Student Honors Projects

The Student Honors Projects collection represent exemplary work in physics at Illinois Wesleyan University. The Ames Library is proud to archive these and other honors projects in Digital Commons @ IWU, the University's online archive of student, faculty and staff scholarship and creative activity.

Novel Swelling Structures and Electromotility Response in Polyelectrolyte Gels
by Dana Deardorff

Electromotility, the bending in response to an electric field, of polyelectrolyte gels in ionic solutions has been identified as a candidate for a potential chemomechanical engines such as muscles. We discovered that the underlying physics of these systems is more complex than previously believed. We found that the bending as a function of time obeys a square root power law. This points strongly towards a diffusion mechanism for the bending. Kinetic evidence for diffusion was independently corroborated by experiments on gels grown or bent in the presence of dyes. We explored the effects of varying poly-ion concentration in the backbone of the polymer and in the surrounding medium. In some cases, the electromotility cannot be described as simple bending.

Deflection of an Electron Beam by Photons
by Danning W. Bloom '64

The purpose of this paper was to review information, both experimental and theoretical, concerning the momentum carried by light and its effect on free electrons.

The Vibrational Behavior of a Cured Carbon Fiber Disk and a Tennis Racket
by Nick Timme '08

In this project the vibrational behavior of a circular cured carbon fiber plate and a tennis racket is examined using a speckle-pattern interferometry system built and designed by students at Illinois Wesleyan University. Specifically, the mode shapes and mode frequencies are presented and discussed. With regards to the carbon fiber plate, the effects of the orthogonal construction of the plate on the vibrational behavior are studied. With regards to the tennis racket, the mode shapes of the racket are imaged for the first time using speckle-pattern interferometry. Furthermore, the effects of commercially available vibration dampers on the vibrational behavior of a tennis racket is also presented. In addition, the challenges of imaging various objects using speckle-pattern interferometry are explained, along with several methods for overcoming these challenges.

Visualizing a Fourth Dimension: Hypercubic Resistor Networks
by Andrew J. Nelson '08

A booming field in physics research today is the search for extra dimensions. This is something that has been thought about and discussed in both the scientific and non-scientific world for a long time. Many physicists are currently attempting to answer the question: "is our world really four dimensional?" The purpose of this research, however, is not to answer that question. The purpose of this work is to help reveal four-dimensional artifacts in our perceived three-dimensional world in order to help a student, even a non-physicist, to understand and visualize how the extra spatial dimensionality, if present, might reveal itself in measurements. To that end, models of non-trivial four-dimensional objects have been constructed that have consequences large enough to be easily measured and understood in an intuitive fashion. In building and analyzing data from two, three, and four-dimensional model systems with non-trivial interactions, large and conceptually transparent consequences of extra spatial dimensions have been discovered.

On the Mechanism of Giant Electromotility in Polyelectrolyte Gels
by Kimberly Ann Branshaw '95

Electromotility, i.e. bending in response to an electric field, of polyelectrolyte gels in ionic solutions has recently been investigated at a few leading academic and industrial labs as a potential chemomechanical engine. We have discovered that the underlying physics in these systems is more complex than previously believed. We have found that the bending, which seems to obey a {t power law, is inconsistent with the simple idea of a bending speed, but is consistent with a diffusion mechanism. Evidence of diffusion was independently provided by experiments on gels grown or bent in the presence of dyes. We have explored the effect of varying poly-ion concentration in the backbone and in the surrounding medium. We have discovered that in some cases, the electromotility cannot be described as simple bending.

A Lattice Gas Approach to the Structure and Dynamics of Electrorheological Fluids
by Jie Chen '93

Electrorheological fluids consist of a colloidal suspension of dielectric particles in a continuous fluid of smaller dielectric constant. Molecular dynamics simulations of these fluids in an applied electric field have recently been shown to produce percolated, columnar structures. No systematic attempt has been made so far to simultaneously include the effects of temperature and the viscous drag due to the continuous fluid. We propose a dipolar lattice gas model for electrorheological fluids and study the resulting structures and dynamics. We attempt to incorporate the effect of the viscosity of the continuous medium by a dynamic ansatz that determines the range over which individual particles can jump in a single simulation event. The temperature is simulated by assigning a probability of jumping to higher energy states in accordance with the Boltzman distribution. We study the equilibrium phases of the system as a function of temperature and find interesting new results. Our new results from finite temperature simulations suggest that there is a gradual phase transformation from a liquid like phase at low electric field or equivalently at high temperature to a solid like phase at high electric field or at low temperature. The simplicity obtained by going to a lattice version will allow us to extend out these simulations even in three dimensions, where little is known about these systems.

Holographic Optical Tweezers: Development and Analysis of the First Holodeck Prototype
by Matthew T. Dearing '00

Tightly focused light can be used to non-invasively trap and manipulate micro-objects, a technique called "optical tweezing." By utilizing the large field gradients present in a focused laser beam, micro-particles-including biological specimens and many other materials-can become confined in all three dimensions. While optical tweezing has existed for over a decade, it has generally been limited to trapping one or two particles at a time. We have developed a technique that uses laser light to assemble large numbers of micro-particles in a highly controllable way. Here we describe, for the first time, the complete implementation of holographic optical tweezer arrays ("HOT" arrays), which offer a new means of simultaneously directing the assembly of particles into any configuration.

Through calculation, and subsequent fabrication of, holographic optical devices, we can sculpt a single laser beam into a fully-configurable array of optical tweezers. Each spot in such an array is then capable of trapping and manipulating one particle, making possible simultaneous control over large collections of micro-objects. Our addition of holographic techniques has extended the basic capabilities of optical tweezing, making it a more viable tool for the assembly of nanodevices and the organization of specimens into user-defined structures.

