Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y17: Flow Near Jamming |
Hide Abstracts |
Room: 402 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y17.00001: Finite size analysis of zero-temperature jamming transition under applied shear stress Hao Liu, XiaoYi Xie, Ning Xu We generate jammed packings of frictionless spheres under constant shear stress by minimizing an enthalpy-like energy. At fixed volume fraction and shear stress, we enumerate jammed states out of a large number of independent minimizations. The yield stress is defined as the shear stress at which the probability of finding jammed states is 50{\%}. We find that the yield stress for three-dimensional systems with harmonic repulsion satisfies the finite size scaling, which implies a diverging length scale approaching the unjamming transition at zero temperature and shear stress. Interestingly, the same length scale is exhibited as well in finite size scaling of typical quantities concerned in the study of jamming at zero shear stress, including the potential energy, pressure, coordination number, and shear modulus. This consistency indicates that the length scale found here is robust and universal for three-dimensional systems with harmonic repulsion. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y17.00002: Pinning Susceptibility Near the Jamming Transition Samer Nashed, Amy Graves, Carl Goodrich, Elliot Padgett, Andrea Liu The study of jamming in the presence of pinned obstacles is of both practical and theoretical interest. In simulations of soft, bidisperse disks and spheres, we pin a small fraction, $n_f$ of particles prior to the equilibration process. The presence of pinned particles is known to lower the critical packing fraction, $\phi_J$, for jamming. Further, around this threshold there is a peak in a quantity which we have termed the ``pinning susceptibility'': $\chi_P = \lim_{n_f \rightarrow 0} \frac{\partial P_J (\phi, n_f)}{\partial n_f}$. In the thermodynamic limit, we have posited that $\chi_P \propto |\Delta \phi |^{-\gamma_P} $. Finite-size scaling calculations, involving careful fits of $P_J$ to logistic sigmoidal functions, yield a value for the critical exponent, $\gamma_P$. This new exponent is proposed to be independent of inter-particle potential. Its dependence on dimensionality (2 vs. 3 dimensions) will be discussed. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y17.00003: Soft(er) solids: Strain softening near jamming Brian Tighe, Julia Boschan, Ell\'ak Somfai Many solids become softer when sheared beyond a threshold strain. The strain softening crossover signals the breakdown of linear superposition and the onset of strain dependent elastic moduli. Using simulations of soft spheres close to their jamming transition, we probe the softening regime to characterize its strain and pressure dependence. We identify a threshold strain that vanishes at unjamming, indicating that marginal solids are easily driven into the softening regime and that softening, rather than linear response, is likely to be observed in experiments. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y17.00004: On the route to shear jamming, are fragile states real? Ling Zhang, Jie Zheng, Jie Zhang Shear jammed states have been discovered in recent experiments (Zhang et al Granular Matter 2010, Zhang et al Soft Matter 2010, and Bi et al Nature 2011). Due to the existence of friction between the system and the third dimension, it is unclear whether a fragile state would still exist along the route of shear jamming if the friction were completely eliminated. In a novel apparatus developed recently at SJTU, the friction is successfully eliminated by letting the particles float on the surface of a shallow water layer, revealing more details of the route of shear-jamming. Using high-precision force-gauge and simple-beam apparatus, we are able to measure small forces of three orders of magnitude below the limit of the photo-elastic resolution between particles and boundaries. In this talk, we are going to report the recent progress towards the understanding of the nature of the fragile states. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y17.00005: Dynamics of Unjammed Emulsions Rodrigo Guerra, Thomas Kodger, David Weitz Light scattering and NMR densitometry measurements of quiescent emulsions have shown that amorphous packings of soft, repulsive droplets unjam at osmotic pressures $10^5$ times larger than the typical droplet thermal energy density: $\frac{3\,k_B T}{4\pi R^3}$. This transition corresponds to the pressure at which the thermal fluctuations of individual droplet positions match the yield strain of the packing and drive the fluidization of the material. We use confocal microscopy to investigate the microscopic dynamics of this fluid-like phase and find them to be fundamentally different from those of conventional glass-forming liquids; cage-breaking dynamics are not evident from droplet mean squared displacements and the effective viscosity of the emulsion, though $10^5$ larger than the background fluid, appears largely insensitive to the confining pressure. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y17.00006: Rheology of Soft Colloids Near Rigidity Onset: Critical Scaling, Thermal and Non-thermal Responses Ye Xu, Anindita Basu, Tim Still, Paulo Arratia, Zexin Zhang, Kerstin Nordstrom, Jerry Gollub, Douglas Durian, Arjun Yodh We study the rheological behavior of colloidal suspensions composed of soft sub-micron-size hydrogel particles across the liquid-solid transition. Specifically, steady-state and frequency-dependent rheometric measurements of three-dimensional mono- and bi-disperse colloidal suspensions are carried out as a function of volume fraction. We found the shear stress versus strain-rate curves exhibit very similar critical scaling features characteristic of jamming transition reported in microfluidic experiments [1] and simulation [2,3]. On the other hand, the observed stresses and shear rates near rigidity onset differ significantly in suspensions with different particle size and stiffness. We understand the difference by normalizing the measured stress and strain-rate data by thermal stress and time scales, as suggested by recent simulation work [2,3]. In this context, the normalized data in our systems reside in a regime wherein thermal effects are important, though suspension rheology across the full range of microgel particle experiments appear to exhibit both thermal and athermal mechanisms. [1] K. N. Nordstrom, et al., Phys. Rev. Lett., 2010. [2] A. Ikeda, et al. Phys. Rev. Lett., 2012. [3] A. Ikeda, et al. Soft Matter, 2013. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y17.00007: Pressure Distribution and Critical Exponent in Statically Jammed and Shear-Driven Frictionless Disks Stephen Teitel, Daniel V{\aa}gberg, Yegang Wu, Peter Olsson We numerically study the distributions of global pressure that are found in ensembles of statically jammed and quasistatically sheared systems of bidisperse, frictionless, disks at fixed packing fraction $\phi$ in two dimensions. We use these distributions to address the question of how pressure increases as $\phi$ increases above the jamming point $\phi_J$, $p\sim |\phi - \phi_J|^y$. For statically jammed ensembles, our results are consistent with the exponent $y$ being simply related to the power law of the interparticle soft-core interaction. For sheared systems, however, the value of $y$ is consistent with a non-trivial value, as found previously in rheological simulations. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y17.00008: Effect of friction on shear jamming Dong Wang, Jie Ren, Joshua Dijksman, Robert Behringer Shear Jamming of granular materials was first found for systems of frictional disks, with a static friction coefficients $\mu_s \simeq 0.6$. Jamming by shear is obtained by starting from a zero-stress state with a packing fraction $\phi_S \leq \phi \leq \phi_J$ between $\phi_J$ (isotropic jamming) and a lowest $\phi_S$ for shear jamming. This phenomenon is associated with strong anisotropy in stress and the contact network in the form of ``force chains,'' which are stabilized and/or enhanced by the presence of friction. We address experimentally how reducing friction affects shear jamming by using either teflon disks of teflon wrapped photoelastic particles. The teflon disks were placed in a wall driven 2D shear apparatus, in which we can probe shear stresses mechanically. Teflon-wrapped disks were placed in a bottom driven 2D shear apparatus (Ren et al., PRL 2013). Both apparatuses provide uniform simple shear. In all low-$\mu$ experiments, the shear jamming occurred, as observed through stress increases on the packing. However, the low-$\mu$ differences observed for $\phi_J - \phi_S$ were smaller than for higher friction particles. Ongoing work is studying systems using hydrogel disks, which have a lower friction coefficient than teflon. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y17.00009: Dissipation and Rheology of Sheared Soft-Core Frictionless Disks Daniel V\aa gberg, Peter Olsson, S. Teitel We use numerical simulations to investigate the effect of different dissipative models on the shearing rheology of massive soft-core frictionless disks in two dimensions. We show that the presence of Newtonian (overdamped) vs Bagnoldian (inertial) rheology is related to the formation of large connected clusters of disks, and that sharp transitions may exist between the two as system parameters vary. In the limit of strongly inelastic collisions, we find that rheological curves collapse to a well-defined limit when plotted against an appropriate dimensionless strain rate. