Eric Sembrat's Test Bonanza

The effects of vibrations on fluids are important in a wide range of scientific and engineering applications such as liquid storage, mixing, convection, pattern formation, and the study of basic fluid instabilities.

Vertical vibrations are the most studied case because the basic (unexcited) state is quiescent in a co-moving reference frame. Horizontally or obliquely vibrated systems, although more resistant to theoretical analysis, may be more relevant to the question of general fluid behavior than the more popular vertically forced Faraday system.

We present the new results on interface instability between miscible liquids when vibrations act either parallel to the interface or under 5-7° angle. The interface is represented as a transitional layer of small but nonzero thickness.  The considered mixtures represent the wide class of fluids: water-alcohol. We demonstrate both experimentally and theoretically not only that interface instability exists in miscible liquids but also strongly affect by the gravity.  The dependence of pattern formation and mixing on vibration forcing is discussed.

Computational modeling of eukaryotic cells moving on substrates is an extraordinarily complex task: many physical processes, such as actin polymerization, action of motors, formation of adhesive contacts concomitant with both substrate deformation and recruitment of actin etc., as well as regulatory pathways are intertwined. Moreover, highly nontrivial cell responses emerge when the substrate becomes deformable and/or heterogeneous. Here we extended a computational model for motile cell fragments, based on an earlier developed phase field approach, to account for explicit dynamics of adhesion site formation, as well as for substrate compliance via an effective elastic spring. Our model displays steady motion vs. stick-slip transitions with concomitant shape oscillations as a function of the actin protrusion rate, the substrate stiffness, and the rates of adhesion. Implementing a step in the substrate’s elastic modulus, as well as periodic patterned surfaces exemplified by alternating stripes of high and low adhesiveness, we were able to reproduce the correct motility modes and shape phenomenology found experimentally. We also predict the following nontrivial behavior: the direction of motion of cells can switch from parallel to perpendicular to the stripes as a function of both the adhesion strength and the width ratio of adhesive to non-adhesive stripes.

The unification of the four fundamental forces remains one of the most important issues in theoretical particle physics. In this talk, I will first give a short introduction to Non-Commutative Spectral Geometry, a bottom-up approach that unifies the (successful) Standard Model of high energy physics with Einstein's General theory of Relativity. The model is built upon almost-commutative spaces and I will discuss the physical implications of the choice of such manifolds. I will show that even though the unification has been obtained only at the classical level, the doubling of the algebra may incorporate the seeds of quantization. I will then briefly review the particle physics phenomenology and highlight open issues and current proposals. In the last part of my talk, I will explore consequences of the Gravitational-Higgs part of the spectral action formulated within such almost-commutative manifolds. In particular, I will study modifications of the Friedmann equation, propagation of gravitational waves and the onset of inflation. I will show how current measurements (Gravity Probe, pulsars, and torsion balance) can constrain free parameters of the model. I will conclude with a short discussion on open questions.

A new observational era in gravitational wave astronomy is poised to begin in this decade, with the upcoming start of Advanced LIGO and Advanced Virgo.  These instruments will be capable of the direct detection of gravitational wave transients, which will yield new insights into the engines powering some of the most energetic astrophysical events: the coalescence of neutron star and/or black hole binary systems, core-collapse supernovae, and isolated neutron star instabilities. I will present the path towards this detection using the second generation of gravitational wave interferometers, and summarize the open analysis challenges, prospects for astrophysical inference and the potential for multi-messenger astronomy with combined information from the electromagnetic and neutrino sectors.

In our standard scenarios galaxy evolution via mergers, supermassive binary black holes appear to be inevitable. Yet, close supermassive binaries, at separations of a parsec or less have remained elusive. Finding such binaries constitutes a test of galaxy evolution models as well as models that invoke them in order to explain a variety of other phenomena. Separations of about a parsec are particularly interesting because they are the progenitors of gravitational wave sources. Moreover, early models suggested that binaries may spend a great deal of time at these separations. Motivate by the above considerations, our group embarked a few years ago on a search for close supermassive binary black holes. I will describe the methodology that we are using, the results we have so far, and our strategy for future observations.

We are continually gathering larger amounts and kinds of data about real systems, and have increasingly higher expectations of detail and fidelity in the models we build of those systems. As we try to incorporate more detail and broader domains into our predictive mathematical models, we become less able to intuit their working principles. While simulation of large-scale models can demonstrate sufficiency of the model to capture a phenomenon, it generally does not lead to an analytical understanding of the minimally sufficient causes of that phenomenon, which may belong to a subset of the model’s many components. Understanding minimal descriptions of behavior is valuable for making principled, predictable, and efficient changes to the behavior, for instance to design a new treatment for a disease.

On the other hand, highly simplified, abstract models are popular ways to represent intuition about mechanisms. These are generally not derived or inferred using systematic principles directly from a detailed model, and so we often remain unsure whether we have found the correct low-dimensional representation of a mechanism. If we have, how do we know how to relate adjustments to the reduced model to corresponding changes in the detailed model without an explicit mapping between the representations?

The qualitative theory of dynamical systems and the methods of asymptotic analysis contain many useful tools for understanding models at multiple scales and levels of representation.  In this talk, I describe how I have interpreted and augmented these methods into practical algorithms that allow their partial automation in software. As a result, high-dimensional dynamics that were previously inaccessible to pen-and-paper analysis can now be understood and visualized through computer-assisted systematic reduction.

This talk will show examples of these methods applied to several biological problems, including the inference of mechanism for plateau potentials in a detailed cardiac myocyte model.

Liquids and solids tend to stick to each other.  When a liquid droplet sticks to a solid surface we call it wetting.  When a solid particle sticks to a solid surface we call it adhesion.  The classic coarse-grained descriptions of these two phenomena are distinct from each other.  Both descriptions assume that solid objects undergo very little deformation during wetting and adhesion.  In this talk, I will show how this assumption breaks down when the solids are soft enough and how wetting and adhesion really are not that different after all.

The spectrum of cosmic rays includes the most energetic particles ever observed. The mechanism of their acceleration and their sources are, however, still mostly unknown. Observing astrophysical neutrinos can help solve this problem. Because neutrinos are produced in hadronic interactions and are neither absorbed nor deflected, they will point directly back to their source. This talk will cover searches for high-energy neutrinos (> 100 TeV) at the IceCube neutrino observatory, which have recently produced the first evidence for a flux beyond standard expectations from neutrinos generated in the Earth's atmosphere. This includes the detection of two events with energies above 1 PeV -- the highest energy neutrinos ever observed. The current status of these astrophysical neutrino searches and prospects for the future will be discussed.

The fractional quantum Hall effect (FQHE) states in the second Landau level have attracted growing interests and intensive theoretical and experimental investigations due to them possibly being non-Abelian states. Recently, we systematically examined the spin polarization of the FQHE states in a series of high quality, low density two dimensional electron systems. Evidence of spin transition was observed, suggesting a more complicated nature of the FQHE ground states in the second Landau level.

When the light interacts with low-dimensional systems, new optical phenomena can arise because of the reduced dimensionality. Classic examples include discrete electronic energy levels quantum dots or plasmon resonances of metallic nanoparticles. In addition to the dimensionality or shape, the light-matter interaction can be further tuned by using optical nonlinearities. Typically, the induced polarization currents depend linearly on the intensity of the radiation field. However, when the linear relationship breaks down new interesting phenomena arise like frequency conversion or intensity dependent refractive index. We combine these new possibilities with the interesting properties of the low - dimensional systems and demonstrate potential for applications ranging from sub-diffraction resolution imaging to on-chip frequency conversion.