Eric Sembrat's Test Bonanza

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.

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

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.

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

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.

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.

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.

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].

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 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.