[gmx-users] Pressure drop and atomic acceleration in nano-channel

[朱文鹏]

Dear all,

I am not sure if it is appropriate to post it here. I am trying to simulate
a water flow through a nano-channel (graphene-based). Nano-channel aligns
in the z-direction connecting holes of two graphene membrane which lies in
the x-y plane. Different lengths of nano-channels are considered, while the
dimensions of simulation box are fixed (5.922nm, 5.8614nm, 20nm in x, y, z
directions). The length of the nano-channel varies from 1nm to 7nm.

The pressure gradient is applied by artificially adding acceleration/force
on each water molecules in the z-direction. The method is using a
gravity-like force to mimic the pressure drop.I would like to investigate
the water flow rate across different nano-channel under the same pressure
drop.

However, I am confused about the relation between the pressure drop
and atomic acceleration/force. It is different in literature about how to
calculate them. There are three different ways as below:

1. (Acceleration)*(Density)=(Pressure drop)/(Length)
Length here is the length of nano-channel

2. (Acceleration)*(Density)=(Pressure drop)/(Length)
Length here is the dimension of simulation box in the z-direction.

3. (Force)*(Number of water molecules)=(Pressure drop)*(Area of simulation
box in the xy-direction)

In my simulations, the second method is used to calculate the atomic
acceleration. The pressure drop is fixed at 200MPa. The results are hard to
understand. The water flow rate increases with the length of the
nano-channel. However, in the Poiseuille flow, the flow rate should be
proportional to the 1/(the length of the nano-channel).

Could you please help me to understand my results? Which method is correct
to calculate the atomic acceleration by the setting pressure drop?

Thank you very much for your attention. I would appreciate it if you have
any comments.

Best,
Jason