Recovering Lost Information in the Digital WorldYonina Eldar, WIS
Room Drory Auditorium 13:15 The conversion of physical analog signals to the digital domain for further processing inevitably entails loss of information.The famous Shannon-Nyquist theorem has become a landmark in analog to digital conversion and the development of digital signal processing algorithms. However, in many modern applications, the signal bandwidths have increased tremendously, while the acquisition capabilities have not scaled sufficiently fast. Furthermore, the resulting high rate digital data requires storage, communication and processing at very high rates which is computationally expensive and requires large amounts of power. In this talk, we present a framework for sampling and processing a wide class of wideband analog signals at rates far below Nyquist by exploiting signal structure and the processing task. We then show how these ideas can be used to overcome fundamental resolution limits in optical microscopy, ultrasound imaging, quantum systems and more. We demonstrate the theory through several demos of real-time sub-Nyquist prototypes and devices operating beyond the standard resolution limits combining high spatial resolution with short integration time.
On energy equilibration in slow fast systemsVered Rom-Kedar
Room Room A 14:15 . In 1949, Fermi proposed a mechanism for the heating of particles in cosmic rays. He suggested that on average, charged particles gain energy from collisions with moving magnetic mirrors since they hit the mirrors more frequently with heads on collisions. Fermi, Ulam and their followers modeled this problem by studying the energy gain of particles moving in billiards with slowly moving boundaries. Until 2010 several examples of such oscillating billiards leading to power-law growth of the particles averaged energy were studied. In 2010 we constructed an oscillating billiard which produces exponential in time growth of the particles energy. The novel mechanism which leads to such an exponential growth is robust and may be extended to arbitrary dimension. Moreover, the exponential rate of the energy gain may be predicted by utilizing adiabatic theory and probabilistic models. The extension of these results to billiards with mixed phase space leads to the development of adiabatic theory for non-ergodic systems. Finally, such accelerators lead to a faster energy gain in open systems, when particles are allowed to enter and exit them through a small hole. The implications of this mechanism on transport in extended systems and on equilibration of energy in closed systems like "springy billiards" will be discussed. The latter application provides a key principle: to achieve ergodicity in slow-fast systems in the adiabatic limit, the fast subsystems should NOT be ergodic.
Three-Dimensional Active Defect LoopsGareth Alexander
Room Drory Auditorium 13:15 We describe the flows and morphological dynamics of topological defect lines and loops in three-dimensional active nematics and show, using theory and numerical modelling, that they are governed by the local profile of the orientational order surrounding the defects. Analysing a continuous span of defect loop profiles, ranging from radial and tangential twist to wedge ±1/2 profiles, we show that the distinct geometries can drive material flow perpendicular or along the local defect loop segment, whose variation around a closed loop can lead to net loop motion, elongation, or compression of shape, or buckling of the loops. We demonstrate a correlation between local curvature and the local orientational profile of the defect loop, indicating dynamic coupling between geometry and topology. To address the general formation of defect loops in three dimensions, we show their creation via bend instability from different initial elastic distortions.
Shaping liquid droplets and elastic membranesZvonimir Dogic
Room Drory Auditorium 13:15 We describe two self-assembly pathways observed in micron-thick colloidal membranes that spontaneously assemble in mixtures of monodisperse colloidal rods and non-adsorbing polymer. In a first example, we study mechanisms by which membrane-embedded 2D liquid droplets acquire unusual non-spherical shapes, suggesting that the interfacial edge domain has spontaneous non-zero edge curvature. These experimental observations can be explained by a simple geometric argument which predicts that the edge curvature towards shorter rod domains softens the resistance of the edge to twist. In a second example, we study the 3D structure of membranes composed of miscible rod-like molecules of differing lengths. Above a critical concentration of shorter rods flat 2D membranes become unstable and assume a bewildering variety of different shapes and topologies. Simple arguments suggest that doping colloidal membranes with miscible shorter rods tunes the membrane’s Gaussian modulus, which in turn destabilizes flat 2D membranes.
Locomotion by shape control in nature and technologyAntonio DeSimone
Room Drory Auditorium 15:00
Optics, Vision, and Evolution, after Mitchell Feigenbaum 1944-2019Jean-Pierre Eckmann
Room Auditorium 11:00 Many people are aware of Feigenbaum's astonishing discovery of the universality of period doubling, and the constant delta=4.66920 which carries his name. In the last 13 years of his life Feigenbaum worked on other subjects, and he wrote the manuscript (in TeX) of a book whose title is "Reflections on a Tube". This is closely related to his life-long interest in optics and aspects of vision. It deals with the optics of images reflected in a cylindrical mirror (usually called anamorphic pictures). He shows that the eye does not interpret ray-tracing, but caustics. But there are two caustics, and therefore, the viewer can actually see two different images. The visual system will often prefer one over the other. The question is the "which" and "why"? Starting from this discovery, Feigenbaum derived other aspects of this observation, dealing with the vision of fish, the "broken" pencil in water, or aspects of the floor of swimming pools. All these examples show two possible images. His study tells me how a simple study in classical optics can lead to interesting questions in perception and the visual system. I will give an overview of this project. As I discussed with him, over those 13 years, many aspects of his work, I have edited his manuscript so it can be published as a book which should appear in a forseeable future.
