Eric Sembrat's Test Bonanza

Even though cosmic rays were discovered more than 100 years ago, their origin remains a mystery. Neutrinos, product of cosmic ray interactions at or near the production site, are the best astrophysical messenger to find the cosmic ray sources. The past few months have seen fast progress in the search for very-high-energy (>100 TeV) astrophysical neutrinos.  IceCube has reported a set of events that are inconsistent with terrestrial origin and have characteristics best explained by an astrophysical origin. In this presentation I will discuss the current status of IceCube's observations including Georgia Tech's role in this work. I will also discuss future prospects for the field of neutrino astrophysics.

Lattices that are on the verge of mechanical instability provide useful models for systems as diverse as architectural structures, crystalline and amorphous solids, sphere packing’s and granular matter, networks of semi-flexible polymers, and protein structure. This talk will explore elastic and mechanical properties and mode structures of model periodic lattices of periodic versions of this lattice, including the origin and nature of zero modes under both periodic (PBC) and free boundary conditions (FBC). It will derive general conditions (a) under which the zero modes under the two boundary conditions are essentially identical and (b) under which phonon modes are gapped with no zero modes in the periodic spectrum but include zero-frequency surface Rayleigh waves in the free spectrum. The gapped states have a topological characterization, similar to that of topological insulators that define the nature of zero-modes at the boundary between systems with different topology.

Textbooks on colloidal phenomena teach us to describe the electrostatic and dispersion interactions at interfaces using single parameters (Hamaker constants, surface potentials or charge densities), an approach which treats surface as uniform. Real surfaces, be they mineral, polymeric, or biological, present heterogeneous surface chemistry that complicates their interactions.  It is recognized that clustered rather than uniform presentation of attractive chemistries, for instance peptide sequences, enhances biological interactions.  Likewise, heterogeneity in charge and acid-base interactive groups has been invoked to explain colloidal instabilities and bacterial adhesion (that DLVO theory fails to predict).  A lack of understanding the energy landscape of real surfaces prevents their description by convolved versions of DLVO and related approaches.  Our lab, has, however, developed a series of model surfaces with well-characterized electrostatic heterogeneity and, through systematic variations in the interface, demonstrated how heterogeneity alters the surface forces. 

This talk addresses the fundamental aspects of heterogeneous surface interactions and then translates the interactions to static and dynamic particle adhesion.  In particular, the influence of the energy and distribution of the heterogeneity will be addressed. The talk will then demonstrate how interactions at heterogeneous interfaces are particularly sensitive to physical interfacial features such as curvature and mechanical softness, and how this sensitivity can be exploited for separation and microfluidic sensing schemes.  Finally, the talk will address the role of heterogeneity in dynamic adhesion, from capture –hold-release protocols to continuous behaviors such as rolling.  These will be demonstrated with particles and spherical bacteria.  Throughout the talk, parallels between synthetic and biological interfaces will be drawn, probing the extent to which charge and other chemical clusters can be treated conceptually as biological receptors.

In designing a new generation of legged robots, it is critical to understand the design principles employed by animals. One of the key steps to successful development of such bio-inspired robots is to systematically extract relevant biological principles, rather than direct copying features of an animal solution, which may be impossible to realize or irrelevant in engineering domain. The talk will introduce several examples that successfully implement bio-inspired design principles learned from animals. Our highlighting example is the development of  the MIT Cheetah, currently running at 13.5mph with an locomotion efficiency rivaling animals. Three research thrusts of the MIT Cheetah are optimum actuator design, biotensegrity structure design, and the momentum balancing control architecture for a fast and stable gallop. Each research component is guided by the biomechanics studies of runners such as dogs and cheetahs capable of fast traverse on rough and unstructured terrains. Through this project, we seek to derive design principles of quadrupedal locomotion that share characteristics with available mechanical and electrical capabilities in order to develop most efficient, robust robots, which will be part of our life in the future.

Human-built machines are usually efficient, fast and powerful, and are largely constructed from stiff materials. In order to cope with complex environments, the most recent robotic devices have begun to incorporate compliant joints and control systems based on impedance rather than force and position monitoring. However, even these advanced machines cannot perform with the robustness and adaptability found in living animals. A major challenge is that the design, fabrication and control of highly deformable structures is still poorly understood. Our research is directed at understanding how the movements of soft animals are controlled and in applying these findings to the development of soft robots. The guiding framework is that morphological computation (embodied intelligence) is essential for soft animals to move and manipulate in the natural world, and that soft machines need to incorporate the same strategies.

