We perform molecular dynamics (MD) simulations of the crystallization process in binary Lennard-J... more We perform molecular dynamics (MD) simulations of the crystallization process in binary Lennard-Jones systems during heating and cooling to investigate atomic-scale crystallization kinetics in glass-forming materials. For the cooling protocol, we prepared equilibrated liquids above the liquidus temperature T l and cooled each sample to zero temperature at rate Rc. For the heating protocol, we first cooled equilibrated liquids to zero temperature at rate Rp and then heated the samples to temperature T > T l at rate R h . We measured the critical heating and cooling rates R * h and R * c , below which the systems begin to form a substantial fraction of crystalline clusters during the heating and cooling protocols. We show that R * h > R * c , and that the asymmetry ratio R * h /R * c includes an intrinsic contribution that increases with the glass-forming ability (GFA) of the system and a preparation-rate dependent contribution that increases strongly as Rp → R * c from above. We also show that the predictions from classical nucleation theory (CNT) can qualitatively describe the dependence of the asymmetry ratio on the GFA and preparation rate Rp from the MD simulations and results for the asymmetry ratio measured in Zr-and Au-based bulk metallic glasses (BMG). This work emphasizes the need for and benefits of an improved understanding of crystallization processes in BMGs and other glass-forming systems.
Bulk metallic glasses (BMGs) are amorphous alloys with desirable mechanical properties and proces... more Bulk metallic glasses (BMGs) are amorphous alloys with desirable mechanical properties and processing capabilities. To date, the design of new BMGs has largely employed empirical rules and trial-and-error experimental approaches. Ab initio computational methods are currently prohibitively slow to be practically used in searching the vast space of possible atomic combinations for bulk glass formers. Here, we perform molecular dynamics simulations of a coarse-grained, anisotropic potential, which mimics interatomic covalent bonding, to measure the critical cooling rates for metal-metalloid alloys as a function of the atomic size ratio σS/σL and number fraction xS of the metalloid species. We show that the regime in the space of σS/σL and xS where well-mixed, optimal glass formers occur for patchy and LJ particle mixtures, coincides with that for experimentally observed metal-metalloid glass formers. Thus, our simple computational model provides the capability to perform combinatorial ...
Predicting the side-chain dihedral angle distributions of nonpolar, aromatic, and polar amino acids using hard sphere models
Proteins, 2014
The side-chain dihedral angle distributions of all amino acids have been measured from myriad hig... more The side-chain dihedral angle distributions of all amino acids have been measured from myriad high-resolution protein crystal structures. However, we do not yet know the dominant interactions that determine these distributions. Here, we explore to what extent the defining features of the side-chain dihedral angle distributions of different amino acids can be captured by a simple physical model. We find that a hard-sphere model for a dipeptide mimetic that includes only steric interactions plus stereochemical constraints is able to recapitulate the key features of the back-bone dependent observed amino acid side-chain dihedral angle distributions of Ser, Cys, Thr, Val, Ile, Leu, Phe, Tyr, and Trp. We find that for certain amino acids, performing the calculations with the amino acid of interest in the central position of a short α-helical segment improves the match between the predicted and observed distributions. We also identify the atomic interactions that give rise to the differen...
We have investigated wavelength-dependent light scattering in biomimetic structures with short-ra... more We have investigated wavelength-dependent light scattering in biomimetic structures with short-range order. Coherent backscattering experiments are performed to measure the transport mean free path over a wide wavelength range. Overall scattering strength is reduced significantly due to short-range order and near-field effects. Our analysis explains why single scattering of light is dominant over multiple scattering in similar biological structures and is responsible for color generation.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2009
We study the response of dry granular materials to external stress using experiment, simulation, ... more We study the response of dry granular materials to external stress using experiment, simulation, and theory. We derive a Ginzburg-Landau functional that enforces mechanical stability and positivity of contact forces. In this framework, the elastic moduli depend only on the applied stress. A combination of this feature and the positivity constraint leads to stress correlations whose shape and magnitude are extremely sensitive to the nature of the applied stress. The predictions from the theory describe the stress correlations for both simulations and experiments semiquantitatively.
Using simulations of glassy systems under steady-state shear, we compare effective temperatures o... more Using simulations of glassy systems under steady-state shear, we compare effective temperatures obtained from static linear response with those from time-dependent fluctuation-dissipation relations. Although these two definitions are not expected to agree, we show that they yield the same answer over two and a half decades of effective temperature. This suggests that a more complete conceptual framework is necessary for effective temperatures in steady-state driven systems.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2014
We perform molecular dynamics simulations to compress binary hard spheres into jammed packings as... more We perform molecular dynamics simulations to compress binary hard spheres into jammed packings as a function of the compression rate R, size ratio α, and number fraction x(S) of small particles to determine the connection between the glass-forming ability (GFA) and packing efficiency in bulk metallic glasses (BMGs). We define the GFA by measuring the critical compression rate R(c), below which jammed hard-sphere packings begin to form "random crystal" structures with defects. We find that for systems with α≳0.8 that do not demix, R(c) decreases strongly with Δϕ(J), as R(c)∼exp(-1/Δϕ(J)(2)), where Δϕ(J) is the difference between the average packing fraction of the amorphous packings and random crystal structures at R(c). Systems with α≲0.8 partially demix, which promotes crystallization, but we still find a strong correlation between R(c) and Δϕ(J). We show that known metal-metal BMGs occur in the regions of the α and x(S) parameter space with the lowest values of R(c) for ...
Testing the Equal-Probability Assumption for Jammed Particle Packings1
... [8] JM Luck and A. Mehta: A Column of Grains in the Jamming Limit: Glassy Dynamics in the Com... more ... [8] JM Luck and A. Mehta: A Column of Grains in the Jamming Limit: Glassy Dynamics in the Compaction Process, Eur. Phys. J. B, Vol.35 (2003), pp.399411. [9] G. Gao, J. Blawzdziewicz and C. O'Hern: Fre-quency Distribution of Mechanically Stable Disk Packings, Phys. Rev. ...
Effective temperatures in a sheared, athermal system
We study fluctuations in a sheared, zero-temperature system numerically. We calculate five differ... more We study fluctuations in a sheared, zero-temperature system numerically. We calculate five different quantities (effective temperatures) that reduce to the true temperature in an equilibrium, thermal system. All five have the same shear-rate dependence, and three have the same value. We find that near the onset of jamming, the relaxation time is the same function of these three temperatures in the sheared system as of the true temperature in an unsheared system. We also test whether there is an effective thermal equilibrium in these systems using two different methods. We place two systems initially at two different effective temperatures in contact, shear them, and then study the time evolution of the effective temperature in each subsystem. We also study a system with a bidisperse particle size distribution and measure the effective temperatures of the small and large particles.
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Papers by Corey Ohern