Packets of Diffusing Particles Exhibit Universal Exponential TailsStas Burov
Room Drory Auditorium 13:15 Brownian motion is a Gaussian process described by the central limit theorem. However, exponential decays of the positional probability density function $P(X,t)$ of packets of spreading random walkers, were observed in numerous situations that include glasses, live cells and bacteria suspensions. We show that such exponential behavior is generally valid in a large class of problems of transport in random media. By extending the Large Deviations approach for a continuous time random walk we uncover a general universal behavior for the decay of the density. It is found that fluctuations in the number of steps of the random walker, performed at finite time, lead to exponential decay (with logarithmic corrections) of $P(X,t)$. This universal behavior holds also for short times, a fact that makes experimental observations readily achievable.
Thermal conductance of one dimensional disordered harmonic chainsBiswarup Ash - WIS
Room Room A 14:15 Heat transfer in solids is usually described in terms of Fourier's law according to which the thermal conductance of a material scales inversely with its length or, equivalently, thermal conductivity is independent of sample length. Theoretical and experimental studies over the past decade have demonstrated that Fourier's law is violated for a variety of one-dimensional systems. Despite the large number of studies of many intriguing models, the validity criteria for Fourier's law remain elusive, and a breakdown of Fouriers law seems to be commonplace. In this talk, I will discus heat conduction mediated by longitudinal phonons in one dimensional disordered harmonic chains to understand the role of different parameters that may affect the scaling of thermal conductance in these systems. Using scaling properties of the phonon density of states and localization in disordered systems, we find non-trivial scaling of the thermal conductance with the system size. Our theoretical findings are corroborated by extensive numerical analysis. We show that, suprisingly, the thermal conductance of a system with strong disorder, characterized by a `heavy-tailed' probability distribution, and with large impedance mismatch between the bath and the system scales normally with the system size, i.e., in a manner consistent with Fourier's law. We identify a dimensionless scaling parameter, related to the temperature scale and the localization length of the phonons, through which the thermal conductance for different models of disorder and different temperatures follows a universal behavior.
The Surprises of a Nanochannel –Yoav Green
Room Room A 14:15 Nanofluidic systems have the potential to revolutionize numerous fields of high practical importance, including desalination, energy harvesting, bio-sensing, fluid based electrical circuits, and more. It is, therefore, not surprising that in the last two decades we are witnessing an increase in nanofluidic-based research. However, realizing the full potential of nanofluidics remains conditional to conquering several significant challenges. Notably, our current understanding of the fundamental physical phenomena that govern ion transport through nanochannels is incomplete and many key questions remain open. Fifteen years ago it was suggested that low-voltage Ohmic response of nanochannel-microchannels systems was dominated by the electrical resistance of the nanochannel, and that the resistances of the adjacent microchannels, were negligible. I will present evidence contradicting this suggestion that has since become paradigm. I will present a new modified paradigm which emphasizes the importance of the microchannels in determining the overall response. Our result suggest the need to conduct fundamental driven research to further reveal the physics of ion-transport at low-voltages so that we can unveil the physics at high-voltages where non-linear electroconvective effects are prevalent. Bio: Yoav Green is currently a senior lecturer in the Department of Mechanical Engineering at Ben-Gurion University. Before that, Yoav was post-doctoral researcher in the Harvard T. H. Chan School of Public Health where he worked in the field of biomechanics. Yoav holds a PhD in mechanical engineering from the Technion - Israel Institute of Technology where his research fields were nanofluidics and electrokinetics. Yoav also holds an MSc in physics (astrophysics and astronomy) from the Weizmann Institute of Science, and BSc in aerospace engineering from the Technion.
Packets of Diffusing Particles Exhibit Universal Exponential TailsStas Burov, Bar-Ilan University
Room Drory Auditorium 13:15 Brownian motion is a Gaussian process described by the central limit theorem. However, exponential decays of the positional probability density function $P(X,t)$ of packets of spreading random walkers, were observed in numerous situations that include glasses, live cells and bacteria suspensions. We show that such exponential behavior is generally valid in a large class of problems of transport in random media. By extending the Large Deviations approach for a continuous time random walk we uncover a general universal behavior for the decay of the density. It is found that fluctuations in the number of steps of the random walker, performed at finite time, lead to exponential decay (with logarithmic corrections) of P(X,t). This universal behavior holds also for short times, a fact that makes experimental observations readily achievable.