Bio


Predicting mechanical strength of materials through theory and simulations of defect microstructures across atomic, mesoscopic and continuum scales. Developing new atomistic simulation methods for long time-scale processes, such as crystal growth and self-assembly. Introducing magnetic field in quantum simulations of electronic structure and transport.

Academic Appointments


Honors & Awards


  • Career Award, National Science Foundation (2006)
  • Presidential Early Career Award, National Science and Technology Council (2004)
  • Young Investigator Award, AFOSR (2006)
  • Beer and Johnston Outstanding New Mechanics Educator Award, American Society for Engineering Education (2008)
  • T. J. R. Hughes Young Investigator Award, ASME (2013)

Professional Education


  • PhD, MIT, Nuclear Engineering (2001)

2016-17 Courses


Stanford Advisees


All Publications


  • Highly stretchable polymer semiconductor films through the nanoconfinement effect SCIENCE Xu, J., Wang, S., Wang, G. N., Zhu, C., Luo, S., Jin, L., Gu, X., Chen, S., Feig, V. R., To, J. W., Rondeau-Gagne, S., Park, J., Schroeder, B. C., Lu, C., Oh, J. Y., Wang, Y., Kim, Y., Yan, H., Sinclair, R., Zhou, D., Xue, G., Murmann, B., Linder, C., Cai, W., Tok, J. B., Chung, J. W., Bao, Z. 2017; 355 (6320): 59-?

    Abstract

    Soft and conformable wearable electronics require stretchable semiconductors, but existing ones typically sacrifice charge transport mobility to achieve stretchability. We explore a concept based on the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. The fully stretchable transistors exhibit high biaxial stretchability with minimal change in on current even when poked with a sharp object. We demonstrate a skinlike finger-wearable driver for a light-emitting diode.

    View details for DOI 10.1126/science.aah4496

    View details for Web of Science ID 000391739900042

    View details for PubMedID 28059762

  • Stability of Eshelby dislocations in FCC crystalline nanowires INTERNATIONAL JOURNAL OF PLASTICITY Ryu, S., Cai, W. 2016; 86: 26-36
  • Spatiotemporal periodicity of dislocation dynamics in a two-dimensional microfluidic crystal flowing in a tapered channel. Proceedings of the National Academy of Sciences of the United States of America Gai, Y., Leong, C. M., Cai, W., Tang, S. K. 2016; 113 (43): 12082-12087

    Abstract

    When a many-body system is driven away from equilibrium, order can spontaneously emerge in places where disorder might be expected. Here we report an unexpected order in the flow of a concentrated emulsion in a tapered microfluidic channel. The velocity profiles of individual drops in the emulsion show periodic patterns in both space and time. Such periodic patterns appear surprising from both a fluid and a solid mechanics point of view. In particular, when the emulsion is considered as a soft crystal under extrusion, a disordered scenario might be expected based on the stochastic nature of dislocation dynamics in microscopic crystals. However, an orchestrated sequence of dislocation nucleation and migration is observed to give rise to a highly ordered deformation mode. This discovery suggests that nanocrystals can be made to deform more controllably than previously thought. It can also lead to novel flow control and mixing strategies in droplet microfluidics.

    View details for PubMedID 27790994

  • Dislocation Structure and Mobility in hcp He-4 PHYSICAL REVIEW LETTERS Borda, E. J., Cai, W., de Koning, M. 2016; 117 (4)
  • Advanced time integration algorithms for dislocation dynamics simulations of work hardening MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Sills, R. B., Aghaei, A., Cai, W. 2016; 24 (4)
  • Solute drag on perfect and extended dislocations PHILOSOPHICAL MAGAZINE Sills, R. B., Cai, W. 2016; 96 (10): 895-921
  • Anisotropic Size-Dependent Plasticity in Face-Centered Cubic Micropillars Under Torsion JOM Ryu, I., Cai, W., Nix, W. D., Gao, H. 2016; 68 (1): 253-260
  • Shape-Controlled, Self-Wrapped Carbon Nanotube 3D Electronics ADVANCED SCIENCE Wang, H., Wang, Y., Tee, B. C., Kim, K., Lopez, J., Cai, W., Bao, Z. 2015; 2 (9)
  • Stochastic behaviors in plastic deformation of face-centered cubic micropillars governed by surface nucleation and truncated source operation ACTA MATERIALIA Ryu, I., Cai, W., Nix, W. D., Gao, H. 2015; 95: 176-183
  • A Bamboo-Inspired Nanostructure Design for Flexible, Foldable, and Twistable Energy Storage Devices NANO LETTERS Sun, Y., Sills, R. B., Hu, X., Seh, Z. W., Xiao, X., Xui, H., Luo, W., Jin, H., Xin, Y., Li, T., Zhang, Z., Zhou, J., Cai, W., Huang, Y., Cui, Y. 2015; 15 (6): 3899-3906

    Abstract

    Flexible energy storage devices are critical components for emerging flexible electronics. Electrode design is key in the development of all-solid-state supercapacitors with superior electrochemical performances and mechanical durability. Herein, we propose a bamboo-like graphitic carbon nanofiber with a well-balanced macro-, meso-, and microporosity, enabling excellent mechanical flexibility, foldability, and electrochemical performances. Our design is inspired by the structure of bamboos, where a periodic distribution of interior holes along the length and graded pore structure at the cross section not only enhance their stability under different mechanical deformation conditions but also provide a high surface area accessible to the electrolyte and low ion-transport resistance. The prepared nanofiber network electrode recovers its initial state easily after 3-folded manipulation. The mechanically robust membrane is explored as a free-standing electrode for a flexible all-solid-state supercapacitor. Without the need for extra support, the volumetric energy and power densities based on the whole device are greatly improved compared to the state-of-the-art devices. Even under continuous dynamic operations of forceful bending (90°) and twisting (180°), the as-designed device still exhibits stable electrochemical performances with 100% capacitance retention. Such a unique supercapacitor holds great promise for high-performance flexible electronics.

