Eric Sembrat's Test Bonanza

PLEASE NOTE: This is a WEBINAR

Loose sand particles can jam together to become a solid. Think of the dry sand on the beach: it supports your weight so you can walk on it – you don't need to swim in sand. Jammed sand indeed so much resists deformation that it even expands when you try to make it flow. This was already discovered by Osborne Reynolds in 1885. In fact, you can make sand particles jam by making them flow in a container that cannot expand. This effect is known as shear jamming and was discovered only recently. We have now studied the remarkable mechanics of these solid, shear-jammed structures. We did this in a model system, composed out of plastic disks. With an inventive new experimental setup, we were able to perform uniform
shear to a two-dimensional assembly of these disks in a container that does not expand. Deforming the collection of disks from a loose state with no forces between the disks, this model sand developed fascinating force structure. The plastic disks are optically sensitive to the forces acting on them, so these forces could be visualized as fringe patterns. With so much detail about the microscopic contacts in hand, we could for the first time fully establish and also quantify the existence of non-linear mechanical behavior of these shear jammed solids. Moreover, even though particles in these shear jammed packings hardly had the space to move, we uncovered completely unexpected dynamics in the force networks as shown in the image,
when they exposed the packing to repeated deformations. These results provide new perspective and important benchmarks for theoretical modeling of these shear jammed solids. And that's good, because at some point engineers may use the concept of shear jamming to build your new house. Perhaps even better is that it also shows the surprise and beauty in materials as common as sand on the beach.

Supermassive black holes are amazingly exotic and yet ubiquitous objects, residing in the centers of essentially all stellar bulges in galaxies. Recent years have seen remarkable advances in our understanding of how these black holes form and grow over cosmic time, and how energy released by active galactic nuclei (AGN) connects the growth of black holes to their host galaxies and large-scale structures. I will review some recent work that explores these connections, with a focus on statistical studies of AGN clustering and the links between black hole growth and and star formation. I will highlight some new insights into how and when AGN "feedback" is important for galaxy evolution, and discuss some prospects for exciting future progress.

The motion of biological systems in fluids is inherently complex, even for the simplest organisms. In this talk, we develop methods to analyze locomotion of both mechanical and biological systems with the aim of rationalizing biology and informing robotic design. We begin by building a visualization framework studying an idealized swimmer, Purcell's three link swimmer, at low Reynolds number. This framework allows us to illustrate the complete dynamics of the system, efficiently design gaits for motion planning, and identify optimal gaits in terms of efficiency and speed. We extend the three-link case to a serpenoid swimmer, or a swimmer with a continuously deformable shape.  

Drawing on the principles behind representing the serpenoid swimmer's shape, we develop a method based on proper orthogonal decomposition (POD) that describes the motion of complex biological systems in a low order manner, so that using only two degrees of freedom adequately describes the animal's motion. We successfully apply this method to species in both high and low Reynolds environments to elucidate different phenomena, including chemotaxing (movement owing to the presence of an attractant), inter- and intra-species comparison in sea urchin spermatozoa and bull spermatozoa, and kinematic responses to increasing viscosity in C. elegans (nematodes). We successfully illustrate the generalized utility of our decomposition method, combined with our visualization framework, to explore and understand fundamental kinematics of a wide range of both natural and man-made systems.

Current study in quantum dynamical evolution of complex systems investigates quantum systems characterized by fluctuations and quantum correlations.  Spin-1 condensates are predicted to generate non-classical states with quantum correlations, specifically squeezed states in the early low depletion limit and highly non-Gaussian distributions in the long term beyond the low depletion limit.  These states are created due to the quantum fluctuations about an unstable equilibrium in the spin-nematic subspaces to which the system is initialized.  In this talk I will discuss the underlying theory along with our measurements of spin-nematic squeezing [1], the later non-Gaussian distributions [2], and our efforts to stabilize the initial unstable equilibrium by periodically perturbing the dynamics.

1.  C.D. Hamley, C.S. Gerving, T.M. Hoang, E.M. Bookjans, and M.S. Chapman, “Spin-Nematic Squeezed Vacuum in a Quantum Gas,” Nature Physics 8, 305-308 (2012).
 2.  C.S. Gerving, T.M. Hoang, B.J. Land, M. Anquez, C.D. Hamley, and M.S. Chapman, “Non-equilibrium dynamics of an unstable quantum pendulum explored in a spin-1 Bose–Einstein condensate,” Nature Communications 3, 1169 (2012)

The U.S. Naval Observatory provides the master clock for the DoD.  To support this mission, we have built and fielded 4 rubidium atomic fountain clocks at our Washington D.C. site.  This ensemble of clocks has been running continuously for slightly less than two years and is contributing to our larger ensemble of atomic clocks.

