Past Colloquia: Spring 2014

"Universal models for coherent flow structures in turbulence"

January 14, 2014

Eric Brown, Yale University

Abstract: Turbulence is of tremendous importance in a wide range of astrophysical, geophysical, and engineering flow problems. Unfortunately, the largest-scale coherent structures such as convection rolls have different structures and dynamics in different flow geometries, so universal descriptions have been elusive. On the other hand, the robustness of these structures holds some promise for universal descriptions. I will present a new approach to universal models based on using robust empirical flow structure shapes as approximate solutions to the Navier-Stokes equations, which leads to low dimensional dynamical systems models for the flow. As an example, I will present results of Rayleigh-Benard convection experiments, in which a container is filled with water and heated from below. Buoyancy drives a flow which organizes into a roll-shaped circulation which spontaneously breaks the symmetry of the system. As a consequence, this roll exhibits a wide range of dynamics including erratic meandering, spontaneous flow reversals, and several oscillation modes. A simple model consisting of stochastic ordinary differential equations quantitatively reproduce these observed flow dynamics. The effects of boundary geometry and different forcings are each represented by different model terms. These results may lead to more general and relatively easy to solve models for turbulent flows with potential applications to climate, weather, and even the turbulent dynamo that is responsible for Earth's magnetic field.

"Dark Energy and Cosmic Sound"

January 21, 2014

Daniel Eisenstein,The Harvard-Smithsonian Center for Astrophysics

Abstract: I will discuss how sound waves racing through the cosmos during the first million years of the Universe provide a robust method for measuring the low-redshift cosmological distance scale and thereby the properties of dark energy. The distance that the sound can travel can be computed to high precision and creates a signature in the late-time clustering of matter that serves as a standard ruler. Galaxy clustering results from the Sloan Digital Sky Survey and SDSS-III reveal this feature and allow us to measure distances to high accuracy, including a new 1% measurement to z=0.57.

"Learning aerodynamics from insects and dreaming up new ways to fly"

January 28, 2014

Leif Ristroph, Courant Institute at New York University

Abstract: When viewed as miniature flying machines, insects are marvels of engineering that can teach us about the challenges and opportunities presented by flapping-wing aerodynamics. In the first part of this talk, I’ll show how a zoomed-in and slowed-down view of insect flight reveals the intricate strategies involved in orchestrating aerial maneuvers and in keeping up-right and on-course in the face of unexpected disturbances. In the second half, I’ll discuss where we are in terms of learning from insects in order to design and build small-scale flapping-wing robots. I’ll present a new aerodynamic apparatus – a kind of wind tunnel for flapping-wing flight – that serves to help inspire and test new flapping-wing concept vehicles. Specifically, I’ll show how this has led us to investigate unusual flying machines that look not like insects or birds but perhaps more like umbrellas, pyramids, UFOs, and jellyfish.

"From Black Holes to Fluid Holograms"

February 4, 2014

Veronika Hubeny, University of Durham

Abstract: This talk gives an overview of the Fluid/Gravity correspondence, which was developed 6 years ago in the context of the gauge/gravity duality. Mathematically, it posits that Einstien's equations of general relativity (with negative cosmological constant) in d+1 dimensions capture the (generalised) Navier-Stokes' equations of fluid dynamics in d dimen- sions. In particular, given an arbitrary fluid dynamical solution, we can systematically construct a corresponding black hole spacetime whose properties mimi that of the fluid flow. After reviewing the basic indredients, motivated partly by special properties of black holes, I will talk about the construction and implications of this remarkable correspondence.

"A Soft X-ray Spectropolarimeter Telescope"

February 11, 2014

Herman Marshall, MIT Kavli Institute

Abstract: We are developing instrumentation for a telescope capable of measuring linear X-ray polarization over a broad-band using conventional spectroscopic optics. Multilayer-coated mirrors are key to this approach, being used as Bragg reflectors at the Brewster angle. By laterally grading the multilayer mirrors and matching to the dispersion of a spectrometer, one may take advantage of high multilayer reflectivities and achieve modulation factors near 100% over the entire 0.2-0.8 keV band. We have a laboratory demonstration of the polarization of a pair of multilayer mirrors and will present progress on work to demonstrate the capabilities of laterally graded multilayer coated mirrors. We also present plans for a suborbital rocket experiment designed to detect a polarization level of <20% for an active galactic nucleus.

