Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J45: Focus Session: Extreme Mechanics |
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Sponsoring Units: GSNP DPOLY Chair: Joel Marthelot, Massachusetts Institute of Technology Room: 216AB |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J45.00001: Graphene Statistical Mechanics Mark Bowick, Andrej Kosmrlj, David Nelson, Rastko Sknepnek Graphene provides an ideal system to test the statistical mechanics of thermally fluctuating elastic membranes. The high Young's modulus of graphene means that thermal fluctuations over even small length scales significantly stiffen the renormalized bending rigidity. We study the effect of thermal fluctuations on graphene ribbons of width W and length L, pinned at one end, via coarse-grained Molecular Dynamics simulations and compare with analytic predictions of the scaling of width-averaged root-mean-squared height fluctuations as a function of distance along the ribbon. Scaling collapse as a function of W and L also allows us to extract the scaling exponent eta governing the long-wavelength stiffening of the bending rigidity. A full understanding of the geometry-dependent mechanical properties of graphene, including arrays of cuts, may allow the design of a variety of modular elements with desired mechanical properties starting from pure graphene alone. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J45.00002: Fanning the Optimal Breeze with an Abanico Grace Goon, Joel Marthelot, Pedro Reis Flexible hand-held fans, or abanicos, are universally employed as cooling devices that are both portable and sustainable. Their to and fro axial motion about one's hand generates an airflow that increases the evaporation rate near the skin and refreshes. We study this problem in the context of fluid-structure interaction, through precision model experiments. We first characterize the elastic properties of a number of commercially available fans and evaluate their aerodynamic performance in a custom built apparatus. The air velocity profile that results from the flapping motion of the fan is characterized for different driving conditions. We then fabricate our own analogue model fans that comprise a thin elastic plate, shaped as a circular section, with an underlying substructure of radial slats. A systematic variation of the geometric and elastic parameters, along with an exploration of the parameter space of the periodic driving motion (amplitude and frequency), allows us to establish optimal design and operational conditions for maximal output of the generated airflow, while minimizing the input power. [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J45.00003: Theoretical and experimental analysis of mylar balloons Antonio Romaguera, Vincent D\'emery, Benny Davidovitch In the present study, we present a theoretical and experimental study of the problem known as the mylar balloon shape. The problem consists of inflating a balloon made of two circular discs of an unstretchable material sewed at the edge. A solution for this problem was given by W. H. Paulsen in 1994 for constrain free. In our analyzes, we fixed the height of the balloon and measure the inflated diameter. As a result, we were able to map the constrained shape in terms of the original mylar balloon's shape. The basic assumption of this problem is that the gravitational, stretching and bending energies are negligible compared with the mechanical energy $-pV$. Controlling the pressure and the height of the balloon, we are able to find the condition where these assumptions fail, specially in the limit $h\to 0$ for fixed $p$. A remarkable feature of this problem is the presence of wrinkles across the equator of the balloon. A precise description for that region must include the large deformation from the flat disc initial condition. We will also present some experimental data on the wrinkle's length and its connection with the pressure and height of the balloon. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J45.00004: Voltage Induced Buckling Instability, a Means for Advanced Functionality within Soft Materials Behrouz Tavakol, Sarah E. Beauchamp, Aschvin Chawan, Douglas P. Holmes Instabilities within structures composed of soft materials may provide advanced functionality. We use the buckling of thin dielectric plates for pumping fluids and controlling the flow rate within microchannels. When exposed to an electric field, a confined dielectric plate buckles out of the plane, and this buckling can stop or enhance the flow rate of surrounding media. Compliant or grease electrodes have conventionally been used to aid in voltage application to both sides of the dielectric film. Here we introduce fluid electrodes, which make this mechanism embeddable into micro devices, enable the buckling at lower voltages, and significantly enhance the rate of deformation. We show that this mechanism can function as a microvalve to control the flow rate, or as a micropump to enhance the flow rate. We also examine buckled shapes of the dielectric film using a scaled-up version with fluid electrodes. These reversible, voltage-induced buckling instabilities can potentially be used in variety of different applications to control or enhance fluid flow in micro devices. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J45.00005: Discontinuous Buckling Luuk Lubbers, Corentin Coulais, Johannes Overvelde, Katia Bertoldi, Martin van Hecke Buckling of beams under uniaxial loading is perhaps the most basic example of an elastic instability. In this talk we show that sufficiently wide beams exhibit discontinuous buckling, an unstable form of buckling where the post-buckling stiffness is negative. We develop a 1D model that matches our experimental and numerical data and identify nonlinearity as the main cause for negative stiffness. We then utilize this non-linearity to create metamaterials that allow us to rationally design the (negative) post-buckling stiffness of metabeams, independently of beam thickness, thereby making it possible to violate Euler`s limit for slender beam buckling. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J45.00006: Bulk Elastic Fingering in Soft Materials Baudouin Saintyves, John Biggins, Zhiyan Wei, Serge Mora, L. Mahadevan, Elisabeth Bouchaud Systematic experiments have been performed in purely elastic polyacrylamide gels in Hele-Shaw cells. We have shown that a bulk fingering instability arises in the highly deformable confined elastomers. A systematic study shows that surface tension is not relevant. This instability is sub-critical, with a clear hysteretic behavior. Our experimental observations have been compared very favorably to theoretical and finite element simulations results. In particular, the instability wavelength and the critical front advance have been shown to be proportional to the distance between the two glass plates constituting the cell. A very important feature is that elasticity doesn't influence this lengthscale, making this instability very generic. We will also show some new results about an elastic counterpart experiment of the famous Saffman-Taylor experiment, where we push a soft gel in a stiff one. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J45.00007: Mechanical properties of 3D printed warped membranes Andrej Kosmrlj, Kechao Xiao, James C. Weaver, Joost J. Vlassak, David R. Nelson We explore how a frozen background metric affects the mechanical properties of solid planar membranes. Our focus is a special class of ``warped membranes'' with a preferred random height profile characterized by random Gaussian variables $h(q)$ in Fourier space with zero mean and variance $<|h(q)|^2 > \sim q^{-m}$. It has been shown theoretically that in the linear response regime, this quenched random disorder increases the effective bending rigidity, while the Young's and shear moduli are reduced. Compared to flat plates of the same thickness $t$, the bending rigidity of warped membranes is increased by a factor $\sim h_v/t$ while the in-plane elastic moduli are reduced by $\sim t/h_v$, where $h_v =\sqrt{ < |h(x)|^2 > }$ describes the frozen height fluctuations. Interestingly, $h_v$ is system size dependent for warped membranes characterized with $m>2$. We present experimental tests of these predictions, using warped membranes prepared via high resolution 3D printing. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J45.00008: Localization in an Idealized Heterogeneous Elastic Sheet Bekele Gurmessa, Andrew B. Croll Localized deformation is ubiquitous in many natural and engineering materials. Often times such deformations are associated to non-homogeneous strain fields in the materials. In this work we demonstrate the response of idealized non-homogenous elastic sheets to uniaxial compression. The idealized/patterned surface layers are created by selective ultraviolet/ozone (UVO) treatment of the top surface of polydimethylsiloxane (PDMS) using TEM grid mask. By controlling the exposure time of the UVO, samples ranging from continuous thin films to sets of isolated small plates were created. We show how local strains vary with location in a patterned sample, leading to a complex localization process Even at low strains. We also see that continuous regions form isotropic undulations upon compression which persist to high strains, well beyond where localization is observed in the patterned regions. Despite the complexity, the localized deformation profile can be adequately described with a simple elastic model when appropriate local boundary conditions are considered. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J45.00009: Primary and secondary bifurcations in compressed elastomeric bilayers with small modulus contrast Anesia Auguste, Lihua Jin, Zhigang Suo, Ryan C. Hayward Elastic materials undergo various kinds of elastic instabilities when subjected to compression. The primary bifurcation behavior for a stiff thin film on a thick soft substrate is wrinkling, whereas for a homogeneous material it is creasing. While ideal bilayered systems with large contrasts in modulus and thickness are well understood, many system in nature and engineering contexts are far from this simple case. We have developed an experimental system to systematically vary the modulus contrast, complemented by finite element simulations, to study the primary and secondary bifurcations in compressed bilayers. We find that below a ratio of film to substrate elastic modulus of approximately 2, the primary bifurcation is creasing. For slightly larger contrasts, the primary bifurcation is wrinkling but there are two distinct types of secondary bifurcations: (1) wrinkles that transition into creases without period-doubling; and (2) wrinkles to creases preceded by period doubling. Understanding surface instabilities in such non-ideal bilayer systems provides important insights on the behavior of biological tissues and other systems with a small modulus contrast. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J45.00010: The Structural Change of Buckling Depending on the Directional Mechanical Heterogeneity of Top Thin Films Dokyeong Kwon, Hyoseon Suh, Domin Kim, Kookheon Char Buckling of thin films on elastomeric substrates such as polydimethylsiloxane (PDMS) is the well-known phenomenon in buckling instability originating from the moduli mismatch between a substrate and a thin film placed on the top. Recently, many studies on the microstructure created by the buckling with flat top films have been reported and physics behind them has almost been well received. However, only a few work has been done for the buckling structure with micropatterned top films and buckling mechanics for patterned top film-PDMS bilayers has not yet been studied in detail. Here, we present the buckling of mechanically heterogeneous, patterned top films placed on top of elastomeric PDMS substrates. Mechanically heterogeneous top films were prepared by polystyrene (PS) films with topographic patterns. Buckling instability was induced by applying mechanical stresses to the PS-PDMS bilayer. Resulting buckling structure showed the structural change depending on the alignment of the top films with respect to the buckling direction. The structural change was analyzed with finite element method calculation, giving insights on the buckling mechanics of top film with complicated patterns placed on PDMS substrates. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J45.00011: High Aspect Ratio Wrinkles Yu-Cheng Chen, Alfred Crosby Buckling-induced surface undulations are widely found in living creatures, for instance, gut villi and the surface of flower petal cells. These undulations provide unique functionalities with their extremely high aspect ratios. For the synthetic systems, sinusoidal wrinkles that are induced by buckling a thin film attached on a soft substrate have been proposed to many applications. However, the impact of the synthetic wrinkles have been restricted by limited aspect ratios, ranging from 0 to 0.35. Within this range, wrinkle aspect ratio is known to increase with increasing compressive strain until a critical strain is reached, at which point wrinkles transition to localizations, such as folds or period doublings. Inspired by the living creatures, we propose that wrinkles can be stabilized in high aspect ratio by manipulating the strain energy in the substrate. We experimentally demonstrate this idea by forming a secondary crosslinking network in the wrinkled surface and successfully achieve aspect ratio as large as 0.8. This work not only provides insights for the mechanism of high aspect ratio structures seen in living creatures, but also demonstrates significant promise for future wrinkle-based applications. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J45.00012: Competition between adhesion and inertia during stick-slip peeling of Pressure Sensitive Adhesives M.-J. Dalbe, R. Villey, M. Ciccotti, P.