On the Friction Behavior of SiO Tip Sliding on the Au(111) Surface: How Does an Amorphous SiO Tip Produce Regular Stick-Slip Friction and Friction Duality?

Friction behaviors of an amorphous SiO tip sliding on the Au(111) surface in atomic force microscopy (AFM) are investigated through molecular dynamics (MD) simulations. We observed a regime of extremely low, close-to-zero friction at low normal loads with clear stick-slip friction signals. The friction is almost independent of the applied normal load below a threshold value.

Will Polycrystalline Platinum Tip Sliding on a Gold(111) Surface Produce Regular Stick-Slip Friction?

Friction measurements by an atomic force microscope (AFM) frequently showed regular stick-slip friction signals with atomic-scale resolutions. Typically, for an AFM metal tip sliding on a metal crystal surface, the microstructure of the tip made from the thermally evaporated metal coating on a silicon cantilever was polycrystalline. Our detailed molecular dynamics(MD) simulations of a polycrystalline Pt tip ( = 10 nm in radius) sliding on an Au(111) surface revealed how the geometry of the polycrystalline tip took effect on the friction behavior at the contact interface.

Nucleation of Frank Dislocation during the Squeeze-Out Process in Boundary Lubrication: A Molecular Dynamics Study.

Liquid-vapor molecular dynamics (LVMD) simulations are performed to reinvestigate the phase transition and solvation force oscillation behavior of a simple argon liquid film confined between two solid surfaces. Our simulations present a novel scenario in which the n → n - 1 layering transitions are accompanied by the formation, climb, and annihilation of Frank partial dislocations during the squeeze-out process under compression. This is indicated by the splitting of the repulsive peaks in the solvation force profile.

A Molecular Dynamics Simulations Study of the Influence of Prestrain on the Pop-In Behavior and Indentation Size Effect in Cu Single Crystals.

The pop-in effect in nanoindentation of metals represents a major collective dislocation phenomenon that displays sensitivity in the local surface microstructure and residual stresses. To understand the deformation mechanisms behind pop-ins in metals, large scale molecular dynamics simulations are performed to investigate the pop-in behavior and indentation size effect in undeformed and deformed Cu single crystals. Tensile loading, unloading, and reloading simulations are performed to create a series of samples subjected to a broad range of tensile strains with/without pre-existing dislocations.

Measurement of Single-Molecule Forces in Cholesterol and Cyclodextrin Host-Guest Complexes.

Biological host molecules such as β-cyclodextrins (β-CDs) have been used to remove cholesterol guests from membranes and artery plaques. In this work, we calibrated the host-guest intermolecular mechanical forces (IMMFs) between cholesterol and cyclodextrin complexes by combining single-molecule force spectroscopy in optical tweezers and computational molecular simulations for the first time. Compared to native β-CD, methylated beta cyclodextrins complexed with cholesterols demonstrated higher mechanical stabilities due to the loss of more high-energy water molecules inside the methylated β-CD cavities.

Ultrafast photoinduced band splitting and carrier dynamics in chiral tellurium nanosheets.

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H and H and the degenerate H valence bands (VB) and the lowest degenerate H conduction band (CB) as well as a higher energy transition at the L-point.

Molecular Understanding of Ion Effect on Polyzwitterion Conformation in an Aqueous Environment.

Polyzwitterions (PZs) are promising materials for the antifouling in reverse osmosis and nanofiltration membrane technology for water treatment. Fundamental understanding of the structure and molecular interactions involving zwitterions is crucial to the optimal design of antifouling in membrane separation. Here we employ the umbrella sampling and molecular dynamics simulations to investigate molecular interactions between sulfobetaine/carboxybetaine zwitterions and different metal ions (Na, K, and Ca) in an aqueous solution.

On the shear dilation of polycrystalline lubricant films in boundary lubricated contacts.

Shearing of a solidified polycrystalline lubricant film confined between two solid surfaces has been studied by molecular dynamics simulations. In the case of a perfect commensurate contact, we observe interlayer slips within the film and shear-induced order-to-disorder transition of lubricant molecules around grain boundaries. This process is accompanied by the nucleation, propagation, and annihilation of dislocations in the solidified film, resulting in repeated dilation and collapse of the lubricant film during the stick-slip motion.

