It is currently possible in an efficient way to calculate the distance between two hard convex objects with a generic shape and to predict their collision [ED4HB]. The first application of such algorithms has been tested on the molecular dynamics of a systems of classical hard ellipsoids of revolution [DeMicheleHE06]. The same methods allows to simulate shaped step interactions[DeMichelSticky06]. The recent introduction of an algorithm for the rotation of objects with a generic inertia tensor completes such approach [vanZonCondMat]. Moreover,following the lines of [BD4HS], it seems to be possible to extend classical Molecular Dynamics simulation to the Brownian Dynamics simulation of interest in the field of colloids and suspensions.
A renewed interest for assemblies of hard shaped objects stems from the field of colloidal science; nevertheless,simulations of the dynamics of hard objects [Allen89,Allen93] needs to be revived and theoretical investigations on the dynamics of shaped objects reconsidered. Besides, we believe that there is a potential interest in several cross-disciplinary scientific and technological fields. Such belief stems from an analysis of the current investigations in several fields:
* theory of molecular liquids: while hard-spheres are widely
understood and form the reference system for many theories of
the dynamics and the statics, there no such understanding for
shaped objects [GrayGubbinsBook]
* granular fluids: simulations of shaped particles are
practically limited to hard needles
* geo-mechanics: soil rheology employs mostly models and
simulations where grains are spherical or, in the most
sophisticated cases, ellipsoidal [Ouadfel99,Ouadfel01].
The new rheology introduced by switching from spheres to
elongated objects should induce to take account of shape as a
must, not as an optional.
* powders: same situation than in geo-mechanics
* fruit & vegetables: damages due to processing, transportation
and handling are studied via simulations of spheres that
represent potatoes, tomatoes, apples [LoodtsMSTh]. Damages are
predicted looking at the collision points on the spheres; from
our experience on ellipsoids, we believe that at least the
collision frequency and the magnitude of the impact is deeply
influenced even by a little elongation of the objects.
Moreover, experiments for the mechanical characterizations
require at least a description in terms of an ellipsoidal
* colloids and nanoassembly: despite the possibility of
controlling the shape of the particles [vanDillen2004], hard
spheres remain the main reference system.
* lyotropic liquid crystals: several overlap criteria
[Allen93,Het99,Blaak99] allow the simulation of specific hard
objects and point out the importance of the shape on the
appearance of new liquid-crystal phases. No general method to
investigate and quantify such effect exists for at least convex
* coarse-grained proteins: from experimental data, at least
Human Serum Albumin can be modeled as an hard ellipsoid with
a repulsive stepwise potential [Sjoberg97].
* go-models [Go83] for folding: rigid sub-sequences of a protein
could be modelled by a single shaped unit speeding up the
* computer science, computer graphics, game design, robotics,
virtual reality: distance calculations and collision prediction
are the core for the realization of virtual environments and
robotic manipulations; again, most algorithm are specific and
not shape-independent [Ju01,Eberly01].
In conclusion, we believe that it is timely to promote a cross-disciplinary meeting on the subject in order to create an enlarged community sharing the same interests on simulating hard bodies
Cristiano De Michele
("Sapienza" University of Rome)
(CNR - Institute for Complex Systems (ISC))