Chirality and its Spontaneous Symmetry Breaking in Two Liquid Crystal Systems

Chirality, or handedness, is a key concept spanning all fields of natural science, from biology to mathematics. Chiral structures can arise from achiral building blocks that lack a handedness if their assembly is unstable to chiral deformations, a phenomenon called spontaneous symmetry breaking. We theoretically study the role of chirality in two systems composed of liquid crystals dissolved or suspended in water, and our results match those obtained experimentally by our collaborators. In the first system, we study achiral liquid crystals whose Frank twist modulus is much lower than their splay and bend Frank moduli and which are confined in capillaries. Under homeotropic anchoring, their ground state configuration undergoes spontaneous chiral symmetry breaking when the twist modulus decreases enough relative to the splay and bend moduli. Under degenerate planar anchoring, a small twist-to-saddle-splay ratio of elastic moduli leads to degenerate twisted configurations even though an undeformed configuration is possible. Measuring the twist profile of an experimental system produces a value for the saddle-splay constant, which has been difficult to achieve previously. Under either boundary condition, domain walls and point defects, whose topological charges depend on chirality, separate domains with different degenerate configurations, and certain ones are energetically preferred over others. In the second system, we study filamentous viruses acting as colloidal liquid crystals under the influence of depletion, which promotes condensation of the viruses into 2D colloidal monolayers. These membranes have tunable chirality and show a rich array of emergent behaviors, including a transition from a circular shape to a striking starfish shape upon changing the chirality of constituent viruses, partial coalescence via domain walls through which the viruses twist by 180 degrees, and phase-separated rafts of a particular size when two virus species with different lengths and opposite chiralities are used. We formulate a simple theory combining Frank elasticity and depletion that shows how entropy and chirality drive the formation and behavior of these diverse structures. Our work may facilitate the design of chiral sensors and reconfigurable materials and suggests that chirality contributes not only biochemically but also physically to the behavior of lipid rafts in biomembranes

A Hopfield-like model with complementary encodings of memories

We present a Hopfield-like autoassociative network for memories representing examples of concepts. Each memory is encoded by two activity patterns with complementary properties. The first is dense and correlated across examples within concepts, and the second is sparse and exhibits no correlation among examples. The network stores each memory as a linear combination of its encodings. During retrieval, the network recovers sparse or dense patterns with a high or low activity threshold, respectively. As more memories are stored, the dense representation at low threshold shifts from examples to concepts, which are learned from accumulating common example features. Meanwhile, the sparse representation at high threshold maintains distinctions between examples due to the high capacity of sparse, decorrelated patterns. Thus, a single network can retrieve memories at both example and concept scales and perform heteroassociation between them. We obtain our results by deriving macroscopic mean-field equations that yield capacity formulas for sparse examples, dense examples, and dense concepts. We also perform network simulations that verify our theoretical results and explicitly demonstrate the capabilities of the network.Comment: 34 pages including 21 pages of appendices, 9 figure

Transurethral radiofrequency collagen denaturation for the treatment of women with urinary incontinence

Peer reviewedPublisher PD

A Formal Modeling and Analysis Framework for Software Product Line of Preemptive Real-Time Systems

International audienceThis paper presents a formal analysis framework to analyze a family of platform products w.r.t. real-time properties. First, we propose an extension of the widely-used feature model, called Property Feature Model (PFM), that distinguishes features and properties explicitly Second, we present formal behavioral models of components of a real-time scheduling unit such that all real-time scheduling units implied by a PFM are automatically composed to be analyzed against the properties given by the PFM. We apply our approach to the verification of the schedulability of a family of scheduling units using the symbolic and statistical model checkers of Uppaal

Crossing Borders, Organizations, Levels and Technologies: IS Collaboration in Humanitarian Relief

Humanitarian non-governmental organizations (NGOs) are increasingly facing complex challenges due to the high frequency of natural disasters and the growing number of actors in the humanitarian relief sector. One of these complex challenges is the management of information. In an attempt to mitigate these challenges, NGOs are increasingly collaborating through inter-organizational structures such as collaboration bodies to find mechanisms to coordinate information technologies. These collaboration bodies facilitate four kinds of “cross” collaboration; 1) cross organization, 2) cross border, 3) cross levels, and 4) cross technology. Within each collaboration body the role and function of a project also takes on special significance as much of the cross collaboration activities are channeled through projects that cross all four types of collaboration. In this paper we examine four case studies set in two collaboration bodies focused on IT in the humanitarian sector