Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W28: Focus Session: Soft-Matter, Biology, & Bioinspiration |
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Sponsoring Units: GSNP Chair: Carmel Majidi, Carnegie Mellon University Room: 336 |
Thursday, March 21, 2013 2:30PM - 2:42PM |
W28.00001: Cavitation in trees monitored using simultaneously acoustics and optics Alexandre Ponomarenko, Olivier Vincent, Philippe Marmottant Under hydric stress, in dry weather conditions, the sap within trees may reach extreme negative pressures and cavitate: bubbles appear, which eventually causes an embolism in the circulation. It has been shown that cavitation is associated with short acoustic emissions, and they can be recorded in the ultrasound range. However the precise origin of each acoustic emission is still not clear. In particular, the acoustic emissions could be not only the consequence of cavitation, but also of the collapse of xylem conduits, or of fractures in the wood. Here we present an original set-up where we can simultaneously record (i) the acoustic emissions, (ii) the location of cavitation events, by imaging the sap channels under light transmission microscopy. We are then able to correlate the sounds to the visible changes in channels, such as the appearance of cavitation bubbles. We hope the results of the present study might help to better understand the acoustic signals emitted by trees, and to obtain further information in the evolution of wood under dry stress conditions. [Preview Abstract] |
Thursday, March 21, 2013 2:42PM - 2:54PM |
W28.00002: Reversible Rigidity Control Using Low Melting Temperature Alloys Wanliang Shan, Tong Lu, Carmel Majidi Inspired by nature, materials able to achieve rapid rigidity changes have important applications for human body protection in military and many other areas. This talk presents the fabrication and design of soft-matter technologies that exhibit rapid reversible rigidity control. Fabricated with a masked deposition technique, the soft-matter composite contains liquid-phase and phase-changing metal alloys embedded in a soft and highly stretchable elastomer. The composite material can reversibly change its rigidity by three orders of magnitude and sustain large deformation. [Preview Abstract] |
Thursday, March 21, 2013 2:54PM - 3:06PM |
W28.00003: ``Lock and key mechanism'' for ligand binding with adrenergic receptors and the arising mechanical effects on the cell membrane Laura Lunghi, Luca Deseri Chemicals hitting the surface of cell aggregates are known to give arise to cyclic Adenosine Mono Phosphate (cAMP), a second messenger that transduces inside the cell the effects of species that cannot get through the cell membrane. Ligands bind to a specific receptor following the so called ``lock and key mechanism''; (beta)-adrenergic receptors are proteins embedded in the lipid bilayer characterized by seven transmembrane helices. Thinning and thickening in cell membranes may be initiated by conformational changes of some of three of the seven domains above. The cell response is linked to the coupling of chemical, conformational and mechanical effects. Part of the cAMP remains intracellular, whereas the remaining fractions migrates outside the cell due to membrane transporters. A new Helmholtz free energy, accounting for receptor and transporter densities, receptor conformation field and membrane elasticity is investigated. It is shown how the density of active receptors is directly related to the conformation field and it enters the resulting balance equation for the membrane stress. Balance laws for fluxes of transporters and receptors, coupled with the former because of the outgoing cAMP flux caused by the transporters, as well as for the diffusive powers must be supplied. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W28.00004: Geometrical study of the deformations of a thin spherical shell inspired by pollen grains. Etienne Couturier, Eleni Katifori, Jacques Dumais, Enrique Cerda Various monocotyledon pollen grains have a geometric design. They are constituted by a stiff thin shell with an n-fold rotationally symmetric softer sector. The mechanic response of these inhomogeneous shells can be approximated as an open shell. Isometric modes are known to be energetically favorable for thin shells when they are possible. Although the literature for the complete sphere, for which these modes are impossible, is extensive, analyses of the deformation of open shells whose isometric deformations are not inhibited, are much more scarce. We focus on the isometric deformation of spheres with n-fold rotationally symmetric openings. The isometric deformation means that the surface remains a constant gaussian curvature surface. Using differential geometry, we obtained an integrable family of surfaces whose gaussian curvature remains approximatively constant. We performed both simulations by tethered mesh methods and experiments with cut ping-pong balls. We observe that first the shell surface deforms without any stretching and is very well described as a part of an approximative constant gaussian curvature surface whose singularities