    View details for DOI 10.1021/acs.nanolett.5b00738

    View details for Web of Science ID 000356316900037

    View details for PubMedID 26011653

  • Intrinsic bauschinger effect and recoverable plasticity in pentatwinned silver nanowires tested in tension. Nano letters Bernal, R. A., Aghaei, A., Lee, S., Ryu, S., Sohn, K., Huang, J., Cai, W., Espinosa, H. 2015; 15 (1): 139-146

    Abstract

    Silver nanowires are promising components of flexible electronics such as interconnects and touch displays. Despite the expected cyclic loading in these applications, characterization of the cyclic mechanical behavior of chemically synthesized high-quality nanowires has not been reported. Here, we combine in situ TEM tensile tests and atomistic simulations to characterize the cyclic stress-strain behavior and plasticity mechanisms of pentatwinned silver nanowires with diameters thinner than 120 nm. The experimental measurements were enabled by a novel system allowing displacement-controlled tensile testing of nanowires, which also affords higher resolution for capturing stress-strain curves. We observe the Bauschinger effect, that is, asymmetric plastic flow, and partial recovery of the plastic deformation upon unloading. TEM observations and atomistic simulations reveal that these processes occur due to the pentatwinned structure and emerge from reversible dislocation activity. While the incipient plastic mechanism through the nucleation of stacking fault decahedrons (SFDs) is fully reversible, plasticity becomes only partially reversible as intersecting SFDs lead to dislocation reactions and entanglements. The observed plastic recovery is expected to have implications to the fatigue life and the application of silver nanowires to flexible electronics.

    View details for DOI 10.1021/nl503237t

    View details for PubMedID 25279701

  • A three-dimensional phase field model for nanowire growth by the vapor-liquid-solid mechanism MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Wang, Y., Ryu, S., McIntyre, P. C., Cai, W. 2014; 22 (5)
  • Modeling a distribution of point defects as misfitting inclusions in stressed solids JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Cai, W., Sills, R. B., Barnett, D. M., Nix, W. D. 2014; 66: 154-171
  • Ideal Shear Strength of a Quantum Crystal PHYSICAL REVIEW LETTERS Landinez Borda, E. J., Cai, W., de Koning, M. 2014; 112 (15)

    Abstract

    Using path-integral Monte Carlo simulations, we compute the ideal shear strength (ISS) on the basal plane of hcp (4)He. The failure mode upon reaching the ISS limit is characterized by the homogeneous nucleation of a stacking fault and it is found to be anisotropic, consistent with Schmid's law of resolved shear stress. Comparing the ISS of hcp (4)He to a large set of classical crystals shows that it closely fits the approximately universal modified Frenkel model of ideal strength. In addition to giving quantitative stress levels for the homogeneous nucleation of extended defects in hcp (4)He, our findings lend support to assumptions in the literature that inherently classical models remain useful for the description of mechanical behavior in quantum crystals.

    View details for DOI 10.1103/PhysRevLett.112.155303

    View details for Web of Science ID 000334597300007

    View details for PubMedID 24785047

  • Efficient time integration in dislocation dynamics MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Sills, R. B., Cai, W. 2014; 22 (2)
  • Efficient Time Integrators for Dislocation Dynamics Simulations Modelling and Simulation in Materials Science and Engineering Sills, R., Cai, W. 2014; 24: 025003
  • Stress dependence of cross slip energy barrier for face-centered cubic nickel JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Kang, K., Yin, J., Cai, W. 2014; 62: 181-193
  • Stress Dependence of Cross Slip Energy Barrier for Face-Centered Cubic Metals Journal of the Mechanics and Physics of Solids Kang, K., Yin, J., Cai, W. 2014; 62: 181
  • Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Won, Y., Gao, Y., Panzer, M. A., Xiang, R., Maruyama, S., Kenny, T. W., Cai, W., Goodson, K. E. 2013; 110 (51): 20426-20430

    Abstract

    Reliably routing heat to and from conversion materials is a daunting challenge for a variety of innovative energy technologies--from thermal solar to automotive waste heat recovery systems--whose efficiencies degrade due to massive thermomechanical stresses at interfaces. This problem may soon be addressed by adhesives based on vertically aligned carbon nanotubes, which promise the revolutionary combination of high through-plane thermal conductivity and vanishing in-plane mechanical stiffness. Here, we report the data for the in-plane modulus of aligned single-walled carbon nanotube films using a microfabricated resonator method. Molecular simulations and electron microscopy identify the nanoscale mechanisms responsible for this property. The zipping and unzipping of adjacent nanotubes and the degree of alignment and entanglement are shown to govern the spatially varying local modulus, thereby providing the route to engineered materials with outstanding combinations of mechanical and thermal properties.

    View details for DOI 10.1073/pnas.1312253110

    View details for Web of Science ID 000328548600031

    View details for PubMedID 24309375

  • Conditional convergence in two-dimensional dislocation dynamics MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Kuykendall, W. P., Cai, W. 2013; 21 (5)
  • Atomistic simulations of grain boundary segregation in nanocrystalline yttria-stabilized zirconia and gadolinia-doped ceria solid oxide electrolytes ACTA MATERIALIA Lee, H. B., Prinz, F. B., Cai, W. 2013; 61 (10): 3872-3887
  • Plasticity of bcc micropillars controlled by competition between dislocation multiplication and depletion ACTA MATERIALIA Ryu, I., Nix, W. D., Cai, W. 2013; 61 (9): 3233-3241
  • Zipping, Entanglement, and the Modulus of Aligned Single-Walled Carbon Nanotube Films Won, Y., Gao, Y., Panzer, Matthew, A., Xiang, R., Maruyama, S., Kenny, Thomas, W., Cai, W. 2013
  • Modeling Dislocation Mechanisms of the Acoustic Nonlinearity in Metallic Crystals 9th International Workshop on Structural Health Monitoring (IWSHM) CASH, W. D., Cai, W. DESTECH PUBLICATIONS, INC. 2013: 1065–1072
  • Modelling plasticity of BCC micro-pillars using dislocation dynamics Acta Materialia Ryu, I., Nix, William, D., Cai, W. 2013; 61: 3233
  • On the existence of Eshelby's equivalent ellipsoidal inclusion solution MATHEMATICS AND MECHANICS OF SOLIDS Kuykendall, W. P., Cash, W. D., Barnett, D. M., Cai, W. 2012; 17 (8): 840-847
  • Nucleation-Controlled Distributed Plasticity in Penta-twinned Silver Nanowires SMALL Filleter, T., Ryu, S., Kang, K., Yin, J., Bernal, R. A., Sohn, K., Li, S., Huang, J., Cai, W., Espinosa, H. D. 2012; 8 (19): 2986-2993

