Weiqing Ren

Simplified and improved string method for computing the minimum energy paths in barrier-crossing events

We present a simplified and improved version of the string method, originally proposed by E et al. [Phys. Rev. B 66, 052301 (2002)] for identifying the minimum energy paths in barrier-crossing events. In this new version, the step of projecting the potential force to the direction normal to the string is eliminated and the full potential force is used in the evolution of the string. This not only simplifies the numerical procedure, but also makes the method more stable and accurate. We discuss the algorithmic details of the improved string method, analyze its stability, accuracy and efficiency, and illustrate it via numerical examples. We also show how the string method can be combined with the climbing image technique for the accurate calculation of saddle points and we present another algorithm for the accurate calculation of the unstable directions at the saddle points.

Transition pathways in complex systems: Application of the finite-temperature string method to the alanine dipeptide

The finite-temperature string method proposed by E, et al. [W. E, W. Ren, and E. Vanden-Eijnden, Phys. Rev. B 66, 052301 (2002)] is a very effective way of identifying transition mechanisms and transition rates between metastable states in systems with complex energy landscapes. In this paper, we discuss the theoretical background and algorithmic details of the finite-temperature string method, as well as the application to the study of isomerization reaction of the alanine dipeptide, both in vacuum and in explicit solvent. We demonstrate that the method allows us to identify directly the isocommittor surfaces, which are approximated by hyperplanes, in the region of configuration space where the most probable transition trajectories are concentrated. These results are verified subsequently by computing directly the committor distribution on the hyperplanes that define the transition state region.

Boundary conditions for the moving contact line problem

The physical processes near a moving contact line are investigated systematically using molecular dynamics and continuum mechanics. Constitutive relations for the friction force in the contact line region, the fluid-fluid interfacial force, and the stresses in the fluid-solid interfacial region are studied. Verification of force balance demonstrates the importance of the normal stress jump across the contact line region. Effective boundary conditions are derived using force balance. It is found that in the flow regime studied, the deviation of the wall contact angle from the equilibrium contact angle is proportional to the velocity of the contact line. The effective continuum model is solved numerically and the behavior of the apparent contact angle and the wall contact angle is studied. It is found that the fluid-fluid interface near the wall exhibits a universal behavior. The onset of the nonlinear response for the contact line motion is studied within the framework of Blake’s molecular kinetic theory.

Energy landscape and thermally activated switching of submicron-sized ferromagnetic elements

Thermally activated switching and the energy landscape of submicron-sized magnetic elements are studied using the string method. For thin films, we found that switching proceeds by two generic scenarios: Domain-wall propagation and reconnection followed by edge domain switching, or vortex nucleation at the boundary followed by vortex propagation through the sample. The second scenario is preferred for thicker films whereas the first is preferred for thin (less than 20 nm) films. The energy landscape of such a system is nicely summarized on the plane spanned by the average magnetization in the in-plane directions. For three-dimensional samples, we found that switching proceeds by vortex propagation through the sample. The implication of the Landau–Lifshitz dynamics is also discussed.

Adaptive minimum action method for the study of rare events

An adaptive minimum action method is proposed for computing the most probable transition paths between stable equilibria in metastable systems that do not necessarily have an underlying energy function, by minimizing the action functional associated with such transition paths. This new algorithm uses the moving mesh strategy to adaptively adjust the grid points over the time interval of transition. Numerical examples are presented to demonstrate the efficiency of the adaptive minimum action method.

Continuum models for the contact line problem

Continuum models are derived for the moving contact line problem through a combination of macroscopic and microscopic considerations. Macroscopic thermodynamic argument is used to place constraints on the form of the boundary conditions at the solid surface and the contact line. This information is then used to set up molecular dynamics to measure the detailed functional dependence of the boundary conditions. Long range molecular forces are taken into account in the form of a surface potential. This allows us to handle the case of complete wetting as well as the case of partial wetting. In particular, we obtain a new continuum model for both cases in a unified form. Two main parameters and different spreading regimes are identified from the analysis of the energy dissipations for the continuum model. Scaling laws in these different regimes are derived. The new continuum model also allows us to derive boundary conditions for the lubrication approximation. Numerical results are presented for the thin film m...