remain outside the shell surface and get closer to the shell surface as the load increases. [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W28.00005: Hysteresis in the creasing instability of hydrogels and elastomers Dayong Chen, Shengqiang Cai, Lihua Jin, Zhigang Suo, Ryan Hayward Soft polymers placed under compressive stress can undergo an elastic creasing instability in which sharp folds spontaneously form on the free surfaces. This process can play an important role in a variety of material failure modes, but has also been harnessed to fabricate dynamic chemical and topographic patterns. Creases have been found to form by nucleation and growth, which we show reflects the influence of surface energy as a barrier for both processes. Hysteresis in the loading and unloading cycles is an important aspect of this process, but has been reported to occur to different degrees in different material systems. Through variations in interfacial energy, we show that for a model elastomeric system, it is self-adhesion within the folding region rather than plastic deformation that gives rise to hysteresis. [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 4:06PM |
W28.00006: Extreme Mechanics of Growing Matter Invited Speaker: Ellen Kuhl Growth is a distinguishing feature of all living things. Unlike standard materials, living matter can autonomously respond to alterations in its environment. As a result of a continuous ultrastructural turnover and renewal of cells and extracellular matrix, living matter can undergo extreme changes in composition, size, and shape within the order of months, weeks, or days. While hard matter typically adapts by increasing its density to grow strong, soft matter adapts by increasing its volume to grow large. Here we provide a state-of-the-art review of growing matter, and compare existing mathematical models for growth and remodeling of living systems. Applications are plentiful ranging from plant growth to tumor growth, from asthma in the lungs to restenosis in the vasculature, from plastic to reconstructive surgery, and from skeletal muscle adaptation to heart failure. Using these examples, we discuss current challenges and potential future directions. We hope to initiate critical discussions around the biophysical modeling of growing matter as a powerful tool to better understand biological systems in health and disease. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W28.00007: A micromechanical viscoelastic model for soft biological tissue Baptiste Coudrillier, Thao D. Nguyen Understanding the viscoelastic behavior of soft collageneous tissue from micromechanical considerations is critical to the characterization of their physiological and pathological response. In this study, we propose to model biological tissue as an aggregate of unit cells (UC). Each UC represents two wavy parallel collagen fibrils cross-linked by intrafibrillar bridges. A fibril consists of two linear springs deforming axially, and interconnected by a linear torsional spring modeling the fibril bending rigidity. When an axial displacement is applied to the unit cell, the uncrimping and stretching of the fibrils cause the ground substance to shear and the intrafibrillar bridges to rotate. This model assumes that the time-dependent behavior of the UC is due to the viscous rotation of the bridges, which are modeled as Maxwell solids. The constitutive equation of the tissue is calculated from the orientation average of the constitutive equation of the unit cell weighted by the probability density function for unit cell distribution. The performance of the model to predict the creep response will be illustrated using the results of an inflation test performed on the human sclera. [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W28.00008: Spatially localized structure-function relations in the elastic properties of sheared articular cartilage Jesse Silverberg, Lawrence Bonassar, Itai Cohen Contemporary developments in therapeutic tissue engineering have been enabled by basic research efforts in the field of biomechanics. Further integration of technology in medicine requires a deeper understanding of the mechanical properties of soft biological materials and the structural origins of their response under extreme stresses and strains. Drawing on the science generated by the ``Extreme Mechanics'' community, we present experimental results on the mechanical properties of articular cartilage, a hierarchically structured soft biomaterial found in the joints of mammalian long bones. Measurements of the spatially localized structure and mechanical properties will be compared with theoretical descriptions based on networks of deformed rods, poro-visco-elasticity, and standard continuum models. Discrepancies between experiment and theory will be highlighted, and suggestions for how models can be improved will be given. [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W28.00009: Highly Deformable Liquid Embedded Soft-Matter Capacitors and Inductors for Stretchable Electronics Andrew Fassler, Carmel Majidi We have developed a family of soft-matter capacitors and inductors that