Dear Seyede, in the case of hydrostatic configuration, inertia forces can be neglected; but if an external force is applied, then the term F in Navier-Stokes equations can have relevant role and it should be considered in computation: convective force + viscous force = gradient pressure + F.  Gianluca

Thank you Dear Gianluca. but my polymeric liquid is not moving and it is stagnant on a flat plate . I want to now the motion equation acting on the fine particles suspended in it when we have F. I know that we have viscose force but I do not know when I am balancing the forces (Viscose and F) should I also have the term Inertia or simply put 0 instead...

Inertia of an object is expressed by its mass. So if you have "m" in your expression already, then you have included it in your treatment. However, if its not there, it could be a wise step to include it as then the treatment will be comparatively better than the former case.

Also I suggest you use Brownian motion treatment for your problem if your main motive to analyse the motion of the suspended particle.

Thank you so much. Yes I think also Brownian motion is to consider