Eric Sembrat's Test Bonanza

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. 

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.

The IceCube Neutrino Observatory has reported a diffuse flux of TeV-PeV astrophysical neutrinos in three years of data. The observation of tau neutrinos in the astrophysical neutrino signal is of great interest in determining the nature of astrophysical neutrino oscillations. Tau neutrinos become distinguishable from other flavors in IceCube at energies above a few hundred TeV, when the particle shower from the initial charged current interaction can be separated from the cascade from the tau decay: the two cascades are called a "double bang" signature. I will discuss the search for tau neutrinos in IceCube, including an analysis which uses the digitized signal from individual IceCube sensors to resolve the two showers, in order to be sensitive to taus at as low an energy as possible. This is the first IceCube search to be more sensitive to tau neutrinos than to any other flavor. I will present the results and prospects for future high energy tau neutrino searches in IceCube and beyond.

In this talk, I will summarize recent results from the South Pole Telescope 2500 deg^2 survey. This mass-limited survey has discovered hundreds of new galaxy clusters at 0 < z < 1.7, allowing an unprecedented view of galaxy cluster evolution. Using follow-up observations from Spitzer, Hubble, Chandra, XMM-Newton, Magellan, VLT, ALMA, ATCA, and Gemini, we are able to study the evolution of the stars, gas, and dark matter in these massive systems. Based on these data, we constrain the evolution of cluster galaxies, the central AGN, the cooling ICM, the heavy metal abundance of the ICM, the dynamical state of the cluster, and various other cluster properties. Looking forward, I will present several new and ongoing surveys which will dramatically change the landscape of galaxy cluster research in coming years.

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.

There is a strong desire, often driven by real or perceived pressures, to publish research in a top tier journal like Science.&nbsp; However, with a rejection rate above 90%, it is a difficult process.&nbsp; When a paper gets rejected without referee comments, it is hard to know why the paper failed to get past the initial screening process.&nbsp; In this talk, I will describe the publication process at Science, within the broader context of developing skills for more effective scientific communication.&nbsp; 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.

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.&nbsp;

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The nascent technique of 4D printing has the potential to revolutionize manufacturing in fields ranging from organs-on-a-chip to architecture to soft robotics. By expanding the pallet of 3D printable materials to include the use stimuli responsive inks, 4D printing promises precise control over patterned shape transformations. With the goal of creating a new manufacturing technique, we have recently introduced a biomimetic printing platform that enables the direct control of local anisotropy into both the elastic moduli and the swelling response of the ink.

We have drawn inspiration from nastic plant movements to design a phytomimetic ink and printing process that enables patterned dynamic shape change upon exposure to water, and possibly other external stimuli. Our novel fiber-reinforced hydrogel ink enables local control over anisotropies not only in the elastic moduli, but more importantly in the swelling. Upon hydration, the hydrogel changes shape accord- ing the arbitrarily complex microstructure imparted during the printing process.

To use this process as a design tool, we must solve the inverse problem of prescribing the pattern of anisotropies required to generate a given curved target structure. We show how to do this by constructing a theory of anisotropic plates and shells that can respond to local metric changes induced by anisotropic swelling. A series of experiments corroborate our model by producing a range of target shapes inspired by the morphological diversity of flower petals.