Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F22: Focus Session: Biological and Bio-inspired Adhesive Polymers I |
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Sponsoring Units: DPOLY DBIO GSNP Chair: Devin Kachan, University of California, Los Angeles Room: 407 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F22.00001: POLYMER PHYSICS PRIZE BREAK |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F22.00002: Toughening elastomers with sacrificial bonds and watching them break Invited Speaker: Costantino Creton Most unfilled elastomers are relatively brittle, in particular when the average molecular weight between crosslinks is lower than the average molecular weight between entanglements. We created a new class of tough elastomers by introducing isotropically prestretched chains inside ordinary acrylic elastomers by successive swelling and polymerization steps. These new materials combine a high entanglement density with a densely crosslinked structure reaching elastic moduli of 4 MPa and fracture strength of 25 MPa. The highly prestretched chains are the minority in the material and can break in the bulk of the material before catastrophic failure occurs, increasing the toughness of the material by two orders of magnitude up to 5 kJ/m$^{\mathrm{2}}$. To investigate the details of the toughening mechanism we introduced specific sacrificial dioxetane bonds in the prestretched chains that emit light when they break. In uniaxial extension cyclic experiments, we checked that the light emission corresponded exactly and quantitatively to the energy dissipation in each cycle demonstrating that short chains break first and long chains later. We then watched crack propagation in notched samples and mapped spatially the location of bond breakage ahead of the crack tip before and during propagation. This new toughening mechanism for elastomers creates superentangled rubbers and is ideally suited to overcome the trade-off between toughness and stiffness of ordinary elastomers. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F22.00003: Bond-breaking in semiflexible networks and the peeling dynamics of a filament from a random array of pinning sites Christian Vaca, Alex J. Levine Recent rheological experiments on cross-linked microtubule networks suggest that the principal dissipative mechanism at low frequencies is cross linker breakage. In such networks, applied stress leads to both the breaking of old cross links and the formation of new ones, allowing the network to maintain its elastic modulus while dissipating energy. We present a model of the underlying microscopic processes in such networks: the force-induced unbinding of a semiflexible filament from an array of randomly distributed pinning sites. These pinning sites dissociate from the filament with a force-dependent rate, as prescribed by e.g., the Bell model. This problem is part of a larger class of nonequilibrium systems that includes the driven motion of three phase contact lines and flux lines in superconductors, in which an elastic object is pulled through a quenched random potential. Using transfer matrix methods and numerical simulations, we explore the distribution of forces on the various pinning sites, and calculate the statistical properties of the filament's peeling dynamics under a constant force applied at one point perpendicular to its length. The mean peeling rate depends on the filament's bending modulus, the elasticity of the pinning sites, and their spatial distribution. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F22.00004: Mussel Adhesion is Significantly Enhanced Due to the Shape and Mechanics of Its Holdfast Kenneth Desmond, Nicholas Zacchia, Herbert Waite, Megan Valentine Mussels permanently adhere to surfaces through a circular plaque that is attached to the animal body via a long thin thread; forming a mushroom-shaped geometry. A plaque just a few millimeters in diameter with a 250-micron diameter thread can withstand large pull forces of a several Newtons without debonding. While the strength of individual chemical bonds plays a role in determining the adhesive strength, the contact mechanics associated with the mushroom shape is also critically important. In fact, numerous other organisms also use mushroom-shaped holdfasts to create strong bonds, suggesting the mushroom geometry is particularly effective for adhesion. To better understand the role of contact mechanics on the adhesive strength of mussels, we study mussel detachment using a custom built load frame capable of pulling on samples along any orientation and measuring the resulting force, while simultaneously imaging the plaque deformation and the glass-plaque interface. We will show that the holdfast shape improves bond strength by an order of magnitude compared to other simple geometries and that force-induced yielding of the mussel plaque improves the bond strength by another two orders of magnitude. These results show that by optimizing for contact mechanics, adhesive strength can be finely tuned for a particular application without changing the interface chemistry. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F22.00005: Equilibrium phase behavior of labile cross inkers in semiflexible networks Devin Kachan, Alex Levine, Robijn Bruinsma The equilibrium phase behavior of cross linkers in a network of semiflexible filaments is complex. The binding of the cross linkers affects the transverse undulations of the filaments leading to a fluctuation-mediate attractive or Casimir interaction between them. If the cross linkers also provide constraint torques to enforce a preferred binding angle between filaments, the resulting networks can have complex spatial distributions of filaments and of cross linkers bound to those filaments. Simulations report both the formation lamellar network structures and the aggregation of cross-linkers in thermal equilibrium. In this talk, we explore the the Casimir interaction between cross linkers bound to a given filament. We report on the spatial correlations between cross linkers bound to a given filament due to their Casimir interactions, and compare these theoretical predictions to the results of Brownian dynamics based finite element simulations of the system. We conclude with a discussion of the implications of these results for the equilibrium structure of semiflexible filament networks with labile cross linkers. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F22.00006: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F22.00007: The Role of Salts in the Evolution of Modern Orb-Webs. Vasav Sahni, Toshikazu Miyoshi, Kelley Chen, Dharamdeep Jain, Sean J. Blamires, Todd A. Blackledge, Ali Dhinojwala The evolution of modern viscid silk webs from ancient cribellate silk webs is associated with a 95{\%} increase in diversity of orb-weaving spiders, and their dominance as predators of flying insects. Yet the transition's mechanistic basis is an evolutionary puzzle. Ancient cribellate silk is a dry adhesive that functions through van der Waals interactions. Viscid threads adhere more effectively than cribellate threads due to high extensibility of their axial silk fibers, and firm adhesion of the viscid glue droplets. The organic and inorganic salts present in viscid glue sequester atmospheric water that plasticizes the axial silk fibers and renders them extensible. Here, we provide direct molecular and macro-scale evidence to show that salts also generate adhesion by directly solvating the glycoproteins, regardless of water content, thus imparting viscoelasticity and enabling the glue droplets to establish good contact. This `dual role' of salts provides a crucial link to the evolutionary transition from cribellate silk to viscid silk. In addition, salts also provide a simple mechanism to adhere even at the extremes of relative humidity, a feat eluding most synthetic adhesives. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F22.00008: Synthetic Adhesive Attachment Discs based on Spider Pyriform Silk Architecture Dharamdeep Jain, Vasav Sahni, Ali Dhinojwala Among the variety of silks produced by spiders, pyriform silk is used in conjunction with the dragline silk to attach webs to different surfaces. Cob weaver spiders employ different architectural patterns to utilize the pyriform silk and form attachment joints with each pattern having a characteristic adhesive performance. The staple pin architecture is a one of the strongest attachment designs employed by spiders to attach their webs. Here we use a synthetic approach to create the a similar patterned architecture attachment discs on aluminum substrate using thermoplastic polyurethane. Measurable pull off forces are generated when the synthetic discs are peeled off a surface. This innovative adhesive strategy can be a source of design in various biomedical applications. [Preview Abstract] |
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