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.

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.

 

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.

 

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.

 

 

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. 

The field of active matter is the result of applying statistical physics to the motion of biological and biomimetic systems, from animal flocks to the cell's cytoskeleton and from robotic swarms to self-propelled colloids. Unlike bird flocks, which can move around freely, cells inside an organism or filaments inside a cell move in a very confined space bounded by curved walls. What is more, the shape of the boundaries can affect the dynamics in dramatic ways. Recently my focus has been on building a theoretical framework to study such problems by combining the concepts of active matter with those of the geometry of curved surfaces. I will discuss what such an approach can teach us about the way active systems respond to the geometry of their environment and what I hope it can teach us about the way such systems deform their environment and regulate their own shape.

DNA nanotechnology, especially scaffolded DNA origami, has emerged into a field that fabricates well-defined nanostructures with unprecedented geometric complexity and precision. This technology is proposed to eventually provide integral components for complex nanomachines and nanofactories. The power of DNA as a nanoscale building material is that it can be designed to self assemble into complex nanostructures that are held together by numerous k_BT-scale (0.025 eV) interactions. This allows DNA-based structures to be both globally stable and locally dynamic. Currently, DNA nanotechnology has a number of applications, including drug delivery, single molecule sensing, and templating of crystalline nanoparticles. However, applications rely largely on static nanomaterial properties. I will discuss the overall current state of the DNA nanotechnology field and our work on developing DNA based nanosensors, whose functionality relies on structural dynamics. 

 

There is a strong desire, often driven by real or perceived pressures, to publish research in a top tier journal like Science.  However, with a rejection rate above 90%, it is a difficult process.  When a paper gets rejected without referee comments, it is hard to know why the paper failed to get past the initial screening process.  In this talk, I will describe the publication process at Science, within the broader context of developing skills for more effective scientific communication.  Aside from publishing in high impact journals, good communication tools are essential for forming scientific collaborations, bypassing research obstacles, avoiding conflicts during scientific presentations and explaining scientific research to funding bodies and the public at large, who are the primary source of financial support for scientific research.

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.