TREND Presentations

Synchronization of Chaotically Oscillating TWTs (Presentation)

Katarzyna Oldak, Case Western Reserve University
(Advisors: Prof. James F. Drake, Dr. Marc Swisdak)

Magnetic reconnection occurs throughout the universe, transferring magnetic energy into kinetic energy by energizing particles found in interstellar plasmas. It drives solar flares and is present in the Earth's magnetotail. Some of the energized particles have been detected, but had energies much greater than what existing theories would predict. However, according to a recent theory of contracting islands, already fast electrons can attain very high velocities by bouncing inside contracting magnetic islands during reconnection. This project investigated the fate of the slower electrons within these contracting islands by studying what happens to the initial Maxwellian distribution of electrons.

Entrainment of Weakly Coupled Oscillators by External Driving (Presentation)

Rose Faghih, University of Maryland
John Platig, Wright State University (Advisors: Profs. Edward Ott, Thomas Antonsen, and Michelle Girvan)

The Kuramoto model describes a large population of weakly coupled oscillators whose frequencies are distributed around some average value. This model is relevant to a host of interesting phenomena, from beating of the heart, to synchronization of firefly flashing, to synchronization in Josephson diode circuits. In our work, we have expanded the Kuramoto model to include an external forcing input to the coupled system. We present parameter studies which show the entrainment properties of the expanded model. In doing this, we hope to add insight into the effects of synchronization in cellular clocks in the brain.

Cell Migration on Chemically and Topographically Modified Surfaces (Presentation)

Abby Goldman, Mount Holyoke College
(Advisors: Prof. Wolfgang Losert, Colin McCann, Meghan Driscoll, Rael Kopace)

In living systems, cells maneuver in a complex three-dimensional environment. For instance, white blood cells, when chasing down invaders, adhere to blood vessel walls and penetrate deep into tissue. We explore how cell affinity for the substrate and the topology of the surface affect the cell motion. We utilize a well-characterized motility model organism, the social amoeba D. discoidium. We capture time-lapsed movies of D. discoidium cells as the move on amine-treated glass, amine-treated acrylic, and use image analysis to track changes in cell speed and directionality. To examine the role of small-scale topology changes, we monitored the cell motion over glass slides coated with gold nanoparticles. In addition, we studied the effect of large-scale topographical modifications by monitoring the cell motion along microscale ramps.

Dynamics of Granular Matter under Localized Stress (Presentation)

Rebecca Taft, Yale University
(Advisors: Prof. Wolfgang Losert, Prof. Core O'Hern, Steven Slotterback, Krisztian Ronaszegi, Andrew Pomerance)

To understand granular flows under stress, we performed experiments in which we push a "penetrometer" into a sample of glass beads and track their movement using 3D imaging techniques. Improvements were made to the existing imaging system by automating the data collection and refining the image processing software. To determine the influence of sample preparation on material failure, we compressed the samples perpendicular and parallel to gravitational loading before inserting the penetrometer. We also performed frictional molecular dynamics (MD) simulations of penetrometer insertion using experiment particle coordinates as initial conditions to predict the failure regions. To account for experimental uncertainty, we first adjusted the positions and radii using a geometric algorithm we designed to bring the system near mechanical equilibrium.

Electrodeposition of Copper with an Alternating Electric Field (Presentation)

Nicholas LeCompte, Worcester Polytechnic Institute
(Advisor: Prof. Daniel Lathrop)

In electrodeposition, two metal electrodes immersed in a salt of that metal have a potential difference applied between them. If this difference is sufficiently high, the anode becomes ionized. This causes a deposition of ions on the cathode. An aggregate of these ions can exhibit diverse morphologies, ranging from fractal tree-like structures to dense, homogeneous clumps. We investigate the effect of an alternating electric field on the morphology of the aggregate, usint copper electrodes in a cupric sulfate solution. The parameters we vary are the frequency and the ratio of AC to DC voltage, and we look at two-dimensional and three-dimensional systems. Quantities such as fractal dimension, growth velocity, tip-splitting frequency, and dendrite thickness are measured, and qualitative observations on the morphologies are made. We also test a hypothesis which predicts that a characteristic length scale of the aggregate is proportional to the inverse square root of the frequency. In addition, chemical properties of the electrocell are examined indirectly from qualities of the aggregate such as color and abrupt morphological transitions.

Diffusion: The Key in Understanding Why Photocathodes Die (Presentation)

Zhigang Pan, University of Maryland
(Advisors: Prof. Patrick O'Shea, Eric J. Montgomery, Kevin L. Jensen)

Robust and highly efficient laser-driven photocathodes are an important source of bright electron beams. Photocathodes are coated with a partial monolayer of cesium to increase quantum efficiency (i.e., the number of electrons per photon). During operation, cesium atoms come up through pores onto the cathode surface, diffuse, and undergo desorption. Having a theoretical model to predict evolution of the cesium surface density is of great interest in furthering the development of high quality photocathodes. Currently, I am developing a model of cesium diffusion through a porous surface. This model would be useful in designing experiments and optimizing photocathode quantum efficiency and lifetime.

Chaotically Oscillating Gate Networks (Presentation)

Sereres Johnston, Andrews University
(Advisors: Prof. Daniel Lathrop and Dr. John Rodgers)

Random Boolean Networks (RBNs) can model complex systems such as neural pathways and gene expression. Usually RBN studies simulate multi-node networks of idealized logic gates, but this TREND project focused on networks of five or fewer 74AC04 CMOS inverters. System parameters such as the power supply voltage, circuit time delay and loop gain were adjusted. Later in the project, AC current with a DC bias was used. Reduced loop gain widened the range in which chaos was observed while combining time delay with gain reduction resulted in intermittency. Self-synchronization through weak external coupling was observed for sets of free-running devices. The networks' sensitivity and their self-synchronizing capability indicate that they or similar systems may be useful as intrusion detectors.

Numerical Model Simulations of a Simple Sensor Network of Time-Delayed Cross-Coupled Semiconductor Lasers (Presentation)

Mark Patrick, University of Maryland
(Advisors: Prof. Rajarshi Roy, Prof. Thomas E. Murphy, Adam Cohen, Bhargava Ravoori, Caitlin Williams)

We examine numerical models of a system of two opto-electronically cross-coupled semiconductor lasers. We implemented a Runge-Kutta integration method to analyze the deviations from the steady state intensities with a band-pass filter in the feedback loop. Through comparison with experiment, we made our model more accurate. We explored the dependence of the dynamics on the coupling strength between the two lasers, as well as the coupling time delays in both the symmetric and asymmetric cases. Numerically, we observed the fast relaxation oscillations of the laser, oscillations on the order of the total delay time, and an unexpected slower bursting behavior that was similar to experiment.

Dynamics of Cross-Coupled Semiconductor Lasers with Time Delays: Experimental Observations (Presentation)

Caitlin Williams, Grove City College
(Advisors: Prof. Rajarshi Roy, Prof. Thomas E. Murphy, Adam Cohen, Bhargava Ravoori, Mark Patrick)

A system of two semiconductor lasers cross-coupled opto-electronically with time delays is a simple prototype element of a sensor network. In this system periodic intensity oscillations are present for coupling strengths above a threshold level. The goal of this project was to understand the dynamics of this system with asymmetric time delays over a wide range of coupling strengths. the laser intensities displayed periodic behavior on two different time scales: one on the odrder of nanoseconds and a second longer time scale of microseconds. Their periods and phases were analyzed using FFT, Hilbrert transforms, and cross-correlation functions.