, adhesion peptides were selectively coupled to short or long PEO tethers within a bimodal brush. Short peptide tethers in a bed of long inert chains did not promote cell attachment. Long peptide tethers with short inert chains resulted in cell attachment comparable to a monomodal brush of long chains. These findings may be of value in designing protein-resistant bioactive surfaces, where nanometer length-scale organization of ligands plays an important role in cell-surface interactions. / by William A. Kuhlman. / Ph.D.