Search arXiv⌕ Search
Home/Archive/arXiv

arXiv · cond-mat/0011412

Molecular Dynamics Results on the Pressure Tensor of Polymer Films

F. Varnik·J. Baschnagel·K. Binder
View original source ↗Read source document ↗Export metadata ↓

Abstract

Polymeric thin films of various thicknesses, confined between two repulsive walls, have been studied by molecular dynamics simulations. Using the anisotropy of the perpendicular and parallel components of the pressure tensor the surface tension of the system is calculated for a wide range of temperature and for various film thicknesses. Three methods of determining the pressure tensor are compared: the method of Irving and Kirkwood (IK), an approximation thereof (IK1), and the method of Harasima (H). The IK- and the H-methods differ in the expression for the parallel component, but not for the normal component of the pressure tensor. Both methods yield a constant normal component throughout the film, as required by mechanical stability, whereas the IK1-method leads to strong oscillations. However, all methods give the same expression for the surface tension.

Explore related subjects

cond-mat.softcond-mat.dis-nn

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

F. Varnik, J. Baschnagel, K. Binder. 2000-11-23. Molecular Dynamics Results on the Pressure Tensor of Polymer Films. https://doi.org/10.1063/1.1288390

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Controlling microalgae populations by phototactic memory

Motile microorganisms navigate environments that vary in space and time by coupling sensory inputs to internal physiological states and active motion. In photosynthetic microorganisms, such coupling can be mediated by phototaxis, yet how temporal sensing combines with spatial variations of the environment to shape population organisation remains poorly understood. To probe this coupling, we expose populations of the phototactic microalga \textit{Chlamydomonas reinhardtii} to structured light landscapes. Under ring-shaped illumination, cells robustly accumulate at the center of the illuminated region, rather than at the light-intensity maximum. Single-cell tracking shows that cells steer relative to local intensity gradients and undergo a non-trivial phototactic response, where, on average, cells outside the ring perform positive phototaxis while cells inside perform negative phototaxis. Population experiments, numerical simulations and analytical modelling show that this trapping cannot be reproduced by an instantaneous switching rule, but requires a finite phototactic memory acting on the transition between positive and negative phototaxis. This internal timescale couples to the geometry of the light field and to the persistence of active motion, enabling controllable spatial localisation without physical confinement. Our results generically show how memory in a single-cell sensory response can control population-level organisation in a living active system.

cond-mat.soft↗
arXiv2026

Emergent odd response in active chiral films

Active chiral fluids can support an anomalous transport coefficient known as odd (or Hall) viscosity. Hydrodynamic descriptions of such fluids typically introduce odd viscosity phenomenologically. How such a response emerges from specific microscopic interactions remains incompletely understood. Here, building on classical shear rheology, we microscopically derive an odd rheological response in active chiral films: thin layers of torque-exerting, elongated particles anchored to a no-slip surface. A canonical realization of such a film is the bacterial carpet, in which flagellated bacteria are tethered head-down to a solid surface while their flagella remain free to spin and inject angular momentum into the surrounding fluid. Using a Smoluchowski kinetic theory for the orientational dynamics of these anchored particles, we derive their stress response to an imposed shear flow. We reveal that shear-induced reorientation leads to a flow-aligned polarization and a transverse surface traction from which the odd-viscosity tensor follows in closed form. This approach stands in contrast with existing microscopic theories, in which the axis of torque injection is insensitive to the flow rather than set by it. Numerical solutions of the kinetic theory further highlight saturation of the transverse traction at strong shear, driven by this orientation dynamics -- signaling departure from linear response. Our results demonstrate how odd viscosity can emerge self-consistently as a coarse-grained rheological signature of active fluid-structure interaction and establish active chiral films as a new controllable setting for odd hydrodynamics.

cond-mat.soft↗
arXiv2026

Inclined surfaces bias bacterial swarming expansion and edge organization

Bacterial swarming is the collective expansion of dense populations of motile cells across hydrated soft surfaces, yet gravity is usually treated as a fixed background condition in standard horizontal assays. We used surface inclination to examine how agar-surface orientation relative to gravity affects the expansion of Enterobacter sp. SM3 swarms. At macroscopic scales, we found that colony expansion depended non-monotonically on inclination, with the strongest spreading at intermediate inclinations and weaker expansion under shallow or near-inverted conditions. Paired up-facing acute and down-facing obtuse configurations with the same tilt magnitude showed different expansion responses, suggesting that swarm expansion depended on both tilt magnitude and which side of the agar surface faced upward relative to gravity. Inclination also produced downhill-biased colony morphologies. For microscopic scales, particle image velocimetry (PIV) analysis showed generally lower mean activity under inclination than in the flat references, with a non-monotonic angular response. The downhill tip showed slower motion, shorter spatial correlations and smaller vortex cores than the upper region. To interpret the macroscopic trends, we extended a two-phase thin-film model by introducing an effective gravity coupling projected normal and tangential to the agar surface. Simulations suggest that the surface-tangential component mainly drives lateral redistribution, whereas the surface-normal component modifies film pressure and helps distinguish up-facing from down-facing configurations.

cond-mat.soft↗