Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer

Anomalous fast dynamics of adsorbate overlayers near an incommensurate structural transition

We investigate the dynamics of a compressively strained adsorbed layer on a periodic substrate via a simple two-dimensional model that admits striped and hexagonal incommensurate phases. We show that the mass transport is superfast near the striped-hexagonal phase boundary and in the hexagonal phase. For an initial step profile separating a bare substrate region (or "hole") from the rest of a striped incommensurate phase, the superfast domain wall dynamics leads to a bifurcation of the initial step profile into two interfaces or profiles propagating in opposite directions with a hexagonal phase in between. This yields a theoretical understanding of the recent experiments for the Pb/Si(111) system.

Friction phenomena and phase transition in the underdamped two-dimensional Frenkel-Kontorova model

Locked-to-sliding phase transition has been studied in the driven two-dimensional Frenkel-Kontorova model with the square symmetric substrate potential. It is found that as the driving force increases, the system transfers from the locked state to the sliding state where the motion of particles is in the direction different from that of driving force. With the further increase in driving force, at some critical value, the particles start to move in the direction of driving force. These two critical forces, the static friction or depinning force, and the kinetic friction force for which particles move in the direction of driving force have been analyzed for different system parameters. Different scenarios of phase transitions have been examined and dynamical phases are classified. In the case of zero misfit angle, the analytical expressions for static and kinetic friction force have been obtained.

Onset of frictional slip by domain nucleation in adsorbed monolayers

It has been known for centuries that a body in contact with a substrate will start to slide when the lateral force exceeds the static friction force. Yet the microscopic mechanisms ruling the crossover from static to dynamic friction are still the object of active research. Here, we analyze the onset of slip of a xenon (Xe) monolayer sliding on a copper (Cu) substrate. We consider thermal-activated creep under a small external lateral force, and observe that slip proceeds by the nucleation and growth of domains in the commensurate interface between the film and the substrate. We measure the activation energy for the nucleation process considering its dependence on the external force, the substrate corrugation, and particle interactions in the film. To understand the results, we use the classical theory of nucleation and compute analytically the activation energy which turns out to be in excellent agreement with numerical results. We discuss the relevance of our results to understand experiments on the sliding of adsorbed monolayers.

Dynamical transitions and sliding friction of the phase-field-crystal model with pinning

We study the nonlinear driven response and sliding friction behavior of the phase-field-crystal (PFC) model with pinning including both thermal fluctuations and inertial effects. The model provides a continuous description of adsorbed layers on a substrate under the action of an external driving force at finite temperatures, allowing for both elastic and plastic deformations. We derive general stochastic dynamical equations for the particle and momentum densities including both thermal fluctuations and inertial effects. The resulting coupled equations for the PFC model are studied numerically. At sufficiently low temperatures, we find that the velocity response of an initially pinned commensurate layer shows hysteresis with dynamical melting and freezing transitions for increasing and decreasing applied forces at different critical values. The main features of the nonlinear response in the PFC model are similar to the results obtained previously with molecular dynamics simulations of particle models for adsorbed layers.

Nonlinear driven response of a phase-field crystal in a periodic pinning potential

We study numerically the phase diagram and the response under a driving force of the phase field crystal model for pinned lattice systems introduced recently for both one- and two-dimensional systems. The model describes the lattice system as a continuous density field in the presence of a periodic pinning potential, allowing for both elastic and plastic deformations of the lattice. We first present results for phase diagrams of the model in the absence of a driving force. The nonlinear response to a driving force on an initially pinned commensurate phase is then studied via overdamped dynamic equations of motion for different values of mismatch and pinning strengths. For large pinning strength the driven depinning transitions are continuous, and the sliding velocity varies with the force from the threshold with power-law exponents in agreement with analytical predictions. Transverse depinning transitions in the moving state are also found in two dimensions. Surprisingly, for sufficiently weak pinning potential we find a discontinuous depinning transition with hysteresis even in one dimension under overdamped dynamics. We also characterize structural changes of the system in some detail close to the depinning transition.

Translocation dynamics with attractive nanopore-polymer interactions

Using Langevin dynamics simulations, we investigate the influence of polymer-pore interactions on the dynamics of biopolymer translocation through nanopores. We find that an attractive interaction can significantly change the translocation dynamics. This can be understood by examining the three components of the total translocation time tau approximately tau1+tau2+tau3 corresponding to the initial filling of the pore, transfer of polymer from the cis side to the trans side, and emptying of the pore, respectively. We find that the dynamics for the last process of emptying of the pore changes from nonactivated to activated in nature as the strength of the attractive interaction increases, and tau3 becomes the dominant contribution to the total translocation time for strong attraction. This leads to nonuniversal dependence of tau as a function of driving force and chain length. Our results are in good agreement with recent experimental findings, and provide a plausible explanation for the different scaling behavior observed in solid state nanopores vs that for the natural alpha-hemolysin channel.

Directional locking effects and dynamics for particles driven through a colloidal lattice

We examine the dynamics of a single colloidal particle driven through a colloidal lattice which can distort in response to the driven particle. We find a remarkably rich variety of dynamical locking phenomena as we vary the angle of the applied drive with respect to the orientation of the colloidal lattice. When the driven colloid locks to certain lattice symmetry directions, its motion is not necessarily aligned with the drive. Applying a transverse force to the driven particle can result in either increased or decreased drag in the driving direction, depending on the angle of the drive. The dynamical locking produces anomalies in both the longitudinal and the transverse velocity vs driving force curves, including steps and regimes of negative differential resistance. As the interaction of the driven particle with the surrounding lattice increases, significant distortion or dislocations in the surrounding media occur, and as a result the directional locking is enhanced. We compare these results to those obtained for driving particles over fixed substrates, and show that a far richer variety of behaviors occurs when the underlying lattice is allowed to distort. We discuss how this system can be used for particle species segregation when the onset of different locking angles occurs at different drives for varied particle charges. We also show that the most pronounced locking phases should be observable at temperatures up to the melting transition of the colloidal lattice.

Collective stochastic resonance in shear-induced melting of sliding bilayers

The far-from-equilibrium dynamics of two crystalline two-dimensional monolayers driven past each other is studied using Brownian dynamics simulations. While at very high and low driving rates the layers slide past one another retaining their crystalline order, for intermediate range of drives the system alternates irregularly between the crystalline and fluidlike phases. A dynamical phase diagram in the space of interlayer coupling and drive is obtained. A qualitative understanding of this stochastic alternation between the liquidlike and crystalline phases is proposed in terms of a reduced model within which it can be understood as a stochastic resonance for the dynamics of collective order parameter variables. This remarkable example of stochastic resonance in a spatially extended system should be seen in experiments which we propose in the paper.