    Abstract

    A unique size-dependent strain hardening mechanism, that achieves both high strength and ductility, is demonstrated for penta-twinned Ag nanowires (NWs) through a combined experimental-computational approach. Thin Ag NWs are found to deform via the surface nucleation of stacking fault decahedrons (SFDs) in multiple plastic zones distributed along the NW. Twin boundaries lead to the formation of SFD chains that locally harden the NW and promote subsequent nucleation of SFDs at other locations. Due to surface undulations, chain reactions of SFD arrays are activated at stress concentrations and terminated as local stress decreases, revealing insensitivity to defects imparted by the twin structures. Thick NWs exhibit lower flow stress and number of distributed plastic zones due to the onset of necking accompanied by more complex dislocation structures.

    View details for DOI 10.1002/smll.201200522

    View details for Web of Science ID 000309454800010

    View details for PubMedID 22829327

  • Singular orientations and faceted motion of dislocations in body-centered cubic crystals PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Kang, K., Bulatov, V. V., Cai, W. 2012; 109 (38): 15174-15178

    Abstract

    Dislocation mobility is a fundamental material property that controls strength and ductility of crystals. An important measure of dislocation mobility is its Peierls stress, i.e., the minimal stress required to move a dislocation at zero temperature. Here we report that, in the body-centered cubic metal tantalum, the Peierls stress as a function of dislocation orientation exhibits fine structure with several singular orientations of high Peierls stress-stress spikes-surrounded by vicinal plateau regions. While the classical Peierls-Nabarro model captures the high Peierls stress of singular orientations, an extension that allows dislocations to bend is necessary to account for the plateau regions. Our results clarify the notion of dislocation kinks as meaningful only for orientations within the plateau regions vicinal to the Peierls stress spikes. These observations lead us to propose a Read-Shockley type classification of dislocation orientations into three distinct classes-special, vicinal, and general-with respect to their Peierls stress and motion mechanisms. We predict that dislocation loops expanding under stress at sufficiently low temperatures, should develop well defined facets corresponding to two special orientations of highest Peierls stress, the screw and the M111 orientations, both moving by kink mechanism. We propose that both the screw and the M111 dislocations are jointly responsible for the yield behavior of BCC metals at low temperatures.

    View details for DOI 10.1073/pnas.1206079109

    View details for Web of Science ID 000309211000029

    View details for PubMedID 22949701

  • Ab initio kinetic Monte Carlo model of ionic conduction in bulk yttria-stabilized zirconia MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Lee, E., Prinz, F. B., Cai, W. 2012; 20 (6)
  • Computing dislocation stress fields in anisotropic elastic media using fast multipole expansions MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Yin, J., Barnett, D. M., Fitzgerald, S. P., Cai, W. 2012; 20 (4)
  • Dislocation dynamics simulation of Frank-Read sources in anisotropic alpha-Fe MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Fitzgerald, S. P., Aubry, S., Dudarev, S. L., Cai, W. 2012; 20 (4)
  • Contribution of dislocation dipole structures to the acoustic nonlinearity JOURNAL OF APPLIED PHYSICS CASH, W. D., Cai, W. 2012; 111 (7)

    View details for DOI 10.1063/1.3699362

    View details for Web of Science ID 000303282403001

  • Stress-driven migration of simple low-angle mixed grain boundaries ACTA MATERIALIA Lim, A. T., Haataja, M., Cai, W., Srolovitz, D. J. 2012; 60 (3): 1395-1407
  • Plasticity of metal nanowires JOURNAL OF MATERIALS CHEMISTRY Weinberger, C. R., Cai, W. 2012; 22 (8): 3277-3292

    View details for DOI 10.1039/c2jm13682a

    View details for Web of Science ID 000299695400001

  • Molecular Dynamics Comprehensive Nuclear Materials Cai, W., Li, J., Yip, S. edited by Konings, R. J., M. Elsevier. 2012: 249–265
  • Molecular Dynamics Comprehensive Nuclear Materials Cai, W., Li, J., Yip, S. edited by Konings, R. Elsevier. 2012: 249–265
  • Equilibrium shape of dislocation shear loops in anisotropic alpha-Fe MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Aubry, S., Fitzgerald, S. P., Dudarev, S. L., Cai, W. 2011; 19 (6)
  • Predicting the dislocation nucleation rate as a function of temperature and stress JOURNAL OF MATERIALS RESEARCH Ryu, S., Kang, K., Cai, W. 2011; 26 (18): 2335-2354
  • Molecular dynamics simulations of gold-catalyzed growth of silicon bulk crystals and nanowires JOURNAL OF MATERIALS RESEARCH Ryu, S., Cai, W. 2011; 26 (17): 2199-2206
  • Energy barrier for homogeneous dislocation nucleation: Comparing atomistic and continuum models SCRIPTA MATERIALIA Aubry, S., Kang, K., Ryu, S., Cai, W. 2011; 64 (11): 1043-1046
  • Entropic effect on the rate of dislocation nucleation PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Ryu, S., Kang, K., Cai, W. 2011; 108 (13): 5174-5178

    Abstract

    Dislocation nucleation is essential to our understanding of plastic deformation, ductility, and mechanical strength of crystalline materials. Molecular dynamics simulation has played an important role in uncovering the fundamental mechanisms of dislocation nucleation, but its limited timescale remains a significant challenge for studying nucleation at experimentally relevant conditions. Here we show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by determining the activation free energy from umbrella sampling. Our data reveal very large activation entropies, which contribute a multiplicative factor of many orders of magnitude to the nucleation rate. The activation entropy at constant strain is caused by thermal expansion, with negligible contribution from the vibrational entropy. The activation entropy at constant stress is significantly larger than that at constant strain, as a result of thermal softening. The large activation entropies are caused by anharmonic effects, showing the limitations of the harmonic approximation widely used for rate estimation in solids. Similar behaviors are expected to occur in other nucleation processes in solids.

    View details for DOI 10.1073/pnas.1017171108

    View details for Web of Science ID 000288894800012

    View details for PubMedID 21402933

  • Dislocation junctions and jogs in a free-standing FCC thin film MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Lee, S., Aubry, S., Nix, W. D., Cai, W. 2011; 19 (2)
  • The stability of Lomer-Cottrell jogs in nanopillars SCRIPTA MATERIALIA Weinberger, C. R., Cai, W. 2011; 64 (6): 529-532
  • Enhancing ionic conductivity of bulk single-crystal yttria-stabilized zirconia by tailoring dopant distribution PHYSICAL REVIEW B Lee, E., Prinz, F. B., Cai, W. 2011; 83 (5)
  • Dislocation contribution to acoustic nonlinearity: The effect of orientation-dependent line energy JOURNAL OF APPLIED PHYSICS Cash, W. D., Cai, W. 2011; 109 (1)

    View details for DOI 10.1063/1.3530736

    View details for Web of Science ID 000286219300159

  • Entropic Effect on the Rate of Dislocation Nucleation Ryu, S., Kang, K., Cai, W. 2011
  • Nanoscale patterning controls inorganic-membrane interface structure NANOSCALE Almquist, B. D., Verma, P., Cai, W., Melosh, N. A. 2011; 3 (2): 391-400

    Abstract

    The ability to non-destructively integrate inorganic structures into or through biological membranes is essential to realizing full bio-inorganic integration, including arrayed on-chip patch-clamps, drug delivery, and biosensors. Here we explore the role of nanoscale patterning on the strength of biomembrane-inorganic interfaces. AFM measurements show that inorganic probes functionalized with hydrophobic bands with thicknesses complimentary to the hydrophobic lipid bilayer core exhibit strong attachment in the bilayer. As hydrophobic band thickness increases to 2-3 times the bilayer core the interfacial strength decreases, comparable to homogeneously hydrophobic probes. Analytical calculations and molecular dynamics simulations predict a transition between a 'fused' interface and a 'T-junction' that matches the experimental results, showing lipid disorder and defect formation for thicker bands. These results show that matching biological length scales leads to more intimate bio-inorganic junctions, enabling rational design of non-destructive membrane interfaces.

    View details for DOI 10.1039/c0nr00486c

    View details for Web of Science ID 000287363500006

    View details for PubMedID 20931126

  • Analysis of the elastic strain energy driving force for grain boundary migration using phase field simulation SCRIPTA MATERIALIA Tonks, M., Millett, P., Cai, W., Wolf, D. 2010; 63 (11): 1049-1052
  • Size and temperature effects on the fracture mechanisms of silicon nanowires: Molecular dynamics simulations INTERNATIONAL JOURNAL OF PLASTICITY Kang, K., Cai, W. 2010; 26 (9): 1387-1401
  • Numerical tests of nucleation theories for the Ising models PHYSICAL REVIEW E Ryu, S., Cai, W. 2010; 82 (1)

    Abstract

    The classical nucleation theory (CNT) is tested systematically by computer simulations of the two-dimensional (2D) and three-dimensional (3D) Ising models with a Glauber-type spin flip dynamics. While previous studies suggested potential problems with CNT, our numerical results show that the fundamental assumption of CNT is correct. In particular, the Becker-Döring theory accurately predicts the nucleation rate if the correct droplet free energy function is provided as input. This validates the coarse graining of the system into a one dimensional Markov chain with the largest droplet size as the reaction coordinate. Furthermore, in the 2D Ising model, the droplet free energy predicted by CNT matches numerical results very well, after a logarithmic correction term from Langer's field theory and a constant correction term are added. But significant discrepancies are found between the numerical results and existing theories on the magnitude of the logarithmic correction term in the 3D Ising model. Our analysis underscores the importance of correctly accounting for the temperature dependence of surface energy when comparing numerical results and nucleation theories.

    View details for DOI 10.1103/PhysRevE.82.011603

    View details for Web of Science ID 000279941800008

    View details for PubMedID 20866625

  • Plasticity of metal wires in torsion: Molecular dynamics and dislocation dynamics simulations JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Weinberger, C. R., Cai, W. 2010; 58 (7): 1011-1025
  • Efficient computation of forces on dislocation segments in anisotropic elasticity MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Yin, J., Barnett, D. M., Cai, W. 2010; 18 (4)
  • Atomistic simulations of surface segregation of defects in solid oxide electrolytes ACTA MATERIALIA Lee, H. B., Prinz, F. B., Cai, W. 2010; 58 (6): 2197-2206
  • Validity of classical nucleation theory for Ising models PHYSICAL REVIEW E Ryu, S., Cai, W. 2010; 81 (3)

    Abstract

    While the classical nucleation theory (CNT) is widely used to predict the rate of first-order phase transitions, its validity has been questioned due to discrepancies with experiments. We systematically test the individual components of CNT by computer simulations of the Ising models and confirm its fundamental assumptions under a wide range of conditions ( h=0.01-0.13J , T=0.44-0.84Tc in two-dimensions and h=0.30-0.60J , T=0.48-0.62Tc in three dimensions). First, CNT accurately predicts the nucleation rate if the correct droplet free energy is provided. Furthermore, theoretical prediction of droplet free energy matches numerical results very well in the two-dimensional (2D) Ising model, if appropriate correction terms are added. This establishes the 2D Ising model as an important reference point where existing theories can predict nucleation rate accurately with no adjustable parameters.

