Physics Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2800
Browse
Search Results
Item Individual and collective dynamics of chemotaxing cells(2011) McCann, Colin Patrick; Losert, Wolfgang; Parent, Carole A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The study of the dynamics of interacting self-propelled entities is a growing area of physics research. This dissertation investigates individual and collective motion of the eukaryote Dictyostelium discoideum, a system amenable to signal manipulation, mathematical modeling, and quantitative analysis. In the wild, Dictyostelium survive adverse conditions through collective behaviors caused by secreting and responding to chemical signals. We explore this collective behavior on size scales ranging from subcellular biochemistry up to dynamics of thousands of communicating cells. To study how individual cells respond to multiple signals, we perform stability analysis on a previously-developed computational model of signal sensing. Polarized cells are linearly stable to perturbations, with a least stable region at about 60 degrees off the polarization axis. This finding is confirmed through simulations of the model response to additional chemical signals. The off-axis sensitivity suggests a mechanism for previously observed zig-zag motion of real cells randomly migrating or chemotaxing in a linear gradient. Moving up in scale, we experimentally investigate the rules of cell motion and interaction in the context of thousands of cells. Migrating Dictyostelium discoideum cells communicate by sensing and secreting directional signals, and we find that this process leads to an initial signal having an increased spatial range of an order of magnitude. While this process steers cells, measurements indicate that intrinsic cell motility remains unaffected. Additionally, migration of individual cells is unaffected by changing cell-surface adhesion energy by nine orders of magnitude, showing that individual motility is a robust process. In contrast, we find that collective dynamics depend on cell-surface adhesion, with greater adhesion causing cells to form smaller collective structures. Overall, this work suggests that the underlying migration ability of individual Dictyostelium cells operates largely independent of environmental conditions. Our gradient-sensing model shows that polarized cells are stable to small perturbations, and our experiments demonstrate that the motility apparatus is robust to considerable changes in cell-surface adhesion or complex signaling fields. However, we find that environmental factors can dramatically affect the collective behavior of cells, emphasizing that the laws governing cell-cell interaction can change migration patterns without altering intrinsic cell motility.Item Modeling and simulation of organic molecular clusters and overlayers on solid surfaces(2011) Liu, Qiang; Weeks, John D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Driven by the rapid development of experimental methods and technology, nano scale physics and chemistry has become more and more important and practicable to study. Monolayers of organic molecules have been studied a lot recently because of many potential applications, such as organic photovoltaic devices (OPV) or organic Liquid Electric Diodes (OLED). It is important to understand and interpret these new experimental advances. At molecular scales, Monte Carlo (MC) simulations and molecular dynamics (MD) are two important methods in computational chemistry and materials science. This dissertation will use these simulation methods along with statistical mechanical theory to study the behavior of single monolayers of organic molecules on solid surfaces. First we give a brief introduction to two dimensional molecular systems. Different from bulk system or single molecules, 2D systems have many unique properties, and attract much experimental and theoretical research attention. Some common methods in experimental and theoretical studies are reviewed. After introducing the properties and experimental results of ACA/Ag(111), we build a lattice gas model and run Monte Carlo simulations to help interpret the experiments. The Pair approximation, a generalization of mean-field theory, is used to calculate the global phase diagrams and put our model into the more general class of spin-1 Ising models. The pair approximation can be used for modeling various monolayer organic molecular systems which correspond to different regions of the parameter space. Then we studied the C60/ZnPc/Ag(111) system, using molecular dynamic simulations. The C60 molecules form unusual chain structures instead of the close packed islands seen on metal surfaces, and we try to provide a theoretical explanation. Finally we use a density functional theory software to calculate the electronic structures of the C60/ZnPc/Ag(111) systems. This calculation predicts a 0.4e charge transfer from substrate to C60 molecule, which we believe is important for the C60 interactions on these surfaces. In general this thesis studies the behavior of organic monolayers and bilayers on metal substrates. This basic work could help us to understand general 2-D system dynamics and electronic properties, and may help us to find new interesting systems with special properties and applications.