Self-propelled Worm-like Filaments: Spontaneous Spiral Formation, Structure, and Dynamics

Worm-like filaments that are propelled homogeneously along their tangent vector are studied by Brownian dynamics simulations. Systems in two dimensions are investigated, corresponding to filaments adsorbed to interfaces or surfaces. A large parameter space covering weak and strong propulsion, as well as flexible and stiff filaments is explored. For strongly propelled and flexible filaments, the free-swimming filaments spontaneously form stable spirals. The propulsion force has a strong impact on dynamic properties, such as the rotational and translational mean square displacement and the rate of conformational sampling. In particular, when the active self-propulsion dominates thermal diffusion, but is too weak for spiral formation, the rotational diffusion coefficient has an activity-induced contribution given by $v_c/\xi_P$, where $v_c$ is the contour velocity and $\xi_P$ the persistence length. In contrast, structural properties are hardly affected by the activity of the system, as long as no spirals form. The model mimics common features of biological systems, such as microtubules and actin filaments on motility assays or slender bacteria, and artificially designed microswimmers

Development and application of a particle-particle particle-mesh Ewald method for dispersion interactions

For inhomogeneous systems with interfaces, the inclusion of long-range dispersion interactions is necessary to achieve consistency between molecular simulation calculations and experimental results. For accurate and efficient incorporation of these contributions, we have implemented a particle-particle particle-mesh (PPPM) Ewald solver for dispersion ($r^{-6}$) interactions into the LAMMPS molecular dynamics package. We demonstrate that the solver's $\mathcal{O}(N\log N)$ scaling behavior allows its application to large-scale simulations. We carefully determine a set of parameters for the solver that provides accurate results and efficient computation. We perform a series of simulations with Lennard-Jones particles, SPC/E water, and hexane to show that with our choice of parameters the dependence of physical results on the chosen cutoff radius is removed. Physical results and computation time of these simulations are compared to results obtained using either a plain cutoff or a traditional Ewald sum for dispersion.Comment: 31 pages, 9 figure

Molecular phenomena in dynamic wetting: superspreading and precursors

Wetting is a multiscale process that can be controlled simultaneously by complex flow patterns on the macroscale and contact line phenomena at the Ångstrom scale. While resolving the latter scale is often circumvented by usage of boundary conditions, there are molecular wetting phenomena in which this approach is infeasible. The focus of this study is to use molecular dynamics simulations to examine two of these phenomena: superspreading, the ultra-rapid wetting of aqueous solutions facilitated by trisiloxane surfactants, and molecular precursors, the development of films of molecular thickness that precede droplets. Molecular simulation resolves the atomistic scale and provides information that is inaccessible from experiment. A challenge in the context of wetting, however, is that dispersion interactions are typically considered short-ranged in molecular simulations, whereas they have long-ranged effects in wetting. To capture these interactions in wetting simulations, the particle-particle particle-mesh algorithm, a long-range solver that is well-established for Coulomb interactions, is extended to dispersion. It is shown that the correct use of this algorithm leads to accurate and efficient simulations.Despite intensive studies on superspreading in the last 20 years, the underlying molecular mechanisms of the process are not understood. That the process is sensitive to various parameters in experiment, and also that previous attempts to model this phenomenon using molecular dynamics simulations failed, motivated the development of a force field dedicated to superspreading. Application in large-scale spreading simulations provides a smooth contact line transition at superspreading conditions. It is shown that this observation offers plausible explanations for experimental findings and a coherent description of the superspreading mechanism.While the dynamics and mass transport mechanisms of molecular precursors are well understood, conditions that lead to precursor formation or different types of precursors are subject to debate. Large-scale spreading simulations, new analysis methods, and excessive free energy computations shed light on these issues and resolve the conflict about the role of the spreading coefficient for precursor formation

Molecular phenomena in dynamic wetting: superspreading and precursors

Wetting is a multiscale process that can be controlled simultaneously by complex flow patterns on the macroscale and contact line phenomena at the Ångstrom scale. While resolving the latter scale is often circumvented by usage of boundary conditions, there are molecular wetting phenomena in which this approach is infeasible. The focus of this study is to use molecular dynamics simulations to examine two of these phenomena: superspreading, the ultra-rapid wetting of aqueous solutions facilitated by trisiloxane surfactants, and molecular precursors, the development of films of molecular thickness that precede droplets. Molecular simulation resolves the atomistic scale and provides information that is inaccessible from experiment. A challenge in the context of wetting, however, is that dispersion interactions are typically considered short-ranged in molecular simulations, whereas they have long-ranged effects in wetting. To capture these interactions in wetting simulations, the particle-particle particle-mesh algorithm, a long-range solver that is well-established for Coulomb interactions, is extended to dispersion. It is shown that the correct use of this algorithm leads to accurate and efficient simulations.Despite intensive studies on superspreading in the last 20 years, the underlying molecular mechanisms of the process are not understood. That the process is sensitive to various parameters in experiment, and also that previous attempts to model this phenomenon using molecular dynamics simulations failed, motivated the development of a force field dedicated to superspreading. Application in large-scale spreading simulations provides a smooth contact line transition at superspreading conditions. It is shown that this observation offers plausible explanations for experimental findings and a coherent description of the superspreading mechanism.While the dynamics and mass transport mechanisms of molecular precursors are well understood, conditions that lead to precursor formation or different types of precursors are subject to debate. Large-scale spreading simulations, new analysis methods, and excessive free energy computations shed light on these issues and resolve the conflict about the role of the spreading coefficient for precursor formation