    View details for DOI 10.1103/PhysRevE.81.030601

    View details for Web of Science ID 000276199300006

    View details for PubMedID 20365686

  • A gold-silicon potential fitted to the binary phase diagram JOURNAL OF PHYSICS-CONDENSED MATTER Ryu, S., Cai, W. 2010; 22 (5)

    Abstract

    We develop an empirical interatomic potential model for the gold-silicon binary system that is fitted to the experimental phase diagram. The model is constructed on the basis of the modified embedded-atom-method formalism and its binary phase diagram is computed by efficient free energy methods. The eutectic temperature and eutectic composition of the model match well with the experimental values. We expect the model to be useful for atomistic simulations of gold-catalyzed growth of silicon nanowires.

    View details for DOI 10.1088/0953-8984/22/5/055401

    View details for Web of Science ID 000273730300008

    View details for PubMedID 21386339

  • Kinetic Monte Carlo simulations of oxygen vacancy diffusion in a solid electrolyte: Computing the electrical impedance using the fluctuation-dissipation theorem ELECTROCHEMISTRY COMMUNICATIONS Lee, E., Prinz, F. B., Cai, W. 2010; 12 (2): 223-226
  • Orientation-Dependent Plasticity in Metal Nanowires under Torsion: Twist Boundary Formation and Eshelby Twist NANO LETTERS Weinberger, C. R., Cai, W. 2010; 10 (1): 139-142

    Abstract

    We show that the plastic deformation of nanowires under torsion can be either homogeneous or heterogeneous, regardless of size, depending on the wire orientation. Homogeneous deformation occurs when 110-oriented face-centered-cubic metal wires are twisted, leading to the nucleation of coaxial dislocations, analogous to the Eshelby twist mechanism. Heterogeneous deformation is predicted for 111 and 100 wires under torsion, localized at the twist boundaries. These simulations also reveal the detailed mechanisms of twist boundary formation from dislocation reactions.

    View details for DOI 10.1021/nl903041m

    View details for Web of Science ID 000273428700024

    View details for PubMedID 20030357

  • The Validity of Classical Nucleation Theory for Ising Models Physical Review E (Rapid Communications) Ryu, S., Cai, W. 2010; 81: 030601
  • Role of surface roughness in hysteresis during adhesive elastic contact PHILOSOPHICAL MAGAZINE LETTERS Kesari, H., Doll, J. C., Pruitt, B. L., Cai, W., Lew, A. J. 2010; 90 (12): 891-902

    Abstract

    In experiments that involve contact with adhesion between two surfaces, as found in atomic force microscopy or nanoindentation, two distinct contact force (P) vs. indentation-depth (h) curves are often measured depending on whether the indenter moves towards or away from the sample. The origin of this hysteresis is not well understood and is often attributed to moisture, plasticity or viscoelasticity. Here we report experiments that show that hysteresis can exist in the absence of these effects, and that its magnitude depends on surface roughness. We develop a theoretical model in which the hysteresis appears as the result of a series of surface instabilities, in which the contact area grows or recedes by a finite amount. The model can be used to estimate material properties from contact experiments even when the measured P-h curves are not unique.

    View details for DOI 10.1080/09500839.2010.521204

    View details for Web of Science ID 000283319100005

  • Improved modified embedded-atom method potentials for gold and silicon MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Ryu, S., Weinberger, C. R., Baskes, M. I., Cai, W. 2009; 17 (7)
  • Modelling dislocations in a free-standing thin film MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Weinberger, C. R., Aubry, S., Lee, S., Nix, W. D., Cai, W. 2009; 17 (7)
  • Mechanics of Crystalline Nanowires MRS BULLETIN Park, H. S., Cai, W., Espinosa, H. D., Huang, H. 2009; 34 (3): 178-183
  • Energy of a Prismatic Dislocation Loop in an Elastic Cylinder MATHEMATICS AND MECHANICS OF SOLIDS Cai, W., Weinberger, C. R. 2009; 14 (1-2): 192-206
  • Dislocation dynamics simulations in a cylinder International Conference on the Fundamentals of Plastic Deformation (DISLOCATIONS) Weinberger, C. R., Aubry, S., Lee, S., Cai, W. IOP PUBLISHING LTD. 2009
  • Comparison of thermal properties predicted by interatomic potential models MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Ryu, S., Cai, W. 2008; 16 (8)
  • Torsion and bending periodic boundary conditions for modeling the intrinsic strength of nanowires JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Cai, W., Fong, W., Eisen, E., Weinberger, C. R. 2008; 56 (11): 3242-3258
  • Comparing the strength of f.c.c. and b.c.c. sub-micrometer pillars: Compression experiments and dislocation dynamics simulations Symposium on Mechanical Behavior of Nanostructured Materials held TMS 2007 Annual Meeting Greer, J. R., Weinberger, C. R., Cai, W. ELSEVIER SCIENCE SA. 2008: 21–25
  • Surface-controlled dislocation multiplication in metal micropillars PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Weinberger, C. R., Cai, W. 2008; 105 (38): 14304-14307

    Abstract

    Understanding the plasticity and strength of crystalline materials in terms of the dynamics of microscopic defects has been a goal of materials research in the last 70 years. The size-dependent yield stress observed in recent experiments of submicrometer metallic pillars provides a unique opportunity to test our theoretical models, allowing the predictions from defect dynamics simulations to be directly compared with mechanical strength measurements. Although depletion of dislocations from submicrometer face-centered-cubic (FCC) pillars provides a plausible explanation of the observed size-effect, we predict multiplication of dislocations in body-centered-cubic (BCC) pillars through a series of molecular dynamics and dislocation dynamics simulations. Under the combined effects from the image stress and dislocation core structure, a dislocation nucleated from the surface of a BCC pillar generates one or more dislocations moving in the opposite direction before it exits from the surface. The process is repeatable so that a single nucleation event is able to produce a much larger amount of plastic deformation than that in FCC pillars. This self-multiplication mechanism suggests a need for a different explanation of the size dependence of yield stress in FCC and BCC pillars.