"Weakly Interacting Particles at the LHC: Searches for new forces, symmetries and dark matter"

February 25, 2014

Christopher Rogan, Harvard University

Abstract: The Large Hadron Collider (LHC) is the world’s most powerful probe of the experimental high-energy frontier, where protons are accelerated and collided at energies previously inaccessible in a laboratory. These particle collisions are recorded and reconstructed by the CMS and ATLAS experiments, whose goals include trying to answer an array of open questions related to the nature of the dark matter that pervades our universe and whether there are new, un-discovered phenomena beyond the existing Standard Model (SM) of particle physics. Often, weakly interacting particles are a central part of these inquiries. In this talk we will briefly review the CMS and ATLAS detectors, focusing on the elements of design that allow them to detect and study events with weakly interacting particles. The part that these ghostly particles play in models of physics beyond the SM, such as supersymmetry (SUSY), will be described along the strategies employed by the CMS and ATLAS experiments to discover them, illustrated through several examples of searches these new phenomena. Finally, the current experimental constraints on physics with new weakly interacting particles from Run I of the LHC will be summarized, along with some perspectives on the approaching Run II.

March 11, 2014

A Celebration of Leonard Eisenbud's 100th Birthday
Cumrun Vafa, Harvard University
Lecture 1: Strings and the Magic of Extra Dimensions

March 12, 2014
Lecture 2: Recent Progress in Toplogical Strings I
Lecture 3: Recent Progress in Topological Strings II

"The Physics of Sand: Emergent Behavior in the Macroworld"

March 25, 2014

Bulbul Chakraborty, Brandeis University

Abstract: Diversity in the natural world emerges from the collective behavior of large numbers of interacting objects. The origin of collectively organized structures over the vast range of length scales from the subatomic to colloidal is the competition between energy and entropy. Thermal motion provides the mechanism for organization by allowing particles to explore the space of configurations. This well-established paradigm of emergent behavior breaks down for collections of macroscopic objects ranging from grains of sand to asteroids. In this macro-world of particulate systems, thermal motion is absent, and mechanical forces are all important. We lack understanding of the basic, unifying principles that underlie the emergence of order in this world. In this talk, I will explore the origin of rigidity of granular solids, and present a new paradigm for emergence of order in these athermal systems.

"Performance through Deformation and Instability"

April 1, 2014

Katia Bertoldi, Harvard University

Abstract: Materials capable of undergoing large deformations like elastomers and gels are ubiquitous in daily life and nature. An exciting field of engineering is emerging that uses these compliant materials to design a active devices, such as actuators, adaptive optical systems and self-regulating fluidics. Compliant structures may significantly change their architecture in response to diverse stimuli. When excessive deformation is applied, they may eventually become unstable. Traditionally, mechanical instabilities have been viewed as an inconvenience, with research focusing on how to avoid them. Here, I will demonstrate that these instabilities can be exploited to design materials with novel, switchable functionalities. The abrupt changes introduced into the architecture of soft materials by instabilities will be used to change their shape in a sudden, but controlled manner. Possible and exciting applications include materials with unusual properties such negative Poisson’s ratio, phononic crystals with tunable low-frequency acoustic band gaps and reversible encapsulation systems.

"Swimming in Sand"

April 8, 2014

Daniel I. Goldman, Georgia Institute of Technology

Abstract: Resistive force theory (RFT) is often used to analyze the movement of microscopic organisms that swim in true fluids. In RFT, a body is partitioned into infinitesimal segments, each which generates thrust and experiences drag as it moves through the medium with a given orientation and direction. Linear superposition of forces from elements over the body allows prediction of swimming kinematics and kinetics. We find that RFT works surprisingly well in dry granular media using empirically determined force-orientation relationships; within a given plane (horizontal or vertical) these relationships are functionally independent of the granular medium. It a variety of situations, RFT quantitatively models the below and above-surface locomotion of animals and robots that operate in the '' frictional fluid " regime (in which frictional forces dominate material inertial forces). In this talk I will discuss examples of granular RFT applied to subsurface swimming: these include prediction of muscle activation wave patterns in the sandfish lizard, elucidation of the benefits of a slender and slick body in desert dwelling reptiles, and, in combination with a geometric approach due to Shapere and Wilczek [PRL, 1987], the discovery of body undulation patterns which generate complex maneuvers, like turning in place.

"Nanoscale Coaxial Probes with Optical, Solar and Sensing Utility"

April 29, 2014

Michael J Naughton, Boston College

Abstract: We discuss a nanoscale coaxial architecture with potential utility in nanophotonics, photovoltaics, visual prosthetics, and biological, chemical and neuro sensing. As subwavelength optical waveguides, these nanostructures can be used in a range of nanoscale manipulations of light, including optical nanoscopy and lithography, high efficiency solar cells, high electrode-density retinal implants and discrete optical metamedia. A modification of the basic structure enables the fabrication of highly sensitive molecular sensors and high resolution optoelectronic neurostimulators/sensors (optrodes). We will report on aspects of these applications, including radial p-n junction "nanocoax" solar cells, and bio, electrochemical and neuro sensing.