I will talk about the construction, operation, and underlying physics of these clocks.   Each clock is a continuously running spectroscopy experiment that measures an atomic frequency to better than 10^-15 in one day.  The performance of these clocks over the previous two years will be presented along with comparisons to international timescales. 

Finally, I will present an experiment where we use this ensemble of clocks to set the most stringent limits on Local Position Invariance (LPI) violations through a “solar null test.”   We make this comparison by looking for variations between atomic clocks based on different atomic references over the year.  The presence or lack of variations driven by the annual variation of the solar gravitational potential sets improved limits on LPI violations and several fundamental constants’ coupling to the gravitational potential [1].

In addition to providing vital clues as to the formation and evolution of black holes, the spin of black holes may be an important energy source in the Universe.  Over the past couple of years, tremendous progress has been made in the realm of observational measurements of spin.   I will describe these efforts with particular focus on the use of X-ray spectroscopy to probe the spin of supermassive black holes in active galactic nuclei (AGN).   I shall describe results from the Suzaku AGNSpin Survey, a Suzaku Key Project that targets five bright and well-known AGN with observations of sufficient depth that black hole spin can be assessed.   For the first time, we are obtaining hints about the distribution of spins across the population of supermassive black holes with some interesting and unexpected consequences.

PLEASE NOTE: This is a WEBINAR

Scientists for years have been trying to better understand the mechanisms that are responsible for transport and mixing in fluid flow.  Mixing is important as it is used in everything from food preparation to energy production to biomedical devices, and is seen in both single and multiphase environments.  While mixing applications are wide ranging, a complete and proper understanding of mixing and transport mechanisms is still lacking.  These mechanisms are influenced by, but not limited to, time varying structures that may be seen in flow.  Addressing this deficit in our knowledge requires improved techniques for quantifying transport and mixing. 

One way in which these transport structures can be more accurately resolved is by investigating time-resolved fluid element trajectories as opposed to the current method of numerical integration of velocity fields.  By following the flow tracers, which have a similar behavior to that of the fluid elements, there is no need for numerical integration, which can introduce noise and error into the trajectories.  These fluid element surrogates may be neutrally buoyant tracer particles but also inertial elements like bubbles or large Stokes number solid particles, which will reveal different types of structures in the flow.  It has been shown that time varying coherent structures in fluid flow can have an important effect on the mixing behavior of a system.  Through the use of fluid element trajectories these time varying structures can be directly studied with the hope that this will aid in the understanding of mixing and transport in three-dimensional flow fields.

PLEASE NOTE: This is a WEBINAR

The transport of particulate material by fluid flow is a problem with far reaching applications. Isotropic particles that are very small and neutrally buoyant behave as Lagrangian tracers and move with the local fluid velocity. However, particles that are large or density mismatched compared to the fluid have different dynamics from the local fluid. The rotational dynamics of anisotropic particles is different from spherical tracers and this fascinating problem is central for many applications ranging from cellulose fibers in paper making to dynamics of ice crystals in clouds. I study the dynamics of single rod-like particle in a turbulent flow between oscillating grids. The position and orientation of rods are measured experimentally using Lagrangian particle tracking with multiple high speed cameras. Rods rotate due to the velocity gradient of the flow and as tracer rods are transported by the flow their orientation becomes correlated with the velocity gradient tensor. This alignment results in suppression of the rotation rates of rods. We have also studied the effects of finite length of rods on the rotation rate in turbulence. As the length of the rods increases the rotation rate variance decreases. In the inertial range the Kolmogorov cascade argument describes the rotation rate of long rods.

Atom interferometers that use pulses of light for coherent control of matter-wave interference can be used for wide ranging studies of light-matter interactions and for realizing precision measurements in atomic physics. We describe an echo type interferometer that utilizes a relatively simple setup to manipulate laser-cooled Rb atoms in a single ground state manifold. We review progress toward a precise determination of the atomic fine structure constant and gravitational acceleration.

*Work supported by CFI, OIT, OCE, NSERC and York University