Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F44: Focus Session: Extreme Mechanics: Snapping, Jumping and Popping |
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Sponsoring Units: GSNP GSOFT Chair: Doug Holmes, Boston University Room: 214D |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F44.00001: Yanking a chain: Lift-off and snapping Pierre-Thomas Brun, Basile Audoly, Alain Goriely, Dominic Vella We revisit the first mechanics problem that everyone meets in high school: a chain on a frictionless pulley. Rather than considering the problem of a mass on at each end of the string, however, we suppose that one end is subject to a constant acceleration. This simple change has some dramatic consequences for the ensuing motion: the chain `lifts off' from the pulley, the free end accelerates faster than the end that is being pulled and finally the chain undergoes a dramatic reversal of curvature reminiscent of the crack or snap of a whip. We present simple experiments, numerical simulations and theoretical arguments that explain some but not all of these phenomena. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F44.00002: Unraveling the chain fountain John Biggins, Mark Warner If a chain is initially at rest in a beaker at a height $h_1$ above the ground, and the end of the chain is pulled over the rim of the beaker and down towards the ground and then released, the chain will spontaneously ``flow'' out of the beaker under gravity. Furthermore, the beads do not simply drag over the edge of the beaker but form a fountain reaching a height $h_2$ above it. I will show that the formation of a fountain requires that the beads come into motion not only by being pulled upwards by the part of the chain immediately above the pile, but also by being pushed upwards by an unexpected reaction force from the pile of stationary chain. I will propose possible origins for this force, argue that its magnitude will be proportional to the square of the chain velocity, and predict and verify experimentally that $h_2\propto h_1$. I will also discuss the case where the pot is tilted, and show, experimentally and theoretically, that the chain rises and falls in an inverted catenary, and discuss the appropriate boundary conditions at the ends of the chain. [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F44.00003: Jumping, snapping and popping at nanometer scale David Haviland The 'jump-to-contact' instability is well known in Atomic Force Microscopy. When a tip attached to a soft cantilever approaches a surface, the large attractive force gradient disrupts the quasi-static force balance and the tip snaps in to contact with the surface. Less appreciated is the converse instability, where a soft liquid-like polymer surface jumps to meet the tip. This nano-scale pop is inaudible, but it does leave a distinctive signature if one carefully monitors the cantilever's steady state dynamics when driven with multiple tones. The nonlinear tip-surface interaction causes intermodulation, or frequency mixing of the drive tones. When many intermodulation products are measured close to the cantilever resonance the spectrum can be transformed to reveal the in-phase and quadrature forces acting on the tip, as a function of oscillation amplitude. We present experimental measurements and theoretical modelling that reveal this surface-jump-to-tip instability. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 9:12AM |
F44.00004: TBD Invited Speaker: Keith Seffen |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F44.00005: The chocolate-egg problem: Fabrication of thin elastic shells through coating Anna Lee, Joel Marthelot, Pierre-Thomas Brun, Pedro M. Reis We study the fabrication of thin polymeric shells based on the coating of a curved surface by a viscous fluid. Upon polymerization of the resulting thin film, a slender solid structure is delivered after demolding. This technique is extensively used, empirically, in manufacturing, where it is known as rotational molding, as well as in the food industry, e.g. for chocolate-eggs. This problem is analogous to the Landau-Levich-Derjaguin coating of plates and fibers and Bretherton's problem of film deposition in cylindrical channels, albeit now on a double-curved geometry. Here, the balance between gravity, viscosity, surface tension and polymerization rate can yield a constant thickness film. We seek to identify the physical ingredients that govern the final film thickness and its profile. In our experiments using organosilicon, we systematically vary the properties of the fluid, as well as the curvature of the substrate onto which the film is coated, and characterize the final thickness profile of the shells. A reduced model is developed to rationalize the process. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F44.00006: Folding of non-Euclidean curved shells Nakul Bende, Arthur Evans, Sarah Innes-Gold, Luis Marin, Itai Cohen, Christian Santangelo, Ryan Hayward Origami-based folding of 2D sheets has been of recent interest for a variety of applications ranging from deployable structures to self-folding robots. Though folding of planar sheets follows well-established principles, folding of curved shells involves an added level of complexity due to the inherent influence of curvature on mechanics. In this study, we use principles from differential geometry and thin shell mechanics to establish fundamental rules that govern folding of prototypical creased shells. In particular, we show how the normal curvature of a crease line controls whether the deformation is smooth or discontinuous, and investigate the influence of shell thickness and boundary conditions. We show that snap-folding of shells provides a route to rapid actuation on time-scales dictated by the speed of sound. The simple geometric design principles developed can be applied at any length-scale, offering potential for bio-inspired soft actuators for tunable optics, microfluidics, and robotics. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F44.00007: Curvature-induced symmetry breaking selects elastic wrinkling patterns Norbert Stoop, Romain Lagrange, Denis Terwagne, Pedro Reis, Joern Dunkel Wrinkling in curved bilayer surfaces is a ubiquitous phenomenon, including embryogenesis, biological tissue differentiation or structure formation in heterogenous thin films. Due to curved substrate and the strong nonlinearities in the elastic strains, predictions for the wrinkling morphology are notoriously difficult to obtain using classical analysis. Here, we derive a generalized Swift-Hohenberg theory to describe these morphologies and their pattern selection. Testing the theory against experiments on spherically shaped surfaces, we find quantitative agreement with analytical predictions for the phase transition curves separating labyrinth, hybrid and hexagonal wrinkling phases. Our approach builds on general differential-geometric principles and can be extended to arbitrarily shaped surfaces. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F44.00008: Bi-stable characteristics of thick-walled domes with applications to soft material snapping Amit Madhukar Bi-stable structures can exhibit interesting mechanical properties which makes them the focus of research in the field of extreme mechanics. Fast transitions can occur between equilibrium states with very little actuation force. One such bi-stable structure is the thick-walled dome. In this work, we apply finite element techniques to examine the stability of such spherical, thick-walled domes undergoing large deformation. We apply the following methods to two structures: a single-layered system as well as bi-layered colloidal microparticles which actuate through pH driven mismatched swelling. The presence of a metastable state is identified by the energy characteristics alone. Monotonically increasing energy represents a mono-stable structure. Bi-stability occurs when we achieve a local energy minimum at some non-zero displacement. Of more interest is the region near the transition of these states where we find a so called pseudo-bi-stable state where small perturbations results in fast transition from the elevated energy state, or snapping. We use our simulations to map out the critical geometric parameters that govern this behavior in order to design a dome to snap. Experimental results are used to validate the simulation results. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F44.00009: Morphing and Snapping of Plates and Shells via Swelling Douglas Holmes, Matteo Pezzulla, Paola Nardinocchi, Steven Shillig Non-homogenous swelling will induce curvature within thin structures - beams will bend and plates will morph into shells. In this work, we examine the dynamics of swelling plates as they deform into shells with either positive or negative Gaussian curvature. The swelling process is driven by a concentration gradient between two partially swollen structures, and the curvature of the final shell is dictated by the geometric arrangement of the swelling materials. The dynamics of this process are driven by diffusion and the geometry of the contact line. We demonstrate that these dynamic deformations can occur over a much faster timescale if the structure is confined. Beginning with a beam bent into an arch, we show how this swelling leads to a snap-through instability with dynamics similar to an arch compressed by a point load. The swelling-induced morphing presented in this talk provides a very simple and controllable way to achieve complex shell structures from simple building blocks. