Interfacial systems with spherical symmetry are ubiquitous in nature and the accurate estimation of local self-diffusion coefficients in these systems is crucial to our understanding of processes such as the partitioning of atmospheric species to aerosol droplets and water transport across cell membranes. In this work, we extend a method originally developed to estimate local diffusion coefficients in systems with flat interfaces to the spherically symmetric case. Specifically, we derive an analytical solution to the linearized Smoluchowski equation in spherical coordinates and utilize molecular dynamics simulations to obtain a parameter required to estimate the local self-diffusion coefficient from the solution. We demonstrate that the derived solution is indeed accurate by comparing it to the numerical solution and also validate that the assumptions under which our solution was derived are not too stringent. We further validate our solution by computing the local diffusion coefficients at different radial positions in bulk SPC/E water and comparing the results to the overall diffusion coefficient obtained from Einstein's mean squared displacement method. Finally, we apply the method to an SPC/E water droplet suspended in its own vapor. We observe that the diffusion coefficient increases from the center of the droplet toward the interface, a result in line with previous results reported for flat interfaces.

Kuzkin's angular momentum balance method is implemented in the LAMMPS SPIN package for atomistic spin-lattice dynamics, along with shifted-force exchange and Neel Hamiltonians parameterized to minimize energy drifts in the simulations. Angular momentum contributions arising from two mechanisms are quantified using this method: particle transport across the boundaries of a periodic simulation domain and external torques applied to the domain by periodic image atoms. When these mechanisms are accounted for, lattice angular mo-mentum is exactly conserved in lattice systems and in spin-lattice systems with isotropic exchange interactions. The calculations show that spin-lattice angular momentum exchange only occurs when the Neel anisotropy energy is added to the exchange energy, and that with this addition, total angular momentum is approximately conserved in the magnetization direction but not in other directions. Inclusion of the Neel anisotropy increases the energy drifts observed in simulations of iron nanoparticles. These drifts are linearly proportional to the magnitude of the anisotropy energy and the simulation time step.

Silicon oxide-doped hydrogenated amorphous carbon (a-C:H:Si:O) is an important form of diamond-like carbon (DLC) for tribological applications, primarily because of its enhanced thermal stability and reduced dependence of friction on environmental humidity. As with other DLCs, its mechanisms of lubrication are still an active area of research, though it is now known that surface passivation and tribofilm growth are important factors. In this study, tribofilm formation for a-C:H:Si:O is examined at the microscale by using steel colloid atomic force microscopy probes as the sliding counterface. This approach provides some inherent advantages over macroscale tribology experiments, namely that the tribofilm thickness and stiffness can be tracked in situ and correlated directly with the friction response. The results of these experiments show that the tribofilm grows rapidly on the steel colloid following a period of counterface wear and high friction. The friction drops more than 80% upon nucleation of the tribofilm, which is attributed to a decrease of more than 80% in adhesion combined with a decrease in the estimated interfacial shear strength of at least 65%. Approximately 80% of the friction decrease occurs before the tribofilm reaches a thickness of 2 nm, suggesting that only the near-surface properties of the tribofilm provide the needed functionality for its effective lubrication mechanisms.

A neural network-assisted molecular dynamics method is developed to reduce the computational cost of open boundary simulations. Particle influxes and neural network-derived forces are applied at the boundaries of an open domain consisting of explicitly modeled Lennard-Jones atoms in order to represent the effects of the unmodeled surrounding fluid. Canonical ensemble simulations with periodic boundaries are used to train the neural network and to sample boundary fluxes. The method, as implemented in the LAMMPS, yields temperature, kinetic energy, potential energy, and pressure values within 2.5% of those calculated using periodic molecular dynamics and runs two orders of magnitude faster than a comparable grand canonical molecular dynamics system.

An iterative implicit solver is implemented in LAMMPS for spin lattice dynamics simulations. An approach to treat multibody interactions, specifically in metallic materials described by the embedded atom method, is presented for the first time in an implicit solver. Magnetization is extremely well preserved in this integrator, with fluctuations eight orders of magnitude smaller than those observed using a timestep of 0.1 fs in the explicit solver currently in LAMMPS. This is true even when larger timesteps than 0.1 fs are used in the implicit integrator, enabling the implicit integrator in some cases to have smaller magnetization error and shorter runtime than the explicit solver for the same overall simulation time. The integrator also conserves energy well. Energy error comparable to or lower than that of the explicit solver is achieved for implicit timesteps up to five times larger than the explicit timestep, with minimal increase in runtime. Errors in energy grow as timestep cubed, while errors in magnetization remain unchanged as timestep increases.