Previously, a generalized Lorentz-Mie scattering theory has been used to model single (non-holographic) optical traps. Here, we develop a simpler and more intuitive approach to examine the trapping potential as a function of particle size, the polarizability of the particle material as compared to that of the surrounding medium, the power of the laser used to trap the particles, and the angular divergence of the optics used for promoting assembly. For this calculation we incorporate an approximate form for the energy density of the laser beam-one that is appropriate both within and outside of the Rayleigh limit. We believe that our conclusions remain viable in the intermediate case, where the particles to be trapped have dimensions on the order of the wavelength of visible light; this regime is of particular interest in applications involving assembly of photonic bandgap materials and other photonically-active structures. Notably, we are the first to address the key question regarding application of holographic optical tweezer arrays, namely the number of particles that can be simultaneously incorporated and manipulated.

There are many potential applications for such techniques; e.g., allowing for the construction of aggregations with tailor-made crystalline symmetries. Defects may be introduced in a controlled way allowing exploration of their role in phase transitions. Even biological specimens could be organized into useful configurations for studying how they behave in large, organized collections. In addition, there is growing interest in electronic devices, which exploit the confinement of electrons onto isolated nanoparticles. The application of our techniques might increase the yield during fabrication of these devices.

Design and Construction of a Radio Frequency Plasma Device
by Matthew Highland '02

We have constructed a radio frequency plasma device to study a wide range of phenomena including, power coupling between the plasma and the antenna and wave propagation. Our system includes a high vacuum chamber with mechanical and diffusion pumps, a radio frequency source magnetic field coils and a matching network.

X-Ray Spectroscopic Mapping of Three Unusual Active Galaxies (NGC 4258, NGC 1097, and NGC 1068)
by Jeremy Kotter '98

One enigmatic class of objects whose structures are interesting and only recently explored are active galactic nuclei (AGN). These are galaxies in which massive black holes sit at the center and accrete matter. The term "active" refers to energetic processes which are not directly attributable to stars and which occur in the innermost portions of galaxies. Astrophysicists have developed general descriptions of AGN, but details about these objects remain incomplete. Notably, the thermal and ionization structures of AGN accretion disks and the geometries of the circum-source clouds which surround the black hole and comprise an important portion of the energy emitting core, as well as the importance of thermal stability to the emission of radiation, is unclear at this time. Therefore, along with my research advisor, Dr. Cynthia Hess, I have studied the unusual active galaxies NGC 4258, NGC 1097, and NGC 1068 in an attempt to shed light upon the morphologies oftheir central regions.

Phase Transitions Occurring in Models of Neighborhood Racial Segregation
by Alexander J. Laurie '03

This thesis is organized as two chapters whose contents are closely related yet quite distinct. The first chapter presents a paper "Role of 'Vision' in Neighborhood Racial Segregation: A Variant of the Schelling Segregation Model," authored by myself and Dr. Jaggi, which has been accepted for publication by the journal Urban Studies and is currently in press (as of April 2003). This chapter introduces the well-known Schelling model of neighborhood segregation, outlines the sociopolitical motivation for our work, and presents the key results that we believe are of interest to social scientists. Chapter two, which ought to be of greater interest to the physics community, presents the results of our investigations into the parallels between the Schelling model and critical phenomena.

Our primary extension of the Schelling model was to include social agents who can authentically 'see' their neighbors up to a distance R, called 'vision'. By exploring the consequences of systematically varying R, we have developed an understanding of how vision interacts with racial preferences and minority concentrations and leads to novel, complex segregation behavior. We have discovered three regimes: an unstable regime, where societies invariably segregate; a stable regime, where integrated societies remain stable; and an intermediate regime where a complex behavior is observed.

Since the primary audience of Urban Studies consists of sociologists and economists, we have not elaborated in the first chapter upon the phase transition which was strongly suggested by the "complex behavior" in the intermediate regime. The purpose of chapter two then, is to elucidate these additional physically interesting aspects of our model. Melting is a textbook example of first order (discontinuous) phase transitions. These are marked by two central features: a sharp temperature at which the transition occurs, and the coexistence of the two phases at that melting point. One can study the first-order phase transition that ice undergoes when melting into water by observing the ice while continuously raising its temperature. However, if you were only able to view the system at certain discrete temperatures, you would only see a either a piece of ice or a puddle of water during each observation. Thus in order to study the potential phase transition occurring in our model, we must be able to control the governing parameters continuously. However, in our original 'discrete' model, R measures how far an agent sees from its own home as an integer number of houses. Since we can only assign discrete values to R, it is meaningless to speak of a phase transition occurring as a function of this variable.

To overcome the limitations of our first model, we introduce a continuous model in chapter two where the range of vision (denoted R2 for notational clarity) can be varied continuously. This model uses a utility function that assigns greater weight to neighbors nearer an evaluating agent. The function used to model this decrease in utility contribution with distance is an exponentially decaying curve. We control the steepness of this curve (and thereby control the agents' vision) using R2. Since R2 can be set to equal any positive real number, we can indeed study the possible phase transition in our simulations' behavior as the function of a continuous variable.

Additionally, the continuous model demonstrates the robustness of the sociologically relevant conclusions drawn in chapter one. Our continuous model, a generalization of a model developed by Wasserman and Yohe (2001), is in fact more realistic than our first model. In particular, we were pleased to discover the same three behavioral regimes and all associated trends in both our discrete model and our continuous model. This confirms that our original results were robust and not merely algorithmic artifacts related to the specific treatment of vision used in our discrete model.