    View details for DOI 10.1073/pnas.0806118105

    View details for Web of Science ID 000259592400016

    View details for PubMedID 18787126

  • Quantum entanglement of formation between qudits PHYSICAL REVIEW A Ryu, S., Cai, W., Caro, A. 2008; 77 (5)
  • Computing image stress in an elastic cylinder JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Weinberger, C. R., Cai, W. 2007; 55 (10): 2027-2054
  • Electronic structure calculations in a uniform magnetic field using periodic supercells JOURNAL OF COMPUTATIONAL PHYSICS Lee, E., Cai, W., Galli, G. A. 2007; 226 (2): 1310-1331
  • Enabling strain hardening simulations with dislocation dynamics MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Arsenlis, A., Cai, W., Tang, M., Rhee, M., Oppelstrup, T., Hommes, G., Pierce, T. G., Bulatov, V. V. 2007; 15 (6): 553-595
  • Brittle and ductile fracture of semiconductor nanowires - molecular dynamics simulations PHILOSOPHICAL MAGAZINE Kang, K., Cai, W. 2007; 87 (14-15): 2169-2189
  • A hybrid method for computing forces on curved dislocations intersecting free surfaces in three-dimensional dislocation dynamics MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING Tang, M., Cai, W., Xu, G., Bulatov, V. V. 2006; 14 (7): 1139-1151
  • Geometric aspects of the ideal shear resistance in simple crystal lattices PHILOSOPHICAL MAGAZINE Bulatov, V. V., Cai, W., Baran, R., Kang, K. 2006; 86 (25-26): 3847-3859
  • Dislocation multi-junctions and strain hardening NATURE Bulatov, V. V., Hsiung, L. L., Tang, M., Arsenlis, A., Bartelt, M. C., Cai, W., Florando, J. N., Hiratani, M., Rhee, M., Hommes, G., Pierce, T. G., de la Rubia, T. D. 2006; 440 (7088): 1174-1178

    Abstract

    At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects called dislocations. First proposed theoretically in 1934 (refs 1-3) to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening, a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions that tie the dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed 'multi-junctions'. We first predict the existence of multi-junctions using dislocation dynamics and atomistic simulations and then confirm their existence by transmission electron microscopy experiments in single-crystal molybdenum. In large-scale dislocation dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication, thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.

    View details for DOI 10.1038/nature04658

    View details for Web of Science ID 000237080000041

    View details for PubMedID 16641992

  • A non-singular continuum theory of dislocations JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Cai, W., Arsenlis, A., Weinberger, C. R., Bulatov, V. V. 2006; 54 (3): 561-587
  • Stochastic simulation of dislocation glide in tantalum and Ta-based alloys JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Deo, C. S., Srolovitz, D. J., Cai, W., Bulatov, V. V. 2005; 53 (6): 1223-1247
  • Adaptive importance sampling Monte Carlo simulation of rare transition events JOURNAL OF CHEMICAL PHYSICS de Koning, M., Cai, W., Sadigh, B., Oppelstrup, T., Kalos, M. H., Bulatov, V. V. 2005; 122 (7)

    Abstract

    We develop a general theoretical framework for the recently proposed importance sampling method for enhancing the efficiency of rare-event simulations [W. Cai, M. H. Kalos, M. de Koning, and V. V. Bulatov, Phys. Rev. E 66, 046703 (2002)], and discuss practical aspects of its application. We define the success/fail ensemble of all possible successful and failed transition paths of any duration and demonstrate that in this formulation the rare-event problem can be interpreted as a "hit-or-miss" Monte Carlo quadrature calculation of a path integral. The fact that the integrand contributes significantly only for a very tiny fraction of all possible paths then naturally leads to a "standard" importance sampling approach to Monte Carlo (MC) quadrature and the existence of an optimal importance function. In addition to showing that the approach is general and expected to be applicable beyond the realm of Markovian path simulations, for which the method was originally proposed, the formulation reveals a conceptual analogy with the variational MC (VMC) method. The search for the optimal importance function in the former is analogous to finding the ground-state wave function in the latter. In two model problems we discuss practical aspects of finding a suitable approximation for the optimal importance function. For this purpose we follow the strategy that is typically adopted in VMC calculations: the selection of a trial functional form for the optimal importance function, followed by the optimization of its adjustable parameters. The latter is accomplished by means of an adaptive optimization procedure based on a combination of steepest-descent and genetic algorithms.

    View details for DOI 10.1063/1.1844352

    View details for Web of Science ID 000227140000006

    View details for PubMedID 15743217

  • Kinetic Monte Carlo method for dislocation migration in the presence of solute PHYSICAL REVIEW B Deo, C. S., Srolovitz, D. J., Cai, W., Bulatov, V. V. 2005; 71 (1)
  • Modelling Dislocations using a Periodic Supercell Handbook of Materials Modelling Cai, W. edited by Yip, S. Springer. 2005
  • Modeling Dislocations using a Periodic Supercell Handbook of Materials Modelling Cai, W. edited by Yip, S. Springer. 2005
  • Mobility laws in dislocation dynamics simulations 13th International Conference on the Strength of Materials (ICSMA 13) Cai, W., Bulatov, V. V. ELSEVIER SCIENCE SA. 2004: 277–281
  • Core energy and Peierls stress of a screw dislocation in bcc molybdenum: A periodic-cell tight-binding study PHYSICAL REVIEW B Li, J., Wang, C. Z., CHANG, J. P., Cai, W., Bulatov, V. V., Ho, K. M., Yip, S. 2004; 70 (10)
  • Ab initio calculations in a uniform magnetic field using periodic supercells PHYSICAL REVIEW LETTERS Cai, W., Galli, G. 2004; 92 (18)

    Abstract

    We present a formulation of ab initio electronic structure calculations in a finite magnetic field, which retains the simplicity and efficiency of techniques widely used in first principles molecular dynamics simulations, based on plane-wave basis sets and Fourier transforms. In addition we discuss results obtained with this method for the energy spectrum of interacting electrons in quantum wells, and for the electronic properties of dense fluid deuterium in a uniform magnetic field.