What exciting times we are living in! Our ability to measure, model, predict, and influence the thermophysical properties of materials at the microand nanoscale is now enabling exciting new advances with potential positive impacts in many important areas such energy, environment, information, medicine, and transportation. This journal, which was begun in 1997 by my post-doctoral advisor, Chang-Lin Tien, under the title “Microscale Thermophysical Engineering” was established at the very beginning of the nanotechnology revolution and it has always been a forum for thought leadership.

We conducted atomic force microscopy (AFM) experiments by sliding hard tetrahedral amorphous carbon (ta-C)-coated and diamond AFM probes against silicon oxide-doped hydrogenated amorphous carbon (a-C:H:Si:O) films. We reproducibly observe a substantial reduction in friction with repeated sliding. This behavior qualitatively resembles the run-in effects generally seen in macroscale frictional sliding on diamond-like carbons (DLCs), including this a-C:H:Si:O film in particular. As the applied normal load is increased with repetitive sliding, the friction reduces in tandem. The lateral stiffness of the nanoscale contact is measured as a function of applied normal load, thus the real contact area and the interfacial shear strength are inferred throughout the sliding experiments. These measurements show that the friction reduction is caused by a reduction in the interfacial shear strength of the contact. We propose that this arises from sliding-induced structural modification of the a-C:H:Si:O film. The calculated shear strengths are more than an order of magnitude higher than estimates from macroscale friction experiments. Additionally, humidity-controlled experiments show no significant humidity dependence of the friction despite a very strong dependence at macroscale. Reasons for these contradictions with macroscale experiments are discussed.

A custom-grown silicon and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) film is subjected to ball-on-flat tribometry under controlled sliding environments (ambient, dry air, and dry N2) at room temperature using a 52100 steel ball. The resulting friction coefficient is below 0.2 in ambient air and below 0.1 in dry N2. Tribofilms on the steel ball with thicknesses in excess of 500 nm are observed. The tribofilms are derived from the a-C:H:Si:O and grow on the steel ball, and display chemical and structural modifications relative to the original a-C:H:Si:O film. Sliding of the tribofilm-coated steel ball against bare silicon results in low friction, highlighting the inherent lubricity afforded by the tribofilm. Tribofilms grown through sliding against a-C:H:Si:O are characterized, post-sliding, with multiple spectroscopic and imaging techniques which collectively demonstrate that the composition and structure of the tribofilm is strongly dependent on the sliding environment. The unusually high tribofilm thickness allows for nanoindentation analysis, which demonstrates that the films are laterally heterogenous and softer than the original a-C:H:Si:O, with moduli and hardness values ranging over three orders of magnitude. Many regions of the tribofilms are extremely soft, with measured hardness values below 100 MPa and reduced Young’s moduli below 1 GPa, and also show a viscous mechanical response. Transmission electron microscopy and electron energy loss spectroscopy (TEM/EELS) characterization of the tribofilm demonstrates that the bulk structure is not graphitic, and indicates the tribofilms are enriched in C−H bonding. Additionally, there is a marked segregation within the tribofilm of Si/O and carbon. It is proposed that a primarily polymeric tribofilm structure can explain the observed mechanical properties.

Mass accommodation coefficient, a parameter that captures molecular transport phenomena at liquid-vapor interfaces, is essential for predicting the growth of liquid droplets during condensation processes but is difficult to obtain experimentally. Molecular simulations have been widely used to obtain accommodation coefficients for planar interfaces, but the applicability of planar accommodation coefficients to the high-curvature interfaces present in very small droplets is not clear. In this work, molecular dynamics simulations are used to compute equilibrium mass accommodation coefficients at different temperatures for small droplets of various fluids, including Lennard-Jones and Buckingham fluids, benzene, butane, methane, methanol, and water. For all fluids studied, the mass accommodation coefficient increases with droplet size to a constant limiting value and decreases with temperature. Furthermore, the accommodation coefficient curvature dependence collapses onto a universal curve when appropriately scaled.