Wetting Transition on Patterned Surfaces: Transition States and Energy Barriers

We study the wetting transition on microstructured hydrophobic surfaces. We use the string method [J. Chem. Phys. 2007, 126, 164103; J. Chem. Phys. 2013, 138, 134105] to accurately compute the transition states, the energy barriers, and the minimum energy paths for the wetting transition from the Cassie-Baxter state to the Wenzel state. Numerical results are obtained for the wetting of a hydrophobic surface textured with a square lattice of pillars. It is found that the wetting of the solid substrate occurs via infiltration of the liquid in a single groove, followed by lateral propagation of the liquid front. The propagation of the liquid front proceeds in a stepwise manner, and a zipping mechanism is observed during the infiltration of each layer. The minimum energy path for the wetting transition goes through a sequence of intermediate metastable states, whose wetted areas reflect the microstructure of the patterned surface. We also study the dependence of the energy barrier on the drop size and the gap between the pillars.

A level-set method for two-phase flows with moving contact line and insoluble surfactant

A level-set method for two-phase flows with moving contact line and insoluble surfactant is presented. The mathematical model consists of the Navier-Stokes equation for the flow field, a convection-diffusion equation for the surfactant concentration, together with the Navier boundary condition and a condition for the dynamic contact angle derived by Ren et al. (2010) 37]. The numerical method is based on the level-set continuum surface force method for two-phase flows with surfactant developed by Xu et al. (2012) 54] with some cautious treatment for the boundary conditions. The numerical method consists of three components: a flow solver for the velocity field, a solver for the surfactant concentration, and a solver for the level-set function. In the flow solver, the surface force is dealt with using the continuum surface force model. The unbalanced Young stress at the moving contact line is incorporated into the Navier boundary condition. A convergence study of the numerical method and a parametric study are presented. The influence of surfactant on the dynamics of the moving contact line is illustrated using examples. The capability of the level-set method to handle complex geometries is demonstrated by simulating a pendant drop detaching from a wall under gravity.

A climbing string method for saddle point search

The string method originally proposed for the computation of minimum energy paths (MEPs) is modified to find saddle points around a given minimum on a potential energy landscape using the location of this minimum as only input. In the modified method the string is evolved by gradient flow in path space, with one of its end points fixed at the minimum and the other end point (the climbing image) evolving towards a saddle point according to a modified potential force in which the component of the potential force in the tangent direction of the string is reversed. The use of a string allows us to monitor the evolution of the climbing image and prevent its escape from the basin of attraction of the minimum. This guarantees that the string always converges towards a MEP connecting the minimum to a saddle point lying on the boundary of the basin of attraction of this minimum. The convergence of the climbing image to the saddle point can also be accelerated by an inexact Newton method in the late stage of the computation. The performance of the numerical method is illustrated using the example of a 7-atom cluster on a substrate. Comparison is made with the dimer method.

Contact line dynamics on heterogeneous surfaces

Contact line dynamics on rough or chemically heterogeneous surfaces has been a subject of great interest. Most previous work focused on the issue of contact angle hysteresis in the static limit. This paper is devoted to the study of contact line dynamics on a chemically patterned surface over a wide range of contact line speed. Numerical simulations are carried out for two immiscible fluids confined in a channel and driven by either the shear motion of the two confining walls or an external force. It is found that in the low-speed regime when the averaged contact line speed U≪γ/β*, with γ being the surface tension of the fluid interface and β* the friction coefficient at the contact line, the behavior of the contact line dynamics is very similar to that of the static limit, namely it undergoes a stick-slip motion and the contact angle exhibits hysteretic behavior. At finite speed, the stick-slip behavior gradually diminishes, and the contact line motion becomes more smooth. The effect of these microscale ...