We have used the caterpillar as a tractable model system to understand how neural commands and nonlinear material properties interact to create useful movements. Some of these concepts have been implemented in a family of simple elastomeric robots (Softworms) that can move with a variety of caterpillar-like gaits. The next challenge is to make these robots climb in complex branched structures. 

The Sec translocon is a central component of cellular pathways for protein translocation and membrane integration.  Using both atomistic and coarse-grained molecular simulations, we investigate the conformational landscape of the translocon and explore the role of peptide substrates in the regulation of the translocation and integration pathways.   Implications of these results for the regulation of Sec-mediated pathways for protein translocation and membrane integration are discussed.

Using force spectroscopy mode of Atomic Force Microscopy (AFM) one can measure physiologically relevant pN forces between an AFM tip and a biomolecule with a mean displacement resolution of about 0.1 nm. The last 15 years have witnessed an explosion of interest in single molecule force spectroscopy fueled by: 1) new possibilities to advance in protein folding, 2) possibilities to elucidate molecular mechanisms of various cellular processes, and diseases, and 3) efforts to understand the nanomechanical properties of proteins, polysaccharides and DNAs in order to design biomimetic and/or mechanically functional materials.

In this talk, I will present several examples of our AFM force spectroscopy data. First, I will concentrate on elucidating early folding events in a simple model protein from changes of molecular compliance and dissipation factors. Using such measurements, we hope to provide basic understanding of early folding events. Next, I will show how mechanical force can influence the rate and mechanisms of an enzymatic cleavage of a single disulfide bond embedded in a protein. Time permitting, I will present the results of mechanical unfolding of on a recombinant protein comprising an NRR domain from mammalian Notch 1. Notch is a transmembrane cell signaling protein, and understanding its mechanical properties at the single molecule level is expected to help elucidating Notch’s role in processes relevant to embryonic development, tissue homeostasis, and some breast cancers. "Times
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Recently, there is revived interest in non-equilibrium dynamics of the nuclear spins partially due to the decoherence issue of the electron spin qubit in semiconductor quantum dots for quantum computation. In this talk, I will first introduce a microscopic theory for the non-equilibrium nuclear spin dynamics controlled by a closed feedback loop mediated by the electron and/or the hole under continuous wave pumping in a quantum dot [W. Yang and L. J. Sham, Phy. Rev. B 85, 235319(2012)], and then present a study on the nuclear-spin-fluctuation induced spin decoherence of an electron (SDE) in an optically pumped quantum dot. The SDE is computed in terms of the steady distribution of the nuclear field formed through the hyperfine interaction with two different nuclear species in the dot. Different from the existing work, where a bilinear hyperfine interaction between the electron (or hole) spin and the nuclear spin is used, we use an effective nonlinear interaction derived from the Fermi-contact hyperfine interaction. Our feedback loop forms a multi-peak steady distribution of the nuclear field in which the SDE shows remarkable collapses and revivals in nanosecond time scale. Such an anomalous SDE results from an interference effect of the electron Larmor precession in a multi-peak effective magnetic field. Finally, I will briefly discuss significant nuclear spin polarizations using the theory by Yang and Sham.

Cosmic rays are predominantly nuclei, in particular protons.  However, the less abundant cosmic-ray electrons and positrons are also important probes of open questions in astrophysics and particle physics.  The Fermi Gamma Ray Space Telescope, designed to study the high-energy universe with gamma rays, is also an excellent electron and positron detector.  Ground-based imaging atmospheric Cherenkov telescopes have also measured cosmic-ray electrons and positrons up to several TeV.  PAMELA, Fermi, and AMS have discovered a surprising excess of positrons between 10 GeV and 350 GeV.  I will describe these measurements and their implications for astrophysics and particle physics, as well as prospects for future measurements.

To tie a shoelace into a knot is a relatively simple affair. Tying a knot in a field is a different story, because the whole of space must be filled in a way that matches the knot being tied at the core. The possibility of such localized knottedness in a space-filling field has fascinated physicists and mathematicians ever since Kelvin’s 'vortex atom' hypothesis, in which the atoms of the periodic table were hypothesized to correspond to closed vortex loops of different knot types. Perhaps the most intriguing physical manifestation of the interplay between knots and fields is the existence of knotted dynamical excitations. I will discuss some remarkably intricate and stable topological structures that can exist in light fields whose hydrodynamic-like evolution is governed entirely by the geometric structure of the field. I will then turn to experimental hydrodynamics: how to make knotted vortex loop configurations in fluids and how they evolve once made.