-P. Cortet, S. Santucci, L. Vanel We consider the classical problem of the instable stick-slip dynamics often observed when peeling a pressure sensitive adhesive, quantifying for the first time experimentally the influence of the peeling angle. This instability is known to be the consequence of a decreasing fracture energy of the adhesive-substrate joint over a certain range of driving velocity: we focus here on the important case where the instability develops at large driving velocity. We show that the shape of the peeling front velocity fluctuations progressively changes from typical stick-slip relaxation oscillations to nearly sinusoidal oscillations as the peeling angle and/or the driving velocity is increased. This transition is accompanied with a change in the dependencies of the limit cycles' period on the control parameters. We show that it results from the competition, in the dynamical equation, between the standard fracture energy and a term --considered here for the first time-- associated to the freestanding tape elasticity and inertia. We manage to predict quantitatively the transition of the instability amplitude and period from the classical Barquins-Maugis quasistatic regime to a purely inertial regime in which the adhesion energy is no more at play in setting the instability limit cycles. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J45.00013: Influence of large strain rheology on the peeling performances of Pressure Sensitive Adhesives Richard Villey, Matteo Ciccotti, Costantino Creton, Pierre-Philippe Cortet, David J. Yarusso The dependence of adhesion energy of Pressure Sensitive Adhesives (PSA) on peeling velocity reduces to a master curve using a time-temperature superposition principle, usually verified by the linear rheology of polymers. This result has guided models predicting peeling energy of PSA to consider the small strain rheology of the glue only, despite it can experience very large strains before debonding. The argument of the time-temperature superposition principle can actually also be applied to large strains and is thus not a stringent one. To clarify the role of large strain rheology during the peeling of PSA, we present experiments on commercial and custom-made tapes supplied by 3M Company. Small and large strain rheology differences are obtained by changing the glass transition temperature, the cross-linking density and the density of entanglements, yet remaining close to commercial PSA. The rheology influence is decoupled from geometrical effects, by examining the nontrivial dependence of the adhesion energy on the peeling angle. Finally, adhesion energy measurements and visualizations of the process zone, over a large range of peeling velocities, are discussed, in the perspective of building a model for the adherence considering the complete rheology of the glue. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J45.00014: What can cracked polymer do Kexin Jiao, Chuanhong Zhou, Punit Kohli, Anish Poudel, Tsuchin Chu Buckling, delamination, and cracking are very well known phenomenon observed in most thin films. They were theoretically explained by the existence of mechanical instability due to the residue stress generated when a thin film is deposited on substrates or undergoing environmental stimulus. Buckled structures at micro- or nano-scale have been of great interests and have been used extensively in many applications including particles self-assembling, surface wettability modification, and micro-electronic device fabrication. However, peeling of a layer from a substrate due to delamination or fractures on a thin film due to cracking is mostly taken as an undesirable result. Therefore, strategies are inspired for preventing or removing these often undesired structures. We found that after being heated above its decomposition temperature and then cooled to room temperature, a PDMS thin film showed micro-fibers of 100$\mu $m width and up to 1.5 cm in length. By studying the formation mechanism, control of the dimensions and of the growth pattern on a substrate for PDMS micro-fibers were realized. Giving credit to their high flexibility and optical transparency, a PDMS micro-fiber were utilized in high resolution near field imaging achieved by attaching a micro-lens on the fiber. Interestingly, a surface covered by PDMS micro-fibers will turn from superhydrophobic into superhydrophilic by further heating providing potential applications in surface wettability modification. In future, we will investigate and simulate the growth of PDMS micro-fiber and look for more possible applications. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J45.00015: Formation of $^3$He droplets in dilute $^3$He-$^4$He solid solutions Chao Huan, Don Candela, Sung Kim, Liang YIn, Jiang-sheng Xia, Neil Sullivan We review the different stages of the formation of $^3$He droplets in dilute solid $^3$He-$^4$He solutions. The studies are interesting because the phase separation in isotopic helium mixtures is a first-order transition with a conserved order parameter. The rate of growth of the droplets as observed in NMR studies [1] is compared with the rates expected for homogeneous nucleation followed by a period of coarsening known as Ostwald ripening.\newline [1] C. Huan {\it et al.}, J. Low Temp. Phys. 162,167 (2011). [Preview Abstract] |
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