    View details for DOI 10.1103/PhysRevLett.92.186402

    View details for Web of Science ID 000221277900044

    View details for PubMedID 15169514

  • Dynamic transitions from smooth to rough to twinning in dislocation motion NATURE MATERIALS Marian, J., Cai, W., Bulatov, V. V. 2004; 3 (3): 158-163

    Abstract

    The motion of dislocations in response to stress dictates the mechanical behaviour of materials. However, it is not yet possible to directly observe dislocation motion experimentally at the atomic level. Here, we present the first observations of the long-hypothesized kink-pair mechanism in action using atomistic simulations of dislocation motion in iron. In a striking deviation from the classical picture, dislocation motion at high strain rates becomes rough, resulting in spontaneous self-pinning and production of large quantities of debris. Then, at still higher strain rates, the dislocation stops abruptly and emits a twin plate that immediately takes over as the dominant mode of plastic deformation. These observations challenge the applicability of the Peierls threshold concept to the three-dimensional motion of screw dislocations at high strain rates, and suggest a new interpretation of plastic strength and microstructure of shocked metals.

    View details for DOI 10.1038/nmat1072

    View details for Web of Science ID 000189345100017

    View details for PubMedID 14991017

  • Massively-parallel dislocation dynamics simulations IUTAM Symposium on Mesocopic Dynamics of Fracture Process and Materials Strength Cai, W., Bulatov, V. V., Pierce, T. G., Hiratani, M., Rhee, M., Bartelt, M., Tang, M. SPRINGER. 2004: 1–11
  • Dislocation Core Effects on Mobility Dislocations in Solids Cai, W., Bulatov, V., V., Chang, J., Li, J., Yip, S. edited by Nabarro, F. R., N., Hirth, J., P. North-Holland Pub. 2004: 1
  • Dislocation Core Effects on Mobility Dislocations in Solids Cai, W., Bulatov, V. V., Chang, J., Li, J., Yip, S. North-Holland Pub. 2004: 1–80
  • Scalable Line Dynamics in ParaDiS, Conference on High Performance Networking and Computing Bulatov, V., Cai, W., Fier, J., Hiratani, M., Pierce, T., Tang, M. 2004
  • Dislocation image stresses at free surfaces by the finite element method Symposium on Thin Films - Stresses and Mechanical Properties X held at the 2003 MRS Fall Meeting Tang, M. J., Xu, G. S., Cai, W., Bulatov, V. MATERIALS RESEARCH SOCIETY. 2004: 29–33
  • Modeling of dislocation-grain boundary interactions in FCC metals Workshop on Modeling and Experimental Validation de Koning, M., Kurtz, R. J., Bulatov, V. V., Henager, C. H., Hoagland, R. G., Cai, W., Nomura, M. ELSEVIER SCIENCE BV. 2003: 281–89
  • Anomalous dislocation multiplication in FCC metals PHYSICAL REVIEW LETTERS de Koning, M., Cai, W., Bulatov, V. V. 2003; 91 (2)

    Abstract

    Direct atomistic simulations of dislocation multiplication in fcc aluminum reveal an unexpected mechanism, in which a Frank-Read source emits dislocations with Burgers vectors different from that of the source itself. The mechanism is traced to a spontaneous nucleation of partial dislocation loops within the stacking fault. Understanding and a quantitative description of this unusual process are achieved through the development of a continuum model for dislocation nucleation based on the coarse-grained dislocation dynamics approach and a minimal amount of atomistic input.

    View details for DOI 10.1103/PhysRevLett.91.025503

    View details for Web of Science ID 000184086000023

    View details for PubMedID 12906487

  • Periodic image effects in dislocation modelling PHILOSOPHICAL MAGAZINE Cai, W., Bulatov, V. V., CHANG, J. P., Li, J., Yip, S. 2003; 83 (5): 539-567
  • Atomistic measures of materials strength and deformation Conference of the NATO-Advanced-Study-Institute on Computational Materials Science Li, J., Cai, W., CHANG, J. P., Yip, S. I O S PRESS. 2003: 359–387
  • Importance sampling of rare transition events in Markov processes PHYSICAL REVIEW E Cai, W., Kalos, M. H., de Koning, M., Bulatov, V. V. 2002; 66 (4)

    Abstract

    We present an importance sampling technique for enhancing the efficiency of sampling rare transition events in Markov processes. Our approach is based on the design of an importance function by which the absolute probability of sampling a successful transition event is significantly enhanced, while preserving the relative probabilities among different successful transition paths. The method features an iterative stochastic algorithm for determining the optimal importance function. Given that the probability of sampling a successful transition event is enhanced by a known amount, transition rates can be readily computed. The method is illustrated in one- and two-dimensional systems.

    View details for DOI 10.1103/PhysRevE.66.046703

    View details for Web of Science ID 000179176300135

    View details for PubMedID 12443376

  • Nodal effects in dislocation mobility PHYSICAL REVIEW LETTERS Bulatov, V. V., Cai, W. 2002; 89 (11)

    Abstract

    We show that, contrary to the prevailing perception, dislocations can become more mobile by zipping together to form junctions. In a series of direct atomistic simulations, the critical stress to move a junction network in a [110] plane of bcc molybdenum is found to be always smaller ( approximately 50%) than that required to move isolated dislocations. Our data support a previously proposed hypothesis about the nature of anomalous slip in bcc transition metals, yet offer a different atomistic mechanism for conservative motion of screw dislocation networks. The same data suggest a hierarchy of motion mechanisms in which lower-dimensional crystal imperfections control the rate of sliding along the low-angle twist boundaries.