The Cahn-Hilliard equation is often used to model the temporospatial evolution of multiphase fluid systems including droplets, bubbles, aerosols, and liquid films. This equation requires knowledge of the fluid-fluid interfacial mobility γ, a parameter that can be difficult to obtain experimentally. In this work, a method to obtain γ from nonequilibrium molecular dynamics is presented. γ is obtained for liquid-liquid and liquid-vapor interfaces by perturbing them from their equilibrium phase fraction spatial distributions, using molecular dynamics simulations to observe their relaxation toward equilibrium, and fitting the Cahn-Hilliard model to the transient molecular simulations at each time step. γ is then compared to a different measure of interfacial mobility, the molecular interfacial mobility M. It is found that γ is proportional to the product of M, the interface thickness, and the ratio of thermal energy to interfacial energy.

Silicon oxide-containing diamond-like carbon (a-C:H:Si:O) films are a promising class of protective coatings for environmentally-demanding applications owing to their lower residual stresses and superior thermal stability and oxidation resistance relative to undoped diamond-like carbon. However, existing versions of a-C:H:Si:O deposited by traditional methods such as plasma-enhanced chemical vapor deposition (PECVD) undergo substantial degradation and oxidation at temperatures above 250 degrees C. This, together with the difficulty of PECVD in depositing conformal coatings on complex geometries such as high-aspect-ratio features, has limited the applicability of a-C:H:Si:O. Here, the unique capabilities of plasma immersion ion implantation and deposition (PIIID) to grow silicon oxide-rich diamond-like carbon materials that are ultrasmooth, continuous, and conformal on high-aspect-ratio topographies are explored. The high concentration of silicon and oxygen in PIIID-grown films (23 +/- 5 at.% and 11 +/- 4 at.%, respectively) is unrivalled for this class of materials, and drastically increases the resistance to oxidation at high temperatures, compared with PECVD-grown films. The results open the path for using a-C:H:Si:O in applications involving exposure of materials to extreme environments.

Harsh environments pose materials durability challenges across the automotive, aerospace, and manufacturing sectors, and beyond. While amorphous carbon materials have been used as coatings in many environmentally-demanding applications owing to their unique mechanical, electrical, and optical properties, their limited thermal stability and high reactivity in oxidizing environments have impeded their use in many technologies. Silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) films are promising for several applications because of their higher thermal stability and lower residual stress compared to hydrogenated amorphous carbon (a-C:H). However, an understanding of their superior thermo-oxidative stability compared to a-C:H is lacking, as it has been inhibited by the intrinsic challenge of characterizing an amorphous, multi-component material. Here, we show that introducing silicon and oxygen in a-C:H slightly enhances the thermal stability in vacuum, but tremendously increases the thermo-oxidative stability and the resistance to degradation upon exposure to the harsh conditions of low Earth orbit (LEO). The latter is demonstrated by having mounted samples of a-C:H:Si:O on the exterior of the International Space Station via the Materials International Space Station (MISSE) mission 7b. Exposing lightly-doped a-C:H:Si:O to elevated temperatures under aerobic conditions or to LEO causes carbon volatilization in the near-surface region, producing a silica surface layer that protects the underlying carbon from further removal. These findings provide a novel physically-based understanding of the superior stability of a-C:H:Si:O in harsh environments compared to a-C:H.

Low thermal stability is a major limitation of diamond-like carbon (DLC) films, especially amorphous hydrogenated carbon (a-C:H) films, inhibiting their use in several applications. Adding silicon and oxygen to a-C:H increases thermal stability, but mechanisms for this increase are unknown. Reactive molecular dynamics (MD) simulations using the ReaxFF potential were performed on undoped a-C:H and a-C:H containing Si and O (a-C:H:Si:O). As in experiments, the simulated a-C:H:Si:O demonstrated increased thermal stability compared to a-C:H. Atomistic thermal degradation pathways were examined to understand the origins of the enhanced thermal stability of a-C:H:Si:O compared to a-C:H. The primary thermal degradation pathway in undoped a-C:H was the breaking of tensile strained C-C bonds resulting in a transformation of sp3 to sp2-hybridized carbon. The presence of Si suppresses this mechanism by decreasing the frequency of occurrence of highly strained C-C bonds in the unannealed structure. This is due to the longer C-Si equilibrium bond length compared to C-C bonds, which allows the Si-doped films to accommodate higher structural disorder.

Silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) coatings are amorphous thin-film materials composed of hydrogenated amorphous carbon (a-C:H), doped with silicon and oxygen. Compared to a-C:H, a-C:H:Si:O exhibits much lower susceptibility to oxidative degradation and higher thermal stability, making a-C:H:Si:O attractive for many applications. However, the physical mechanisms for this improved behavior are not understood. Here, the thermally induced structural evolution of a-C:H:Si:O was investigated in situ by X-ray photoelectron and absorption spectroscopy, as well as molecular dynamics (MD) simulations. The spectroscopy results indicate that upon high vacuum annealing, two thermally activated processes with a Gaussian distribution of activation energies with mean value E and standard deviation σ take place in a-C:H:Si:O: (a) ordering and clustering of sp2 carbon ( E ± σ = 0.22 ± 0.08 eV) and (b) conversion of sp3- to sp2-bonded carbon ( E ± σ = 3.0 ± 1.1 eV). The experimental results are in qualitative agreement with the outcomes of MD simulations performed using a ReaxFF potential. The MD simulations also indicate that the higher thermal stability of a-C:H:Si:O compared to a-C:H (with similar fraction of sp2-bonded carbon and hydrogen content) derives from the significantly lower fraction of strained carbon-carbon sp3 bonds in a-C:H:Si:O compared to a-C:H, which are more likely to break at elevated temperatures.

Superlattices and nano phononic crystals have attracted significant attention due to their low thermal conductivities and their potential application as thermoelectric materials. A widely used expression to calculate thermal conductivity, presented by Klemens and expressed in terms of the relaxation time by Callaway and Holland, originates from the Boltzmann transport equation. In its most general form, this expression involves a direct summation of the heat current contributions from individual phonons of all wavevectors and polarizations in the first Brillouin zone. In common practice, the expression is simplified by making an isotropic assumption that converts the summation over wavevector to an integral over wavevector magnitude. The isotropic expression has been applied to superlattices and phononic crystals, but its validity for different supercell sizes has not been studied. In this work, the isotropic and direct summation methods are used to calculate the thermal conductivities of bulk Si, and Si/Ge quantum dot superlattices. The results show that the differences between the two methods increase substantially with the supercell size. These differences arise because the vibrational modes neglected in the isotropic assumption provide an increasingly important contribution to the thermal conductivity for larger supercells. To avoid the significant errors that can result from the isotropic assumption, direct summation is recommended for thermal conductivity calculations in superstructures.

Dual control volume grand canonical molecular dynamics is used to perform the first calculation of fluid-fluid interfacial mobilities. The mobility is calculated from one-dimensional random walks of the interface by relating the diffusion coefficient to the interfacial mobility. Three different calculation methods are employed: one using the interfacial position variance as a function of time, one using the mean-squared interfacial displacement, and one using the time-autocorrelation of the interfacial velocity. The mobility is calculated for two liquid-liquid interfaces and one liquid-vapor interface to examine the robustness of the methods. Excellent agreement between the three calculation methods is shown for all the three interfaces, indicating that any of them could be used to calculate the interfacial mobility.

A nonequilibrium molecular dynamics method to induce fluid flow in nanochannels, the insertion-deletion method (IDM), is introduced. IDM inserts and deletes particles within distinct regions in the domain, creating locally high and low pressures. The benefits of IDM are that it directly controls a physically meaningful quantity, the mass flow rate, allows for pressure and density gradients to develop in the direction of flow, and permits treatment of complex aperiodic geometries. Validation of IDM is performed, yielding good agreement with the analytical solution of Poiseuille flow in a planar channel. Comparison of IDM to existing methods indicates that it is best suited for gases, both because it intrinsically accounts for compressibility effects on the flow and because the computational cost of particle insertion is lowest for low-density fluids.

In this work we discuss the idea of one-way acoustic signal isolation in low dimensional nanoelectromechanical oscillators. We report a theoretical study showing that one-way conversion between in-phase and anti-phase vibrational modes of a double layer graphene nanoribbon is achieved by introducing spatio-temporal modulation of system properties. The required modulation length in order to reach full conversion between the two modes is subsequently calculated. Generalization of the method beyond graphene nanoribbons and realization of a NEMS signal isolator are also discussed.