Driven Dynamics of Long-Time Bond-Breaking Events.

We present a predict-correct trajectory propagation (PCTP) method for simulating nonequilibrium driven dynamics of the long-time bond-breaking event in an atomic force microscope (AFM). Whereas the parallel replica method can extend the time scale of molecular dynamics (MD) simulations of infrequent-event systems with high parallel efficiency, the second aspect of the time scale difficulty in MD simulations for slow-driven systems, namely, the unphysically high attempt frequency of an infrequent event, cannot be resolved by this method. Here, we take a gold nanojunction under mechanical pulling as a simulation system and demonstrate that the PCTP simulation is capable of capturing key transition dynamics of bond breaking predicted by accurate MD simulations in the activationless regime, such as at cryogenic temperature with high pulling rates.

Squeezing and stick-slip friction behaviors of lubricants in boundary lubrication.

The fundamental questions of how lubricant molecules organize into a layered structure under nanometers confinement and what is the interplay between layering and friction are still not well answered in the field of nanotribology. While the phase transition of lubricants during a squeeze-out process under compression is a long-standing controversial debate (i.e.

Molecular Simulations of the Hydration Behavior of a Zwitterion Brush Array and Its Antifouling Property in an Aqueous Environment.

We carried out umbrella sampling and molecular dynamics (MD) simulations to investigate molecular interactions between sulfobetaine zwitterions or between sulfobetaine brushes in different media. Simulation results show that it is more energetically favorable for the two sulfobetaine zwitterions or brushes to be fully hydrated in aqueous solutions than in vacuum where strong ion pairs are formed. Structural properties of the hydrated sulfobetaine brush array and its antifouling behavior against a foulant gel are subsequently studied through steered MD simulations.

Computational simulations of solvation force and squeezing out of dodecane chain molecules in an atomic force microscope.

Understanding the squeeze out behaviors of liquid films at nanometer scale in an atomic force microscope (AFM) has been a significant interest since the 1990s. We carry out all-atom static-mode AFM simulations in a liquid-vapor molecular dynamics ensemble to investigate the solvation force oscillation and squeeze out mechanisms of a confined linear dodecane fluid between a gold AFM tip and a mica substrate. Solvation force oscillations are found to be associated with the layering transition of the liquid film and unstable jumps of the AFM tip.

Effect of Layer Charge on CO and HO Intercalations in Swelling Clays.

The effect of layer charge on the intercalation of supercritical carbon dioxide (scCO)-HO mixture in Na-montmorillonite clay interlayers under T = 323 K and P = 90 bar geologic sequestration conditions has been further investigated. This effect includes the charge amount and its location (within either octahedral or tetrahedral layers due to isomorphic substitutions). Two clay models with different layer charges are used in this study.

Contact stiffness and damping of liquid films in dynamic atomic force microscope.

The mechanical properties and dissipation behaviors of nanometers confined liquid films have been long-standing interests in surface force measurements. The correlation between the contact stiffness and damping of the nanoconfined film is still not well understood. We establish a novel computational framework through molecular dynamics (MD) simulation for the first time to study small-amplitude dynamic atomic force microscopy (dynamic AFM) in a simple nonpolar liquid.

Molecular Dynamics Simulations of a Poly(ethylene glycol)-Grafted Polyamide Membrane and Its Interaction with a Calcium Alginate Gel.

Molecular dynamics simulations are carried out to investigate the antifouling property of a polyethylene glycol (PEG)-grafted polyamide (PA) membrane. Our specific interest is the computational study of the interaction between a grafted PEG coating and an alginate gel foulant by a steered molecular dynamics approach. Simulation results show that the PEG coating can hold a tightly bound hydration water layer.

Methane aqueous fluids in montmorillonite clay interlayer under near-surface geological conditions: a grand canonical Monte Carlo and molecular dynamics simulation study.