can be stretched to several times their natural length. These circuit elements are composed of microchannels of a liquid-phase Gallium-Indium-Tin alloy (Galinstan) embedded in a soft silicone elastomer (Ecoflex$^{\scriptsize{\textregistered}}$ 00-30). As the elastomer stretches, the embedded liquid channels deform, causing the capacitance and inductance to change monotonically. The relative changes in capacitance and inductance are experimentally measured as a function of stretch in three directions. The relationships found show potential for these devices to be used as strain sensors and tunable electronic filters. Additionally, theoretical predictions derived using finite elasticity kinematics are consistent with these experimentally found relationships. [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W28.00010: Delayed Fluid-Driven Fractures on Soft Gels Mark Schillaci, Joshua Bostwick, Karen Daniels A droplet of surfactant spreading on a weak gel substrate ($\sim 10$ Pa) can produce fractures on the gel surface, which originate at the contact-line and propagate outwards in a star-burst pattern. Experiments show that the number of arms is controlled by the ratio of the surface tension differential to the gel's shear modulus. We interpret the number of fractures formed in the context of a linear elastic model arising from the uncompensated, Young-Dupre (out-of-plane) force acting at the contact-line. However, we also observe that there is an inherent variability in both the number of fractures formed and the delay for fractures to form. In the regime where single fractures form, we observe a range of delay values consistent with a thermally-activated process. The mean delay time is set by the modulus of gel substrate, decreasing for weaker substrates. In the regime where multiple fractures form, we observe that all fractures appear simultaneously and the long delays are suppressed. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W28.00011: Soft-Matter Resistive Sensor for Measuring Shear and Pressure Stresses Daniel Tepayotl-Ramirez, Peter Roberts, Carmel Majidi Building on emerging paradigms in soft-matter electronics, we introduce liquid-phase electronic sensors that simultaneously measures elastic pressure and shear deformation. The sensors are com- posed of a sheet of elastomer that is embedded with fluidic channels containing eutectic Gallium- Indium (EGaIn), a metal alloy that is liquid at room temperature. Applying pressure or shear traction to the surface of the surrounding elastomer causes the elastomer to elastically deform and changes the geometry and electrical properties of the embedded liquid-phase circuit elements. We introduce analytic models that predict the electrical response of the sensor to prescribed surface tractions. These models are validated with both Finite Element Analysis (FEA) and experimental measurements. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W28.00012: Manufacturing of Liquid-Embedded Elastomers for Stretchable Electronics Rebecca Kramer, Carmel Majidi, James Weaver, Robert Wood Future generations of robots, electronics, and assistive medical devices will include systems that are soft, elastically deformable, and may adapt their functionality in unstructured environments. This will require soft active materials for power circuits and sensing of deformation and contact pressure. As the demand for increased elasticity of electrical components heightens, the challenges for functionality revert to basic questions of fabrication, materials, and design. Several designs for soft sensory skins (including strain, pressure and curvature sensors) based on a liquid-embedded-elastomer approach have been developed. This talk will highlight new ``soft MEMS'' manufacturing techniques based on wetting behavior between gallium-indium alloys and elastomers with varying microtextured surface topography. [Preview Abstract] |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W28.00013: Elastoswellability: Will it bend or will it buckle? Douglas Holmes, Anupam Pandey Soft mechanical structures such as biological tissues and gels exhibit motion, instabilities, and large morphological changes when subjected to external stimuli. Swelling is a robust approach for inducing structural change as it occurs naturally in humid environments and can be easily adapted for industrial design. Small volumes of fluid that interact favorably with a material can cause large, dramatic, and geometrically nonlinear deformations including beam bending, plate buckling, and surface wrinkling. In this talk we address an overarching question regarding swelling-induced deformations: will the structural change occur globally, or will it be confined to the material's surface? We introduce a materials and geometry defined transition point that describes a fluid-structure's characteristic ``elastoswellability'' lengthscale. By locally swelling unconstrained slender beams and plates with solvents of varying solubility, we identify a transition between local surface wrinkling and global structural bending. [Preview Abstract] |
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