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y17.00010: High-velocity drag friction in granular media near the jamming point Yuka Takehara, Ko Okumura Drag friction that acts on a disk in a two-dimensional granular medium is studied at high packing fractions in a closed horizontal cell used in Ref. [1]. We concentrate on a high-velocity region, in which the dynamic component of the force, obtained as average of strongly fluctuating force, clearly scales with velocity squared. We change the packing fraction to experimentally access the rheology near the jamming point and we find that the dynamic force and fluctuation of the force tend to diverge as the packing fraction approaches the jamming point. This is in contrast with the case of soft colloids in which the stress is finite at the jamming point. In addition, we develop a simple theory, which takes into account a collective collision around the disk and is equipped with a length scale diverging towards the jamming point. This theory explains well the experimental data. Unexpectedly, the static component of the force, the total force and the fluctuation of the total force also diverge towards the jamming point, with virtually the same exponent.\\[4pt] [1] Y.Takehara, S. Fujimoto and K. Okumura, High-velocity drag friction in dense granular media, \textit{EPL}, \textbf {92} (2010) 44003. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y17.00011: Velocity fluctuations in hopper flow near the clogging transition Charles Thomas, Douglas Durian Dynamic arrest in granular systems continues to elude a comprehensive description. We consider granular flow from a hopper as a quintessential example of a system which can spontaneously evolve from a freely flowing state to a jammed state. With a large enough opening of size D, grains flow out freely. When D is smaller, however, grains flow for a period and then stop, and the entire hopper has clogged. A critical opening size Dc is defined as the smallest D for which the flow will never clog, and marks the clogging transition. We systematically investigate the grain motion in a quasi-2D hopper as a function of D when D > Dc. Using a high-speed camera, we track the particles and find their instantaneous velocities. We report on the fluctuations of these particle velocities relative to their time-averaged velocity. In other systems, this has been seen to grow on approach to jamming. Additionally, we describe the time scales associated with the intermittency of the flow. Diverging time scales are also a key characteristic of a system near jamming. Furthermore, a clog can be considered an intermittent event of indefinite duration. The similarities and contrasts between the clogging and the jamming transitions will further illuminate systems which undergo dynamic arrest. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y17.00012: Flow of Weakly Vibrated Granular Media Geert Wortel, Olivier Dauchot, Martin van Hecke We experimentally study the response of a granular material that is subjected to vibration and shear -- a combination that leads to very rich behavior. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y17.00013: The Role of Anisotropy in Hopper Flows Audrey Melville, Yaqi Hou, Junyao Tang, Joshua Dijksman, Robert Behringer In this work, we examine granular flows in a quasi-two-dimensional hopper. We use two high-speed cameras to record granular flows composed of photoelastic disks. Our dual camera approach provides synchronized particle tracking data and the force response of each particle. The photoelastic measurements allow us to extract a measure for the local anisotropy of the stress field. Using this data, we probe the relationship between the local flow dynamics with local measurements of the stress anisotropy, particle density, and pressure in the system. Current work includes correlating these quantities in the context of a shear jamming picture. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y17.00014: Evolutionarily designing the least obstructive hopper - with application on 3D printing Guo-Jie Gao, Corey O'Hern, Shigenobu Ogata Placing an obstacle near an orifice of a granular hopper has been shown to facilitate the gravitational granular flow through the orifice by a factor of 100 [I. Zuriguel et al., Phys. Rev. Lett. 107, 278001 (2011)]. Using multiple obstacles, we want to further clarify the physics behind this phenomenon, and study if this approach can further improve the control of the granular flow rate. We develop molecular dynamics (MD) simulations to study the discharging of frictionless grains, and figure out the best design of placing obstacles that discharges densely stored grains most efficiently using an evolutionary procedure that progressively exhausts all possible placements of multiple obstacles including the one recovering the original design in the cited literature. We emphasize the impact of applying our results to 3D printing that makes solid objects of virtually any shape using granular materials. [Preview Abstract] |
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