The grand-canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations are performed to investigate the methane aqueous fluids in Na-montmorillonite clay interlayer under near-surface geological temperature and pressure conditions (T = 300 K and P = 20-50 bar). The chemical potentials of water and methane under these T/P conditions are calculated using the Widom's insertion method. These chemical potentials are used in the GCMC simulations to determine the contents of different species in the clay interlayer, especially in those that correspond to the equilibrium stable spacing distances.

Molecular dynamics simulations of polyamide membrane, calcium alginate gel, and their interactions in aqueous solution.

We perform molecular dynamics (MD) simulations to investigate the cross-linked polyamide (PA) membrane, the aggregation of alginate molecules in the presence of Ca(2+) ions, and their molecular binding mechanism in aqueous solution. We use a steered molecular dynamics (SMD) approach to simulate the unbinding process between a PA membrane and an alginate gel complex. Simulation results show that Ca(2+) ions are strongly associated with the carboxylate groups in alginate molecules, forming a web structure.

Solvation force simulations in atomic force microscopy.

Solvation force oscillation in octamethylcyclotetrasiloxane (OMCTS) versus the distance between an atomic force microscope (AFM) tip and mica substrate has been studied through molecular dynamics simulations. A driving spring model in a liquid-vapor molecular ensemble is used to explore the force oscillation mechanism. It has been found that OMCTS fluid in tip-substrate contact has a strong tendency to form a layered structure, starting from n = 8 layers.

A comparative study by the grand canonical Monte Carlo and molecular dynamics simulations on the squeezing behavior of nanometers confined liquid films.

Grand canonical Monte Carlo (GCMC) and liquid-vapor molecular dynamics (LVMD) simulations are performed to investigate the squeezing and phase transition of a simple liquid argon film confined between two solid surfaces. Simulation results show that the LVMD simulation is capable of capturing the major thermodynamic equilibrium states of the confined film, as predicted by the GCMC simulations. Moreover, the LVMD simulations reveal the non-equilibrium squeeze out dynamics of the confined film.

Hydrated polyamide membrane and its interaction with alginate: a molecular dynamics study.

The properties of the hydrated amorphous polyamide (PA) membrane and its binding with alginate are investigated through molecular dynamics simulations. The density of the hydrated membrane, surface morphology, and water diffusion near and inside the membrane are compared to other studies. Particular focus is given to the steered molecular dynamics (SMD) simulation of the binding between the PA membrane and an alginate model.

Hydration force between mica surfaces in aqueous KCl electrolyte solution.

Liquid-vapor molecular dynamics simulations are performed to study the interaction forces between two mica surfaces in an aqueous KCl electrolyte solution. Strong repulsive hydration force is obtained within a distance of ~2 nm between the two mica surfaces, which cannot be explained by the continuum theory of double-layer repulsion. We find that this short-range repulsive hydration force is much stronger than the double-layer force between mica surfaces.

Hydrophobic drying and hysteresis at different length scales by molecular dynamics simulations.

We performed molecular dynamics simulations to investigate hydrophobic interactions between two parallel hydrophobic plates immersed in water. The two plates are separated by a distance D ranging from contact to a few nanometers. To mimic the attractive hydrophobic force measurement in a surface force experiment, a driving spring is used to measure the hydrophobic force between two hydrophobic plates.

Stick-slip friction and energy dissipation in boundary lubrication.

Shearing of a simple nonpolar film, right after the liquid-to-solid phase transition under nanometer confinement, is studied by using a liquid-vapor molecular dynamics simulation method. We find that, in contrast with the shear melting and recrystallization behavior of the solidlike phase during the stick-slip motion, interlayer slips within the film and wall slips at the wall-film interface are often observed. The ordered solidified film is well maintained during the slip.

Force oscillation and phase transition of simple fluids under confinement.

We use molecular dynamics simulations to explore the force oscillation mechanism of a simple fluid confined between two surfaces. Force profiles obtained through simulations are qualitatively similar to those in surface force measurements. Our results demonstrate that the layering transition is an abrupt, liquid-to-solid phase transition.