Bulletin of the American Physical Society
81st Annual Meeting of the APS Southeastern Section
Volume 59, Number 18
Wednesday–Saturday, November 12–15, 2014; Columbia, South Carolina
Session DD: Statistical and Non Linear Physics |
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Chair: Michel Pleimling, Virginia Tech University Room: Richland II |
Thursday, November 13, 2014 1:15PM - 1:27PM |
DD.00001: Transition between different search patterns in human online search behavior Xiangwen Wang, Michel Pleimling We investigate the human online search behavior by analyzing data sets from different search engines. Based on the comparison of the results from several click-through data-sets collected in different years, we observe a transition of the search pattern from a L\'{e}vy-flight-like behavior to a Brownian-motion-type behavior as the search engine algorithms improve. This result is consistent with findings in animal foraging processes. A more detailed analysis shows that the human search patterns are more complex than simple L\'{e}vy flights or Brownian motions. Notable differences between the behaviors of different individuals can be observed in many quantities. [Preview Abstract] |
Thursday, November 13, 2014 1:27PM - 1:39PM |
DD.00002: Effect of field quenches on the relaxation dynamics of vortex lines in disordered type-II superconductors Hiba Assi, Ulrich Dobramysl, Michel Pleimling, Uwe T\"{a}uber Understanding the dynamics of vortex matter in disordered type-II superconductors is vital for material optimization in technological applications. We model the vortices in the London limit as interacting elastic Lines subject to point-like or extended pinning centers, and employ a Langevin molecular dynamics algorithm to simulate their dynamics. We consider experimentally-motivated initial conditions and analyze the effects of sudden magnetic field changes on the non-equilibrium relaxation dynamics of this system. A magnetic field quench is accomplished by instantaneous addition or removal of vortices from the sample. We disentangle the effects of the competing repulsive vortex interactions and pinning to sample defects, and compare the system's relaxation properties in the presence of either randomly-placed point disorder or correlated columnar defects by investigating the two-time mean square displacement and height-height autocorrelation function, the mean radius of gyration, and the fraction of pinned line elements. [Preview Abstract] |
Thursday, November 13, 2014 1:39PM - 1:51PM |
DD.00003: Effect of current quenches on the dynamics of magnetic flux lines in type-II superconductors Harsh Chaturvedi, Hiba Assi, Ulrich Dobramysl, Michel Pleimling, Uwe T\"auber The mixed phase in type-II superconductors is characterized by the presence of mutually repulsive magnetic flux lines with normal cores. While external currents drive these vortices via the Lorentz force, material defects (point-like or extended) in the superconducting material act as pinning centers for them. We describe the disordered vortex system with an elastic line model, whose dynamics we investigate numerically by means of Langevin Molecular Dynamics. We specifically study the effects of sudden changes of the driving current on the time evolution of the mean flux line gyration radius and the associated transverse displacement correlation functions. Within the moving phase, we obtain fast exponential relaxation to a new non-equilibrium stationary state. Upon quenching from the moving into the pinned glassy phase, we observe algebraically slow relaxation kinetics, with the associated breaking of time translation invariance and indications of aging scaling behavior. [Preview Abstract] |
Thursday, November 13, 2014 1:51PM - 2:03PM |
DD.00004: Effect of Particle Shape on Stress-Strain Measurements of Granular Materials Sara Berry, Ted Brzinski, Karen Daniels Granular materials are composed of many discrete athermal particles, which exhibit liquid like properties below some critical packing fraction and solid like properties, such as yield stress, above. Physics research has generally considered granular materials composed of spherical grains. Most natural grains are not spherical, but rather odd shapes that often break symmetries and exhibit various contact modes. Therefore, in addition to circles, we study quasi-2D elliptical, pentagonal, and star-shaped particles. This set of particles were chosen to break rotational symmetry, introduces point-face and face-face contacts, and even interlock. We measure the yield stress as a function of packing fraction for each type of grains. Our experiment consists of 820 particles floating on an air table, compressed bi-axially. Utilizing force sensors along the walls and an overhead camera, we are able to determine the bulk pressure and the packing fraction. We find that the critical packing fraction is shape dependent. However, this shape dependence can be scaled out of the pressure versus packing fraction relation. [Preview Abstract] |
Thursday, November 13, 2014 2:03PM - 2:15PM |
DD.00005: Granular Flow into Model Grain Silos Eli Owens, James Satterfield It has long been observed that the pressure at the bottom of a granular container, for instance a grain silo, saturates as the height of the container increases relative to its width. However, the precise effect grain shape has on the buildup of sidewall pressure is not well understood. Using a model silo, we investigated the influence of grain shape on sidewall pressure during the filling process. Our silo is 125 cm tall and 16 cm in diameter and is filled with either corn or peas via a cone shaped hopper. This hopper has a lower opening 4 cm in diameter and fills the silo at a constant rate. As the silo fills, we monitor the pressure the grains exert on a 7 cm$^2$ section of the wall. We see that the corn and the peas behave differently. When using the corn, the pressure reaches a peak value and then decays with time. We attribute this decay to the irregular shape of the corn which causes grains higher in the silo to jam and shield the lower grains from the weight above. However, this decay is not as pronounced when using the peas. Since the peas are round, they can more easily rearrange than the irregular corn particles and are not as effective at screening the pressure. [Preview Abstract] |
Thursday, November 13, 2014 2:15PM - 2:27PM |
DD.00006: Comments on a Particular Class of Generalized Trigonometric Functions 'Kale Oyedeji, Ronald E. Mickens The standard trigonometric functions, cos(x) and sin(x), satisfy the relation (sinx)$^2 +$(cosx)$^2=$1. The pair of generalized trigonometric functions, cos(p,q,x) and sin(p,q,x), are defined such that they satisfy the condition \begin{equation} [\sin(p,q,x)]^2 + [\cos(p,q,x)]^2 = 1, \end{equation} with the requirements \begin{equation} \cos(p,q,x) = -(d/(dx))\sin(p,q,x), \end{equation} \begin{equation} p = ((2n)/(2m + 1)) q = ((2k)/(2l + 1)) \end{equation} where (n,m,k,l) are non-negative integers and, m\textless n and l\textless k. These functions are a subclass of those presented in the book by J. Lang and D. Edmunds (Eigenvalues, Embeddings and Generalized Trigonometric Functions, Springer 2011). We discuss several of the mathematical properties of these functions and indicate their relevance for determining solutions to the systems modeled by nonlinear oscillators. Note that while the solutions and their first-derivatives exist, second- and higher-order derivatives may become singular for particular values of x. [Preview Abstract] |
Thursday, November 13, 2014 2:27PM - 2:39PM |
DD.00007: Evolution and Behavior of Aqueous Foam Stabilized by Polycationic Amphiphilic Surfactants Seth Heerschap Aqueous foams are close packing of gas bubbles stabilized by surface active molecules. They exhibit rich topology and rheological behavior mostly attributed to their complex structure. In this project we study the impact of the molecular structure of the surfactants on foam stability. We compare the coarsening rate (bubble growth rate) of foam produced with five different amphilphilic surfactant molecules: Single headed, single tailed anionic and cationic surfactants (SDS, CTAB and DTAB) and triple cationic headed and double tailed surfactants (M-1,14,14 and M-P,14,14). In addition molecules with different hydrocarbon tail lengths (CTAB and DTAB) were tested and their rupture and coarsening rates were measured. Results show significant difference in coarsening and rupture rate between amphiphilic surfactants of different tail lengths. Differences due to surfactant head/tail structure appear to be much more subtle and inconclusive. [Preview Abstract] |
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