Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B54: Extreme Deformation I: Cavitation and YieldingFocus
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Sponsoring Units: DPOLY GSOFT DFD DBIO Chair: Edwin Chan, National Institute of Standards and Technology Room: BCEC 254A |
Monday, March 4, 2019 11:15AM - 11:27AM |
B54.00001: Laser-induced Cavitation Dynamics of Polydimethylsiloxane with Varying Cross-Linking Density and Molecular Weight Sacchita Tiwari, Yue Zheng, Amir Kazemi-Moridani, Kelly McLeod, Ipek Sacligil, Christopher Barney, Alfred Crosby, Gregory Tew, Shengqiang Cai, Jae-Hwang Lee High-strain-rate mechanical properties of a cross-linked polymeric model system are relevant to understanding the dynamics and damage mechanisms of various biological tissues under HSR mechanical stimuli. We present the characterization of two material systems: 1) a commercially available polydimethylsiloxane (Sylgard 184) prepared with varying crosslinking times at fixed temperature; 2) a UV-curable polydimethylsiloxane having different controlled molecular weights. For high-strain-rate characterization, we performed laser-induced cavitation with ablation seeds. The ablation seed in a specimen was vaporized without dielectric breakdown, and produced a rapidly expanding cavity. The expansion dynamics of the laser-induced cavity was observed using ultrafast imaging. The dataset obtained from time-dependent radii of cavities was numerically analyzed, and the material's HSR mechanical parameters were identified. This study can lead us to establish a high-strain-rate mechanical characterization method for soft materials including various tissues. |
Monday, March 4, 2019 11:27AM - 11:39AM |
B54.00002: Modeling high-strain-rate microcavitation in soft materials: the role of material response Anastasia Tzoumaka, David Henann High-strain-rate inertial microcavitation has been shown to be an effective method for mechanical characterization of soft materials. To model inertial microcavitation Rayleigh-Plesset-based approaches are commonly used to capture the cavitation dynamics; however, these approaches are limited to simple viscoelastic constitutive models for the soft material, such as the Kelvin-Voigt model, since the implementation of more complex viscoelastic models requires the integration of complex mathematical expressions. To circumvent this limitation, we have developed a finite-element-based numerical simulation capability for inertial microcavitation that enables the incorporation of more complex constitutive laws. In this talk, we consider nonlinear elastic and power-law viscous constitutive laws and present a deeper investigation of the role of the elasticity and rheology of the soft material on the consequent cavitation dynamics. We apply our simulation capability by comparing computational results with experimental data from high-strain-rate inertial microcavitation of polyacrylamide and collagen gels in order to mechanically characterize these materials at high strain rates. |
Monday, March 4, 2019 11:39AM - 11:51AM |
B54.00003: Crack Geometry Dependence on Kinetic Energy and Loading Rate in High-Speed Cavitation Matthew Milner, Shelby Hutchens Temporary cavitation occurs when a projectile impacts a soft material causing a large radial expansion of the wound tract immediately after the projectile passes. Though cavity size is known to scale with the kinetic energy of the projectile, the fracture-governed damage accompanying this large deformation remains poorly understood. Using a custom designed table-top ballistic cavitation device, we replicate the temporary cavity phenomenon in soft tissue simulants on a small scale by applying a fast, high-pressure pulse of air through a needle. Temporary cavities produced via air pulse (characterized by energy density, loading rate, and needle size) isolate the damage accompanying large and dynamic stretches from that associated with more complicated impact dynamics such as projectile tumble. We find that increasing kinetic energy density and loading rate both result in greater crack area, while transforming the fracture geometry from a single planar crack to multiple radial cracks. As a result, the dependence of accumulated damage, quantified with crack surface area, increases super-linearly with kinetic energy while the temporary cavitation volume is verified to remain approximately linear over the range of energies tested. |
Monday, March 4, 2019 11:51AM - 12:27PM |
B54.00004: Inertial Microcavitation in Soft Matter Invited Speaker: Christian Franck The last two decades have seen significant advances in the manufacturing and design of soft matter materials with tunable control across orders of magnitude in length scale and elastic modulus. Characterization of the mechanical behavior of this new class of emerging complex soft materials has been challenging, especially in the inertial regime at strain rates beyond 100/s. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B54.00005: The Sound of Light: Using optical breakdown to drive extreme mechanical excitations Athanasios Athanassiadis When a high-power laser is focused to a small spot in a fluid, nonlinear interactions at the focus can excite a plasma that expands explosively and emits a strong mechanical shock wave into the ambient medium. This phenomenon – called optical breakdown – can generate peak pressures exceeding 1 MPa a centimeter from the source, with pressure pulses typically lasting less than 1 microsecond. In liquids, optical breakdown is accompanied by the growth of a vapor bubble that rapidly expands from micron to millimeter scales. Together, the ultrafast shock and subsequent bubble expansion can be used to probe the dynamic mechanical response of materials at short time scales and large stress scales. In this talk, I will demonstrate how I have leveraged optical breakdown to remotely measure the mechanical properties of submerged solids. I will show how the mechanical excitation can be tuned optically, and discuss how this technique can be adapted to measure the mechanical response of soft media. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B54.00006: Molecular Dynamics Simulation of Polymeric Systems Under Shock Deformation John P Mikhail, Gregory C Rutledge Currently, mechanisms by which polymeric systems respond to extreme deformation conditions, in particular deformation due to shock waves, are not fully understood. This work uses molecular dynamics (MD) simulations of atomistically detailed models of both homogeneous and heterogeneous polymeric materials (with structural variations on the nanometer scale) to study the underlying physics of system response to shock waves. Simulations are performed using both equilibrium ("Hugoniostatted") and nonequilibrium MD. Semicrystalline polymers with different configurations and degrees of crystallinity are examined, with shocks applied either isotropically or directionally with respect to the crystalline-amorphous interface. The Hugoniot curves are calculated, along with shock velocity and particle velocity. Analysis of the vibrational state of the systems identifies normal modes, along with changes in the vibrational signature pre- to post-shock. This analysis enables validation against experimental results obtainable via Raman or IR spectroscopy and provides insight into deformation mechanisms. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B54.00007: Rheological properties of small-molecule liquids in elastohydrodynamic lubrication Mark Owen Robbins, Vikram Jadhao There is an ongoing debate concerning the rheological model that accurately captures the flow of small-molecule liquids in elastohydrodynamic lubrication (EHL). In EHL, liquids experience extreme pressures (>0.5 GPa) and strain rates (>100,000 s^-1). Rheological properties of squalane, a representative EHL fluid, are investigated using nonequilibrium molecular dynamics simulations for pressures up to 1.2 GPa, strain rates between 10^5 – 10^10 s^-1, and temperatures up to 100 C. Simulation results are consistent with experimental data for a broad range of equilibrium and nonequilibrium conditions. At high temperatures and low pressures, where Newtonian viscosity of squalane is low, shear-thinning associated with molecular alignment is observed, which can be described by power-law fluid models. As Newtonian viscosity rises above ~1 Pa-s, shear-thinning is increasingly dominated by thermally-activated flow processes. In these conditions, the stress-strain-rate behavior from simulations and available experimental data is consistent with the Eyring equation for over 10 decades in strain rate. The macroscopic rheological properties are correlated with underlying molecular orientation and dynamics. |
Monday, March 4, 2019 1:03PM - 1:15PM |
B54.00008: A Molecular View: the Mechanical Behavior of Polymer Networks Ziyu Ye, Robert Riggleman Crosslinked polymer networks swollen in solvent, such as hydrogels, have gained much attention for their unique structural and mechanical properties that mimic natural materials and biological tissues. Since the network structure is linked to material properties, it is important to understand the dynamic behavior of the network during mechanical deformation. As many synthetic and natural soft materials are highly inhomogeneous, an important step is to elucidate how structural heterogeneities, such as network defects and phase boundaries, affect the response of the network to deformation. We use coarse-grained molecular dynamics to investigate the response of swollen polymer networks to deformation and the influence of network heterogeneities on the response. We examine the influence of network structures on soft materials’ bulk mechanical properties using tensile deformation to study both reversible and irreversible responses in gels as a function of polymer concentration, strand length, and network defects. We find that the network formation process will dictate local network structure, which is highly correlated to the dynamic material response under deformation. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B54.00009: Expansion Instabilities of Tube-Like Defects in Polymer Gels Subjected to Hydrostatic Pressure Christopher Barney, Yue Zheng, Shengqiang Cai, Alfred Crosby Cavitation in soft solids is defined as the unstable expansion of a void within a body subjected to a negative hydrostatic pressure. Classically this expansion has been modeled assuming an initially spherical cavity, but the advent of needle-induced cavitation makes an initially cylindrical geometry experimentally relevant. Tube-like defects of varying aspect ratio are first created at the tip of a needle by incorporating a retraction step to the needle insertion process after puncturing soft polymer gels. Instability-like expansion of the cylindrical defects is then experimentally observed at pressures comparable to those expected of a spherical geometry. Complementary modeling shows that the change in initial geometry has little effect on the critical cavitation pressure. Together these measurements demonstrate that the onset pressure of instability-like expansion of cylindrical voids in soft gels does not deviate considerably from the classical spherical model which enables the confident measurement of local elastic properties in soft gels and biological tissues. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B54.00010: Cavitation to Study Brain Mechanics and Tissue Interface Strength Carey Dougan, Yue Zheng, Christopher Barney, Sualyneth Galarza, Shengqiang Cai, Alfred Crosby, Shelly Peyton Cavitation is the rapid expansion of an instability within a material. There is a considerable need to study cavitation in biological tissue, as cavitation-related damage has been implicated in explosive blast injuries on military personnel. Post-mortem analysis of human brains exposed to blasts revealed scarring at boundaries between white and gray matter, at the outermost layer, and at blood vessel/tissue interfaces. Our technique introduces a single bubble at the tip of a needle at a specific location and depth to determine localized brain properties and the impact of tissue boundaries on cavitation damage path. Finite element modeling concludes that if a tissue-tissue interface is weaker than the needle injection path, the cavitation damage propagates along the interface. Our data suggests that interfaces synergistically facilitate fracture propagation at critical pressures 5 kPa less than required to cavitate bulk thalamus. When cavitation occurs near the corpus callosum, we observe a fracture propagation along the corpus callosum and cerebral cortex interface. In deeper regions, fracture separates the amygdala from the thalamus region of brain. This approach allows us to study cavitation damage paths to better understand the mechanism for brain injury. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B54.00011: Volume-controlled Cavity Expansion for Probing of Local Elastic Properties in Soft Materials Shabnam Raayai, Zhantao Chen, Tal Cohen Ability to measure the mechanical properties of soft biological materials in vivo can enable physicians in offering more accurate diagnosis of diseases such as cancer. Needle-based cavity expansion techniques are thus capturing the attention of researchers for measurement of the local nonlinear elastic properties in soft materials. Here we introduce a volume-controlled cavity expansion procedure that builds on the Cavitation Rheology technique [1] without relying on the maximum recorded pressure. We show that by employing an effective cavity radius based on our volume measurements we can consistently collapse the experimental results onto the theoretical predictions, regardless of the specific damage or instability mechanism exhibited by the material. We confirm the applicability of this technique by using PDMS samples, presenting good agreement with results obtained via conventional techniques with less than 5% of scatter. Moreover, since this method does not require visual tracking of the cavity, it can be applied to measure the nonlinear elastic response in opaque samples. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B54.00012: The yielding transition of soft colloids Stefano Aime, Domenico Truzzolillo, Laurence Ramos, Luca Cipelletti We investigate the yielding transition of dense suspensions of colloids interacting via a soft repulsive potential by simultaneously measuring their mechanical response and microscopic dynamics under an oscillating shear deformation. |
Monday, March 4, 2019 2:03PM - 2:15PM |
B54.00013: Plasticity effects in thin film wrinkling: Wrinkling behavior of plastic films bonded to elastomers with large strain mismatch Rahul Gopalan Ramachandran, Junyu Yang, Sameer Damle, Spandan Maiti, Sachin Velankar We examine the mechanics of composite films comprising a SEPS elastomer layer sandwiched between two thinner surface layers of plastic (polyethylene). Upon stretching such composite films to over twice their length and then releasing, the plastic surface films develop a highly wrinkled surface texture. The mechanism for this texturing is that during stretching, the plastic layers yield and stretch irreversibly whereas the elastomer stretches reversibly. Thus upon releasing, the plastic layers buckle due to compressive stress imposed by the elastomer. Although the wrinkling process appears somewhat similar to the wrinkling of a stiff elastic film bonded to a soft elastic substrate, our experiments and simulations show that plasticity plays a major role at all stages: (1) during stretching, the plastic layer yields in tension; (2) during recovery, the plastic layer first yields in-plane in compression and then buckles; (3) post-buckling, plastic hinges are formed at high-curvature regions. Homogeneous wrinkles are predicted only within a finite window of material properties: if the yield stress is too low, the plastic layers yield in-plane without wrinkling, whereas if the yield stress is too high, non-homogeneous wrinkles are predicted. |
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