I want to simulate the experiment of microneedle extraction of cytoplasm, but how to achieve the pressure difference between the cell and the microneedle, so that the cytoplasm can flow out smoothly.
What command is more appropriate?
What timescales are you interested in? The real extraction process involves relatively large regions of space. You may be better off characterizing steady-state flow using one of the methods generally used in LAMMPS.
Choosing the right method depends very highly on the boundary conditions, and on what you plan on obtaining from this simulation beyond the animation itself. Unless your model is so much more detailed than is typically done with DPD, you are basically simulating flow of a model fluid. I don’t think it would be fair to call it “cytoplasm” unless you have a fair representation of the proteins dissolved in it.
See this older thread about a very similar question: