Eric Sembrat's Test Bonanza

The physics of granular flow is of widespread practical and fundamental interest, and is also important in geology and astrophysics. One challenge to understanding and controlling behavior is that the mechanical response is nonlinear, with a forcing threshold below which the medium is static and above which it flows freely. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems brought close to jamming, where intermediate-time motion is correlated in the form of intermitted string-like swirls. This will be briefly reviewed in the context of glassy liquids and colloids, and more deeply illustrated with experiments on three different granular systems. This includes air-fluidized beads, where jamming is approached by density and airspeed; granular heap flow, where jamming is approached by depth from the free surface; and dense suspensions of NIPA beads, where jamming is approached by both density and shear rate. Emphasis will be given to measurement and analysis methods for quantifying heterogeneities, as well as the scaling of the size of heterogeneities with distance to jamming -- which we show to have have universal form for all three experimental systems.

Many organisms fly in order to survive and reproduce. I am fascinated by the mechanics of flying birds, insects, and autorotating seeds. Their development as an individual and their evolution as a species are shaped by the physical interaction between organism and surrounding air. It is critical that the organism’s architecture is tuned for propelling itself and controlling its motion. Flying macroscopic animals and plants maximize performance by generating and manipulating vortices. These vortices are created close to the body as it is driven by the action of muscles or gravity, then are ‘shed’ to form a wake (a trackway left behind in the fluid). I study how the organism’s architecture is tuned to utilize the fluid dynamics of vortices. Here I link the aerodynamics of insect wings to that of bat, maple seed and bird wings. The methods used to study all these flows range from robot fly models to maple seeds flying in a vertical wind tunnel to freeze dried swift wings tested in a low turbulence wind tunnel. The study reveals that animals and plants have converged upon the same solution for generating high lift: a leading edge vortex that runs parallel to the leading edge of the wing, which it sucks upward. Why this vortex remains stably attached to flapping animal and spinning plant wings is elucidated and linked to kinematics and wing morphology. While wing morphology is quite rigid in insects and maple seeds, it is extremely fluid in birds. Here I show how such ‘wing morphing’ significantly expands the performance envelope of birds during both gliding and flapping flight. Finally I will show how these findings have inspired the design of new flapping and morphing micro air vehicles.

 lentinklab.stanford.edu

Nature and technology abound with fluid interfaces such as the surfaces of oil droplets in water or the membrane surfaces of living cells.  These interfaces are typically crowded with adsorbed particles, proteins or other large molecules, which are effectively confined to a two-dimensional fluid.  This two-dimensional system, though, has a twist: it can spontaneously change its curvature and thereby substantially alter the interactions among the bound particles or proteins.  In biology, there are many examples where proteins change the shape of a membrane – a key part of a cell’s ability to exchange materials with its exterior (via endocytosis).  Despite the many known examples, there remain quite basic questions about how proteins and membrane curvature work together.   In this talk, I will describe our experiments with a family of membrane-binding proteins known as BAR, which have a strong affinity for highly curved membranes.  BAR proteins are shaped like a banana, which suggests a geometric mechanism for altering membrane shape – but in fact the mechanism remains controversial.  By measuring the binding affinity of BAR as a function of mechanical tension applied to the membrane, we aim to derive new insights into how the BAR protein and its soft, two-dimensional substrate work together.

PLEASE NOTE: This is a WEBINAR

A problem of practical interest in control theory is to stabilize a nonlinear dynamical system through the action of a feedback control.  The stabilization problem can be embedded as an optimal control problem leading one to solve a Hamilton-Jacobi (HJ) PDE.  The associated HJ equation is a first order nonlinear singular PDE and, under suitable conditions, can be solved locally using power series methods.  In this talk, I will present a numerical method that extends the domain of validity of the power series approach.  The method relies on patchy vector field techniques, level set methods, and a Cauchy-Kowalevski continuation algorithm.  The method will be illustrated on 2D-3D control systems.

PLEASE NOTE: This is a WEBINAR

Accurate modeling of blood flow provides insights into arterial stent design, surgical planning, and analysis of stroke risk. Unfortunately, fully detailed modeling of the cardiovascular system is computationally impossible due to the enormous number of blood vessels in the body.  Instead, a common technique is to choose a small subset of arteries to model in detail while accounting for the "un-modeled" parts of the cardiovascular system through boundary conditions.  A popular tactic for deriving such a boundary condition is to analytically solve a simple blood flow model in an idealized self-similar tree of arteries.  This technique, termed the "structured tree" boundary condition, is computationally cheap but is not broadly applicable since it assumes temporal periodicity.  We have developed a generalized version of the structured tree condition that lacks this restriction and is applicable to general transient flow regimes.  We will discuss the derivation of this condition and its nontrivial numerical implementation.  Additionally, computational results and a comparison to the original structured tree condition for periodic problems will be provided.

