Abstract:

In 2013, the IceCube collaboration announced discovery of a population of astrophysical neutrinos with energies up to a few PeV, consistent with isotropic arrival. The origin of these neutrinos is heavily debated and plausible scenarios include galactic and extragalactic astrophysical accelerators and annihilation of extremely massive dark matter, or some combination of mechanisms. It is very likely, whatever the origin of these neutrinos, their production will be accompanied by the production of gamma rays at similar energies. These gamma rays are observable if the sources of these neutrinos is within the gamma-ray horizon. The High Altitude Water Cherenkov Observatory (HAWC) is sensitive to gamma rays up to 100 TeV and now has a year of data at full sensitivity. HAWC data will illuminate the origin of the IceCube neutrinos by observing or ruling out nearby accelerators. I will discuss the HAWC instrument and the efforts to identify large-scale isotropic gamma-ray emission.

 

color:black">Upon approaching the glass transition a liquid gets extremely sluggish without obvious structural changes. Despite decades of work, the physical origin of this glassy slowdown remains controversial. A common explanation relies on the increasing roughness of the underlying free-energy landscape, but the theoretical and experimental underpinnings of this scenario are still lacking. In this talk, I will survey recent advances that let us unambiguously identify and track the growing amorphous order, a manifestation of the rarefaction of metastable states in the rugged landscape. I will further explore the crucial role this order plays in driving the glassy slowdown.

Mechanical metamaterials have novel elastic and acoustic properties--negative Poisson's ratios and compressibilities, phononic bandgaps, bistability and acoustic lensing--which derive from their structure. Properties may be made robust by linking them to the system's topological state, in which the global structure determines and protects a particular mechanical response, equivalent to the behavior of electronic systems such as topological insulators. Topologically nontrivial states may be achieved in virtually any marginally rigid (isostatic) structure and at any scale: hinged frames, jammed packings, 3D-printed structures, origami/kirigami, self-assembled lattices and oscillator networks.

The immediate effect of topologically polarizing such a system is to create protected floppy edge modes. The ultimate goal is to manufacture systems with arbitrary programmed mechanical responses that are robust against disorder and fluctuations. I will describe two recent advances: (1) Creating materials with bulk topological modes and (2) Exploiting global mechanical instabilities to alter the topological state. In the first case, I describe lattices (the equivalent of Weyl semimetals) that possess topologically-protected bulk zero modes, leading to a sinusoidal elastic instability at incommensurate wavelength. In the second case, I consider systems with global elastic instabilities and show that the nature of such an instability determines much of the lattice's mechanical and acoustic properties, such as the structure of its edge modes. Finally, I show that extending this instability into the nonlinear regime can alter the topological polarization, hence tuning the edge stiffness by many orders of magnitude.

Abstract:

Liquid crystals are best known for their use in displays, but their interest extends far beyond. This phase of matter, intermediate between liquid and solid, is composed by anisotropic molecules which spontaneously align in space. When the molecules cannot achieve a perfect order, they form topological defects, “mathematical” objects which can be used as physical objects for many purposes. I show two examples of how liquid crystal defects can inspire concepts for new materials. The first example is a bistable system, obtained by confining liquid crystals in a micron-sized cubic scaffold.  The device can switch between “bright” and “dark” metastable states, thanks to the interaction of the defects with the scaffold. The second example is a self-assembled  structure of liquid crystal defects that act as micro-lenses. The structure resembles an insect’s compound eye, able to focus objects at different distances and sensitive to the polarization of light. 

In materials science, the control over the spatial arrangement of colloids in soft matter hosts implies control over a wide variety of materials properties, ranging from the system’s rheology, to its optics, to its catalytic activity. To direct particle assembly, colloids are often manipulated using external fields to steer them into well-defined structures at given locations. We have been developing alternative strategies based on fields that arise when a colloid is placed within soft matter to form an inclusion that generates a potential field in its host. Such potential fields allow particles to interact with each other. If the soft matter host is deformed in some way, the potential allows the particles to interact with the global system distortion. The concept is quite general, and applied within any medium in which distortions cost energy. We have explored these ideas in three media: curved fluid interfaces, where particles interact with the host interface via capillarity; confined nematic liquid crystals, where particles interact with the host director field via elastic interactions, and deformed lipid bilayers, where particles interact o tense membranes. These example systems have important analogies and pronounced differences which we seek to understand and exploit.