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F44.00010: Interface adhesion between 2D materials and elastomers measured by buckle delamination Christopher Brennan, Nanshu Lu A major application for 2D materials is creating electronic devices, including flexible and wearable devices. These applications require complicated fabrication processes where 2D materials are either mechanically exfoliated or grown via chemical vapor deposition and then transferred to a host substrate. Both processes require intimate knowledge of the interactions between the 2D material and the substrate to allow for a controllable transfer. Although adhesion between 2D materials and stiff substrates such as silicon and copper have been measured by bulge or peeling tests, adhesion between 2D materials and soft polymer substrates are hard to measure by conventional methods. Here we propose a simple way of measuring the adhesion between 2D materials and soft, stretchable elastomers using mature continuum mechanics equations. By creating buckle delamination in 2D atomic layers and measuring the buckle profile using an atomic force microscope, we can readily extract 2D-elastomer adhesion energy. Here we look at the adhesion of MoS2 and graphene to PDMS. The measured adhesion values are found insensitive to the applied strains in the substrate and are one order smaller than 2D-silicon oxide adhesion which is mainly attributed substrate surface roughness differences. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F44.00011: Harnessing snap-through instability for shape-recoverable energy-absorbing structure Sung Kang, Sicong Shan, Jordan Raney, Pai Wang, Francisco Candido, Jennifer Lewis, Katia Bertoldi Energy absorbing materials and structures are used in numerous areas for maintaining structural integrity, protection and comfort. To absorb/dissipate energy from shock/vibration, one generally relies on processes such as plastic deformation and damping as the case of metal foams and suspensions. Because plastic deformation and damping induce irreversible change in the energy-absorbing systems such as shape changes and degradation of damping elements by heat dissipation, it would be desirable to develop a new energy-absorption mechanism with reversibility. Furthermore, it would be desirable to implement energy-absorption mechanisms whose behavior is not affected by the rate of loading. Here, we report a shape-recoverable system that absorbs energy without degradation by harnessing multistability in elastic structures. Using numerical simulations, we investigate geometrical parameters that determine the onset of the snap-through and multi-stability. We subsequently manufacture structures with different geometrical parameters and sizes using a scalable direct-write 3D printing approach. We experimentally demonstrate reversible energy-absorption in these structures at strain rates over three orders of magnitudes, with reduced peak acceleration under impact by up to one order of magnitude compared with control samples. Our findings can open new opportunities for scalable design and manufacturing of energy-absorbing materials and structures. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F44.00012: Non-porous Elastic Sheets with Negative Poisson's Ratio Farhad Javid, Evelyne Smith-Roberge, Matthew Innes, Ali Shanian, Katia Bertoldi Negative Poisson's ratio (NPR) materials---materials that contract (expand) in transverse directions when compressed (stretched) uniaxially---have attracted significant interest both because of their unusual properties and their many potential applications. However, complex fabrication processes, high porosity, and low structural stiffness of most of the proposed NPR materials have significantly limited their practical applications. In this work, a novel NPR material is designed by coupling the in- and out-of-plane (popping) deformations in an elastic sheet with a periodic distribution of dimples. As a result, such NPR material has zero porosity, relatively high structural stiffness, and can be made from both hard and soft materials using industrial fabrication techniques. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F44.00013: Elastic and capillary failure of particle stabilized droplets Niveditha Samudrala, Raphael Sarfati, Jin Nam, Eric Dufresne Colloidal surfactants robustly stabilize fluid interfaces against spontaneous phase separation. Like molecular surfactants, they improve the thermodynamic and kinetic stability of the interface. However, particle stabilized interfaces are also thought to exhibit enhanced mechanical stability. Here, we investigate the mechanics of colloid-stabilized droplets using micro-pipette aspiration. We observe two distinct modes of failure: a classic buckling of the particle-laden interface similar to the buckling of a thin elastic shell, and a capillary failure where the encapsulated fluid is sucked out through the porous shell. To elucidate the underlying physics, we quantify the critical tension to drive each of these phenomena as a function of the size of the droplets, particles, and micro-pipette. [Preview Abstract] |
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