Eric Sembrat's Test Bonanza

Recent work from Marten Scheffer and colleagues has made bold claims.

"Complex dynamical systems, ranging from ecosystems to financial markets and the climate, can have tipping points at which a sudden shift to a contrasting dynamical regime may occur [1]. Although predicting such critical points before they are reached is extremely difficult, work in different scientific fields is now suggesting the existence of generic early-warning signals that may indicate for a wide class of systems if a critical threshold is approaching." In a paper, now in Press in Critical Care Medicine, Scheffer and colleagues (including me) argue that these results may be applicable in medicine [2].  I will discuss this work from the context of my own interest in bifurcations, problems associated with alternans rhythms [3], and transition to and risk stratification for sudden cardiac death.

 [1] "Early-warning signals for critical transitions" Nature 461, 53 (2009) by Marten Scheffer et al.

 [2] "Slowing down of recovery as generic risk marker for acute transitions in chronic diseases" by Olde-Rikkert et al. Critical Care Medicine (2016)

 [3] "Predicting the onset of period-doubling bifurcations in noisy cardiac systems" T. Quail, A. Shrier, L. Glass, PNAS 112, 9358-9363 (2015)

 

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.

The enjoyment of music and art are uniquely human experiences. Yet we still do not understand the attributes that lead us to appreciate some artistic works and not others. In this talk I will address how concepts in mathematics and physics can help us to think about these matters. Chaos refers to irregular time series that are generated following a definite set of deterministic rules. A fractal is an image, in which magnification of a small region is similar to the whole. I will give examples of how the concepts of chaos and fractals can be exploited to propose simple computer algorithms that can be used to generate sequences of sounds and images. I will also show how random patterns of dots can be manipulated to generate displays that are visually interesting, and that can be used as an input to probe the physiological processes underlying visual perception. The talk will challenge you to think about what you hear and see, and how you do it.

 

 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.