Despite their natural abundance and wide industrial applications, such as red blood cells and clay, disks are the least studied colloidal systems compared to geometries like spheres and rods. We have established methods to fabricate and control the size, aspect ratio, and polydispersity of disks and systematically investigate their effects on discotic liquid crystal phase transitions. This talk will focus on surface controlled shape design of discotic microparticles using phase changing emulsions and the observation of the discotic smectic phase using nanoplates with identical thickness. Recent results such as discotic liquid crystals emulsions, gelation via ionic exchange, depletion attraction induced liquid crystal phase transition, iridescence from layered structure, and nematic hydrogels will briefly presented. Comprehensive understanding of the colloidal discotics in terms of complex fluids behaviors and liquid crystal transitions will help establish theory for model atomic liquid crystals and develop industrial applications.

Biography

Professor Cheng obtained his PhD degree from the Physics Department of Princeton University in 1999. He has his MS degree from the Institute of High Energy Physics (Beijing) in 1993 and BS degree from the Modern Physics Department of University of Science and Technology of China in 1990. He was a postdoctoral fellow of ExxonMobil Research and Engineering Company (Annandale, New Jersey, USA), and Harvard University (with Prof. Dave Weitz). He joined Texas A&M University as an Assistant professor in the Artie McFerrin Department of Chemical Engineering in August 2004 and was promoted to Associate professor in 2010.  He is also a faculty member of the Materials Science and Engineering Program and the Professional Program in Biotechnology of TAMU. Professor Cheng’s expertise is in the area of complex fluids and soft condensed matter physics. He has published over 60 papers in journals including Nature, Science, and Physical Review Letters.

One of the long term challenges in human health and disease is the control of pathogens, such as antibiotic-resistant forms of bacteria. In this talk, we will briefly describe two directions where soft condensed matter physics based approaches have been useful.

1)     Bacterial biofilms are structured multi-cellular communities that are notoriously resistant to antibiotics. We translate bacteria movies into searchable databases of bacterial behavior via methods adapted from colloid physics, and find an unexpected diversity in motility driven by Type IV pili across different bacterial species.  The associated phenomena include ‘stick-slip’ motion analogous to earthquake dynamics, and self-organization in early biofilm development reminiscent of capitalist economies.

2)     We examine the mechanism of a range of pore-forming polypeptides, including antimicrobial peptides, cell penetrating peptides, viral fusion peptides, and apoptosis proteins, and show how a combination of geometry, coordination chemistry, and soft matter physics can be used to approach a unified understanding.  

A quantum processor,  using quantum states of light and matter, holds the promise of performing calculations and simulations that are not practical by a classical processor. Many of the key components for a quantum processor have been demonstrated by various research groups and we can expect these components to be integrated into basic quantum processors in the near future. However, there remain significant technical challenges in scaling the system size and making the system robust and flexible. In GTRI ‘s Quantum Information Systems (QIS)  Group, we use atomic ions as our quantum system and are developing scalable technologies for trapping ions and manipulating the quantum information the ions hold as well as tools to run an eventual processor. I will give an overview of the technologies being developed at GTRI and the efforts to make a quantum processor user friendly.

I present inelastic neutron scattering data from one- two- and three-dimensional insulating magnetic materials at low temperatures that do not display a coherent resonant mode of excitation. Instead, momentum resolved spectra take the form of bounded continua. I interpret the spectra as evidence for fractionalization of a spin flip into distinct quasi-particles.

The unifying feature of the quantum magnets examined is a ground state that does not break rotational or translational symmetries – conventional Neel order having been disfavored by competing interactions and/or low dimensionality. I discuss the nature of the quasi-particles based on the neutron scattering data, the underlying lattice structure, and the spin Hamiltonian. 

The talk features inelastic neutron scattering data from novel instrumentation at the NIST Center for Neutron Research and the ORNL Spallation Neutron Source, which is dramatically improving our ability to probe atomic scale dynamics in condensed matter.

 * Also at NIST and ORNL. Work at IQM is supported by DoE, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Grant No. DE-FG02-08ER46544

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