Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L3: Novel Phenomena in Granular Systems with Complex Interactions |
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Sponsoring Units: DCMP Chair: Igor Aronson, Argonne National Laboratory Room: Colorado Convention Center Korbel 2A-3A |
Tuesday, March 6, 2007 2:30PM - 3:06PM |
L3.00001: Thresholds and Dynamics for Oscillating Granular Layers Invited Speaker: The onset and dynamics of flow in shallow horizontally oscillating granular layers are studied and compared to the behavior of avalanches. The variation with depth of the starting acceleration for the oscillating layer matches (approximately) the corresponding variation of the tangent of the starting angle for avalanches in the same container at low frequencies, but deviates as the frequency is increased. However, the threshold behavior depends significantly on the measurement protocol. Just above threshold, the motion decays with time as the material re-organizes over a minute or so, causing the apparent threshold to increase. Once excited, the rheology of the material is found to vary in time during the cycle in surprising ways. If the maximum inertial force (proportional to the container acceleration amplitude) is slightly higher than that required to produce flow, the flow velocity grows as soon as the inertial force exceeds zero in each cycle, but jamming occurs long before the inertial force returns to zero. At higher acceleration, the motion is fluid- like over the entire cycle. However, the fraction of the cycle during which the layer is mobile is typically far higher than what one would predict from static considerations or the behavior of the inclined layer. Finally, we consider the flow profiles as a function of both the transverse distance across the cell at the free surface, and also as a function of the vertical coordinate in the boundary layer near the sidewall. These profiles have time-dependent shapes, and are therefore significantly different from profiles previously measured for avalanche flows. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:42PM |
L3.00002: Particle Shape and Dynamics of Granular Matter: Swarming to Swirling Invited Speaker: We will discuss a series of experiments performed with granular rods, dimers, and flexible chains on a vibrated plate to illustrate the effect of particle shape on self-organization. A non-spherical shape is shown to lead to not only states which resemble nematic and smectic phases but also causes novel dynamics [1]. The ratchet mechanism which leads to vortex motion in a collection of rods on a vibrated plate and drift motion in a bouncing dimer will be discussed [2, 3]. The friction at the point of contact between particle and the substrate, and the coupling about the center of mass of a non-spherical is proposed to lead to observed motion. Exploiting this mechanism we construct mechanical self-propelled particles (SPP) using rods with asymmetric mass distributions. We then investigate the SSP number fluctuations, flow fields, and orientation order inside a container as a function of number density and excitation, and compare their statistics with recent models of active nematic particles and living cells.\\ 1. ``Vortices in vibrated granular rods," D.L. Blair, T. Neicu, and A. Kudrolli, Phys. Rev. E 67, 031303 (2003).\\ 2. ``Anisotropy driven dynamics in vibrated granular rods," D. Volfson, A. Kudrolli, and L.S. Tsimring, Phys. Rev. E 70, 051312 (2004).\\ 3. ``Dynamics of a bouncing dimer," S. Dorbolo, D. Volfson, L. Tsimring, and A. Kudrolli, Phys. Rev. Lett. 95, 044101 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 4:18PM |
L3.00003: Externally driven magnetic granular layers at a liquid/air interface: self-organization, flows and magnetic order Invited Speaker: Collective dynamics and pattern formation in ensembles of magnetic microparticles suspended at the liquid/air interface and subjected to an alternating magnetic field are studied. Experiments reveal a new type of nontrivially ordered dynamic self-assembled structures (``snakes'') emerging in such systems in a certain range of field magnitudes and frequencies. These remarkable structures are directly related to surface waves in the liquid generated by the collective response of magnetic microparticles to the alternating magnetic field. In addition, a large-scale vortex flows are induced in the vicinity of the dynamic structures. Some features of the self-localized snake structures can be understood in the framework of an amplitude equation for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density. Self-assembled snakes have a complex magnetic order: the segments of the snake exhibit long-range antiferromagnetic ordering mediated by the surface wave, while each segment is composed of ferromagnetically aligned chains of microparticles. A phenomenological model describing magnetic behavior of the magnetic snakes in external magnetic fields is proposed. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:54PM |
L3.00004: Nonlinear dynamics of semiflexible magnetic filaments in an external ac magnetic field Invited Speaker: Chains of magnetic particles exist in nature (magnetotactic bacteria, magnetic colloids) and can be created artificially by linking magnetic particles with some polymer (PAA,DNA). Theoretical description of magnetic filaments is based on models of semiflexible polymers extended by incorporation of the effects of body torques due to long-range magnetic interactions. On the basis of these models different phenomena are described - buckling due to body torques, self-propulsion in an AC field, tumbling in the shear flow, orientation of ferromagnetic filaments in the direction perpendicular to an AC field, liquid flow excited by oscillating in an AC field tips of magnetic filaments floating on the surface of the liquid and others. Connection of equilibrium shapes of magnetic filaments with solutions of elastica problem is established. Different regimes of magnetic response of the suspension of magnetic filaments are analyzed by taking into account the thermal noise. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:30PM |
L3.00005: Nematic Ordering in a Population of Growing and Dividing Rod-like Cells Invited Speaker: Morphogenesis is one of the most important themes in biology, and it is also central to nonequilibrium physics. The fundamental issue is to understand how local interactions of elementary components lead to collective behavior and the formation of a highly organized system. In nature this self-organization is found on many different scales, from single cells to schools of fish and herds of animals. Collective behavior leads to significant selective advantages for living organisms. At low density, communication among cells occurs mainly due to chemotaxis, the mechanical response of cell to the gradients of chemicals emitted by other cells. At higher densities, steric exclusion effects may strongly affect their collective behavior. In this work we focus on the mechanical interaction among non-motile bacteria in engineered biofilms. These biofilms are formed by growing two-dimensional bacterial colonies in a highly controlled microfluidic environment. We combine experimental observations and analysis with discrete-element molecular dynamics simulations and theoretical modeling to provide mesoscopic description of the biofilm growth. Our results reveal how cell growth and colony expansion trigger the formation of the orientational (nematic) order in the biofilms. [Preview Abstract] |
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