    View details for DOI 10.1103/PhysRevLett.89.115501

    View details for Web of Science ID 000177676400015

    View details for PubMedID 12225147

  • Molecular dynamics simulations of motion of edge and screw dislocations in a metal Meeting of the International Union of Matierals Research Societies (IUMRS) Chang, J. P., Cai, W., Bulatov, V. V., Yip, S. ELSEVIER SCIENCE BV. 2002: 111–15
  • Kinetic Monte Carlo approach to modeling dislocation mobility Meeting of the International Union of Matierals Research Societies (IUMRS) Cai, W., Bulatov, V. V., Justo, J. F., Argon, A. S., Yip, S. ELSEVIER SCIENCE BV. 2002: 124–30
  • Dislocation motion in BCC metals by molecular dynamics International Conference on the Fundamentals of Plastic Deformation CHANG, J. P., Cai, W., Bulatov, V. V., Yip, S. ELSEVIER SCIENCE SA. 2001: 160–163
  • Point defect interaction with dislocations in silicon International Conference on the Fundamentals of Plastic Deformation Justo, J. F., de Koning, M., Cai, W., Bulatov, V. V. ELSEVIER SCIENCE SA. 2001: 129–132
  • Kinetic Monte Carlo modeling of dislocation motion in BCC metals International Conference on the Fundamentals of Plastic Deformation Cai, W., Bulatov, V. V., Yip, S., Argon, A. S. ELSEVIER SCIENCE SA. 2001: 270–273
  • Anisotropic elastic interactions of a periodic dislocation array PHYSICAL REVIEW LETTERS Cai, W., Bulatov, V. V., CHANG, J. P., Li, J., Yip, S. 2001; 86 (25): 5727-5730

    Abstract

    A method for calculating the anisotropic elastic energy of a dislocation dipole in a periodic cell is derived in which the infinite image summation is absolutely convergent. The core energy of a screw dislocation in Si, extracted from atomistic simulation, is shown to be manifestly system size invariant. Existence of special cell geometry where complete cancellation of elastic interactions occurs is demonstrated.

    View details for Web of Science ID 000169373000022

    View details for PubMedID 11415343

  • Parameter-free modelling of dislocation motion: the case of silicon PHILOSOPHICAL MAGAZINE A-PHYSICS OF CONDENSED MATTER STRUCTURE DEFECTS AND MECHANICAL PROPERTIES Bulatov, V. V., Justo, J. F., Cai, W., Yip, S., Argon, A. S., Lenosky, T., de Koning, M., de la Rubia, T. D. 2001; 81 (5): 1257-1281
  • Commentary on Atomistic Simulations of Materials Strength and Deformation: Prospects for Mechanistic Insights, Materials Science for the 21st Century Li, J., Cai, W., Chang, J., Yip, S. 2001; A: 220-233
  • Atomistic and Mesoscale Modeling of Dislocation Mobility PhD Thesis Cai, W. Massachusetts Institute of Technology. 2001
  • Periodic Boundary Conditions for Dislocation Dynamics Simulations in Three Dimensions Bulatov, V., V., Rhee, M., Cai, W. 2001
  • Minimizing boundary reflections in coupled-domain simulations PHYSICAL REVIEW LETTERS Cai, W., de Koning, M., Bulatov, V. V., Yip, S. 2000; 85 (15): 3213-3216

    Abstract

    We propose a time-dependent boundary condition coupling an atomistic simulation system to linear surroundings such that reflection of elastic waves across the boundary is minimized. Interdomain interactions expressed in terms of memory kernel functions within linear-response theory are treated in a natural dynamical manner, albeit numerically. The approach is shown to give significantly reduced phonon reflections at the domain boundaries relative to existing coupling methods. In addition, we demonstrate that the framework is also effective in the context of static relaxation of displacement fields associated with embedded inhomogeneities.

    View details for Web of Science ID 000089807800037

    View details for PubMedID 11019304

  • Efficient free-energy calculations by the simulation of nonequilibrium processes COMPUTING IN SCIENCE & ENGINEERING de Koning, M., Cai, W., Antonelli, A., Yip, S. 2000; 2 (3): 88-96
  • Intrinsic mobility of a dissociated Dislocation in silicon PHYSICAL REVIEW LETTERS Cai, W., Bulatov, V. V., Justo, J. F., Argon, A. S., Yip, S. 2000; 84 (15): 3346-3349
  • Vacancy interaction with dislocations in silicon: The shuffle-glide competition PHYSICAL REVIEW LETTERS Justo, J. F., de Koning, M., Cai, W., Bulatov, V. V. 2000; 84 (10): 2172-2175
  • Efficient Free-Energy Calculations by the Simulation of Nonequilibrium Processes Computing in Science and Engineering de Koning, M., Cai, W., Antonelli, A., Yip, S. 2000; 2: 88
  • Dynamics of dissociated dislocations in Si: A micro-meso simulation methodology Symposium on Multiscale Modelling of Materials, at the 1998 MRS Fall Meeting Cai, W., Bulatov, V. V., Justo, J. F., Yip, S., Argon, A. S. MATERIALS RESEARCH SOCIETY. 1999: 69–75
  • Kinetic Monte Carlo method for dislocation glide in silicon 5th International Conference on Advanced Materials (IUMRS) Cai, W., Bulatov, V. V., Yip, S. SPRINGER. 1999: 175–83
  • Kink asymmetry and multiplicity in dislocation cores PHYSICAL REVIEW LETTERS Bulatov, V. V., Justo, J. F., Cai, W., Yip, S. 1997; 79 (25): 5042-5045