Simulated three-component granular segregation in a rotating drum

Molecular dynamics simulation: a tool for exploration and discovery using simple models

Emergent phenomena share the fascinating property of not being obvious consequences of the design of the system in which they appear. This characteristic is no less relevant when attempting to simulate such phenomena, given that the outcome is not always a foregone conclusion. The present survey focuses on several simple model systems that exhibit surprisingly rich emergent behavior, all studied by molecular dynamics (MD) simulation.The examples are taken from the disparate fields of fluid dynamics, granular matter and supramolecular self-assembly. In studies of fluids modeled at the detailed microscopic level using discrete particles, the simulations demonstrate that complex hydrodynamic phenomena in rotating and convecting fluids—the Taylor–Couette and Rayleigh–Bénard instabilities—cannot only be observed within the limited length and time scales accessible to MD, but even allow quantitative agreement to be achieved. Simulation of highly counter-intuitive segregation phenomena in granular mixtures, again using MD methods, but now augmented by forces producing damping and friction, leads to results that resemble experimentally observed axial and radial segregation in the case of a rotating cylinder and to a novel form of horizontal segregation in a vertically vibrated layer. Finally, when modeling self-assembly processes analogous to the formation of the polyhedral shells that package spherical viruses, simulation of suitably shaped particles reveals the ability to produce complete, error-free assembly and leads to the important general observation that reversible growth steps contribute to the high yield. While there are limitations to the MD approach, both computational and conceptual, the results offer a tantalizing hint of the kinds of phenomena that can be explored and what might be discovered when sufficient resources are brought to bear on a problem.

Recent advances in simulations of grains in a rotating drum

The discrete elements method (DEM) has been widely used in the past decade to study a wide variety of granular systems. The use of numerical simulations constitutes an interesting alternative to the experiment as they can shed new light on a phenomenon as they can overcome experimental obstacles. A lot of granular phenomena can be studied in 2D or with a limited number of grains but the peculiar phenomenon of axial segregation (or banding) is 3-dimensional by nature and requires a large number of grains. Only very recently has it been made possible to simulate 3D systems on a large scale. This highlight reviews recent work on this topic and attempts to show what knowledge is gained from DEM numerical simulations. The perspectives on the future benefit of this method as well as the challenges it faces are discussed.