Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G51: Invited Session: Frontiers of Soft Matter II |
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Sponsoring Units: GSOFT GSNP Chair: M. Cristina Marchetti, Syracuse University Room: Grand Ballroom C1 |
Tuesday, March 3, 2015 11:15AM - 11:51AM |
G51.00001: Entangled active matter: from ants to living cells Invited Speaker: Francoise Brochard-Wyart We introduce the field of ``Entangled Active Matter'' where the building blocks are transiently bound. We will point out strong similarities between aggregates of ants and cells! We will use multicellular aggregates, a model system for tissues. We characterize the tissue mechanical properties using pipette aspiration technique. The aggregate exhibits a viscoelastic response. We observe aggregate reinforcement with pressure, which may results in pulsed contractions or ``shivering.'' We interpret this reinforcement as a mechano-sensitive active response of the acto-myosin cortex. We describe the spreading of aggregates on rigid and soft substrates, varying both intercellular and substrate adhesion. We find both partial and complete wetting, with a precursor film forming a cellular monolayer in a liquid or gas phase. We model the dynamics of spreading from a balance between active cellular driving forces and permeation of cells to enter into the film. Finally we study the motility of aggregates induced by chemical or rigidity gradients, or spontaneous: on soft substrate, the precursor film is unstable, leading to a symmetry breaking and a global motion of the aggregate. We describe the shapes of migrating aggregates, the flow and the force field responsible of the motion. We monitored the center of mass motion and we characterize the stick-slip motions. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G51.00002: Free-standing thermalized graphene: a hard/soft hybrid Invited Speaker: David Nelson Understanding deformations of macroscopic thin plates and shells has a long and rich history, culminating with the Foeppl-von Karman equations in 1904. These highly nonlinear equations are characterized by a dimensionless coupling constant (the ``Foeppl-von Karman number'') that can easily reach vK $=$ 10$^7$ in an ordinary sheet of writing paper. Since the late 1980's, it has been clear that thermal fluctuations in microscopically thin elastic membranes fundamentally alter the long wavelength physics, leading to a negative thermal expansion coefficient, and a strongly scale-dependent bending energy and Young's modulus. Recent experiments from the McEuen group at Cornell that twist and bend individual atomically-thin free-standing graphene sheets (with vK $=$ 10$^{13}$!) call for a theory of the mechanical deformation of thermally excited membranes with large Foeppl-von Karman number. We present here results for the bending and pulling of thermalized graphene ribbons and tabs in the cantilever mode. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 1:03PM |
G51.00003: Liquids Gone With the Wind Invited Speaker: David Qu\'er\'e Self-propelling fluidic devices naturally result from some asymmetry of wettability, geometry or temperature. Here we consider the case of motions arising from the air around, forced by some trick to flow in an asymmetric way. We first consider vapor flows generated in a Leidenfrost situation, and made anisotropic by textures decorating the hot substrate. We discuss how the force and speed arising from these rectified vapor flows can be optimized. Then, we observe drops on a fiber placed in a symmetric wind. In a well-defined window of wind speed, the drop is found to self-propel along the fiber, which is analyzed. We also show that this effect makes drops moving in opposite direction bounce on each other, which generates fascinating 1-D dynamics.\\[4pt] In collaboration with Guillaume Dupeux, Philippe Bourrianne, Dan Soto, H\'el\`ene de Maleprade, Pierre-Brice Bintein, Hadrien Bense and Christophe Clanet. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:39PM |
G51.00004: Directed assembly in soft matter by energy stored in inclusions Invited Speaker: Kathleen Stebe We have been exploiting fields that arise spontaneously when microparticles are placed in contact with deformable matter to direct assembly. In one context, we use capillary interactions that occur between anisotropic microparticles at fluid interfaces. The fluid interface deforms owing to the particle presence, creating an area field that bears the signature of the particle shape and wetting. We use curvature fields to direct particles to migrate, orient and assemble. In another context, we exploit elastic energies and defect fields that arise in liquid crystals. When a nematic liquid crystal is confined using surfaces with complex topographies and well-defined anchoring energies, the director field and associated defect fields can be molded to store elastic energy which can be used to steer particles into rich assemblies. In a final context, we study the interaction of colloidal particles on giant vesicles under tension, in which particles interact via bending energies to interact with each other. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 2:15PM |
G51.00005: The Extreme Mechanics of Soft Structures Invited Speaker: Pedro Reis I will present a series of experimental investigations on the rich behavior of soft mechanical structures, which, similarly to soft materials, can undergo large deformations under a variety of loading conditions. Soft structures typically comprise slender elements that can readily undergo mechanical instabilities to achieve extreme flexibility and reversible reconfigurations. This field has came to be warmly known as ‘Extreme Mechanics’, where one of the fundamental challenges lies in rationalizing the geometric nonlinearities that arise in the post-buckling regime. I shall focus on problems involving thin elastic rods and shells, through examples ranging from the deployment of submarine cables onto the seabed, locomotion of uniflagellar bacteria, crystallography of curved wrinkling and its usage for active aerodynamic drag reduction. The main common feature underlying this series of studies is the prominence of geometry, and its interplay with mechanics, in dictating complex mechanical behavior that is relevant and applicable over a wide range of length scales. Moreover, our findings suggest that we rethink our relationship with mechanical instabilities which, rather than modes of failure, can be embraced as opportunities for functionality that are scalable, reversible, and robust. [Preview Abstract] |
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