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[ESPResSo] General Suitability of Espresso for Fine Particles

From: Lorenzo Isella
Subject: [ESPResSo] General Suitability of Espresso for Fine Particles
Date: Tue, 9 Oct 2007 13:03:00 +0200

Dear All,
I think I now have a smattering of the basics of Espresso and I have
to start thinking how and if to use it for my research.
I have browsed the web and found espresso applications for polymers,
ions, proteins,  and so on, but my task is really to simulate the
Langevin dynamics of exhaust fine particles (think of them as
carbonaceous particles whose diameter ranges from 2 to 600 nanometers,
the larger ones created by the agglomeration of the small ones) to
investigate agglomeration.
These particles are typically suspended in air, there may or may not
be convection from a carrier flow.
People typically assume that their motion is ruled by a Langevin
equation, and that these particles stick when they collide, giving
rise to complicated structures I would like to investigate.
(1) First of all, am I right to say that the dynamics in the Langevin
thermostat as implemented in Espresso simulates stochastic particle
paths? This is my understanding of the Langevin thermostat in general,
but I am also obviously concerned about the implementation.
(2) Can e.g. the fene or the harmonic potential be twisted to simulate
this "sticking upon collision"?
Basically I need a strong binding potential with a short interaction
range, the interaction range being identified with the particle
radius. If not, is there any conceptual problem in tabulating it?
(3)Back to the particle (stochastic) trajectories: the treatment of
the friction and noise terms is particularly delicate. In my case,
this noise stands for the effects of air molecules kicking the
particles. Depending on the air temperature, the air mean-free path
could be larger or smaller than the particle radius and this has to be
taken into account. Can I "tune" the noise term in Langevin equation?
(4)Related to the previous questions: let us say you have a set of
single particles, each of them separately obeying a Langevin equation
with a certain noise.
After colliding and giving rise to a certain agglomerate, the noise
acting on the agglomerate will NOT in general be the sum of the noises
on the individual particles, due to shielding effects (inner particles
may be difficult to reach by air molecules). Can this be somehow
accounted for in Espresso?

Of course I would have other questions about how to simulate these
systems, but these are really the fundamental ones.
Some of them are not about Espresso in particular. It looks to me a
very versatile tool, but I have not been able to find studies
employing it in my field.
Many thanks for any answer I will get.


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