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Re: [ESPResSo-users] DPD
From: |
Dr. Jens Smiatek |
Subject: |
Re: [ESPResSo-users] DPD |
Date: |
Thu, 11 Sep 2014 16:38:16 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.7.0 |
Dear all,
just as a remark:
@Stefan: You are right. Lower densities will result in the Boltzmann
regime where the standard Marsh, Backx, Ernst-Theory will not be
applicable anymore.
Anyway, we have found that the tunable-slip boundaries can be
effectively tuned and analytically described for number densities
between 3 - 10 (in agreement with the DPD method itself).
Otherwise, the collision numbers per particle in one timestep are too
low and a description within a mean-field approach fails.
Therefore I suggest that using a number density of 3 for DPD is more or
less mandatory for (more or less) analytical control.
You should always remember the fact that the DPD method produces a very
low Schmidt number which corresponds more to a gas than a fluid.
With regard to this, lower densities are also in terms of a physical DPD
interpretation as a solvent doubtful.
And last but not least...
C'mon guys, everything below 100K particles is a small simulation ;-)
Best,
Jens
On 09/11/2014 04:20 PM, Stefan Kesselheim wrote:
> Dear Dusan,
>
> On Sep 11, 2014, at 4:07 PM, Dudo <address@hidden> wrote:
>
>> Dear Jens,
>>
>> thank you very much for the lead.
>>
>> Well, so I've compiled in a feature for "TUNABLE_SLIP"
>> and I have set up interactions: inter $cid $idpolym tunable_slip
>> $temperature $gamma_L $r_cut_L $timestep .....
>> I have turned the flag to 2: constraint cylinder center $cx $cy $cz axis
>> $cnx $cny $cnz radius $crad length $clength direction $cdirection type $cid
>> reflecting 2 .....
>> as Stefan suggested, and I'm trying with the parameters from your paper on
>> polyelectrolytes J. Phys Chem B 2010..
>> Now I see you have used explicit solvent with density 3.75, while modelling
>> a chain of 20 beads.
>> In my case this would mean zillions of solvent particles..
>> Well at the begining Chris asked me, why would I do add explicit particles -
>> so.. is there a way around?
> The particle numbers that you will need are necessary are considerable but
> OK. For a chain of 20 beads, the radius of gyration is probably around 5, and
> thus you need a box size of 10, and 20 is still doable. With a density of 1,
> you'll have 1k to 8k particles. That should be enough to perform the
> measurements, especially as you probably don't need too many steps to get a
> meaningful mobility.
>
> In principle you can use even lower densities, but at some point the mean
> free path of the particles becomes comparable to the box size, and then you
> are in a region where you should be sure what you are doing.
>
> The strength of the hydrodynamic interaction, anyways, depends in the
> "hydrodynamic radius" of the particles. This is the size of a sphere with the
> same diffusion coefficient as an isolated particle with the same viscosity.
> If this quantity is too small (~1/(6 pi visc D), then hydrodynamics will be
> weak.
>
> Cheers and good luck
> Stefan
--
================
Dr. Jens Smiatek
Institute for Computational Physics
University of Stuttgart
Allmandring 3
70569 Stuttgart
Germany
Office: 1.032
Phone: +49-(0)711/685 63757
E-Mail: address@hidden
- [ESPResSo-users] DPD, Dudo, 2014/09/10
- Re: [ESPResSo-users] DPD, Christoph Junghans, 2014/09/10
- Re: [ESPResSo-users] DPD, Dudo, 2014/09/10
- Re: [ESPResSo-users] DPD, Ulf Schiller, 2014/09/10
- Re: [ESPResSo-users] DPD, Jiajia Zhou, 2014/09/11
- Re: [ESPResSo-users] DPD, Dudo, 2014/09/11
- Re: [ESPResSo-users] DPD, Stefan Kesselheim, 2014/09/11
- Re: [ESPResSo-users] DPD, Dr. Jens Smiatek, 2014/09/11
- Re: [ESPResSo-users] DPD, Dudo, 2014/09/11
- Re: [ESPResSo-users] DPD, Stefan Kesselheim, 2014/09/11
- Re: [ESPResSo-users] DPD,
Dr. Jens Smiatek <=
- Re: [ESPResSo-users] DPD, Ulf Schiller, 2014/09/11