 

 

Epithelial cells are mostly quiescent when they are mature and uninjured, but they undergo collective migration during morphogenesis, cancer metastasis, and wound repair. We have recently reported (Nature Materials, Park et al, 2015) that, during differentiation, airway epithelial cells in air-liquid interface culture undergo a transition from a fluid-like, mobile “unjammed” state toward a solid-like, immobile “jammed” state. This transition toward the jammed state is substantially delayed in cells from asthmatic donors, compared with cells from normal donors. Furthermore, mature, jammed cells undergo a transition toward the unjammed state when they are subjected to compressive stress that mimics bronchoconstriction, a process that occurs during asthma exacerbations. These jamming and unjamming transitions are accompanied by unique changes in cell shape that are associated with intercellular forces.

 

The dynamic clustering of globular particles in suspensions exhibiting competing short-range attraction and long-range repulsion such as in protein solutions has gained a lot of interest over the past years. We investigate theoretically the influence of clustering on the dynamics of globular particle dispersions [1]. To this end, we systematically explore various pair potential models by a combination of state-of-the-art analytic methods in conjunction with computer simulations where the solvent-mediated hydrodynamic interactions are likewise included. Our theoretical results show that the cluster peak (intermediate-range-order peak) is present also in the hydrodynamic function characterizing the short-time dynamics, in accord with experimental data [2]. Enhanced short-range attraction leads to a smaller self-diffusion coefficient and a larger dispersion viscosity. The behavior of the (generalized) sedimentation coefficient is more intricate, e.g. showing non-monotonic interaction strength dependence.

 [1] J. Riest and G. Nägele, Short-time dynamics in dispersions with competing short-range attraction and long-range repulsion, Soft Matter 11, 9273 (2015).

[2] Collaboration with D. Godfrin (MIT), Y. Liu (NIST) and N. Wagner (UDEL), work in progress.

 

The progress in neutrino physics over the past fifteen years has been tremendous: we have learned that neutrinos have mass and change flavor. This discovery won the 2015 Nobel Prize. I will pick out one of the threads of the story-- the measurement of flavor oscillation in neutrinos produced by cosmic ray showers in the atmosphere, and further measurements by long-baseline beam experiments. In this talk, I will present the latest results from the Super-Kamiokande and T2K (Tokai to Kamioka) long-baseline experiments, and will discuss how the next generation of high-intensity beam experiments will address some of the remaining puzzles.

Outermost occupied electron shells of chemical elements resemble monopoles, dipoles, quadrupoles, and octupoles corresponding to filled s-, p-, d-, and f-atomic orbitals. Theoretically, elements with hexadecapolar outer shells could also exist, but none of the known stable elements have filled g-orbitals. On the other hand, the research paradigm of “colloidal atoms” displays complexity of physical behavior of colloidal particles exceeding that of their atomic counterparts, allowing for switching between colloidal elastic dipole and quadrupole configurations using weak external stimuli. This lecture will describe colloidal elastic hexadecapoles formed by polymer microspheres dispersed in a liquid crystal, a nematic fluid of orientationally ordered molecular rods. The solid microspheres locally perturb the uniform molecular alignment of the nematic host, inducing hexadecapolar and other elastic multipoles that drive highly anisotropic colloidal interactions. We uncover physical underpinnings behind the spontaneous formation of colloidal elastic hexadecapoles and describe the ensuing particle bonding inaccessible to colloids studied previously. The lecture will conclude with discussion of practical applications that can be enabled by combining unique properties of metal and semiconductor nanoparticles with facile switching of self-assembled ordered superstructures that they exhibit in nematic hosts.