Coupling between the MPCD fluid and embedded objects. (a) 2D sketch of collision step. Here immersed solid spheres are shown in red and a wall in grey. Small dots in the white region indicate the fluid particles and dots in the spheres and in the wall are virtual particles, which are located in a thin layer of thickness d_W = a₀ in the wall and in a thin shell of thickness dS=3a0 in the spheres. The dashed lines show the collision cell grid (from Ref. [14]). (b) 2D sketch of model polymers in MPCD fluid: polymers (light and dark green), MPCD fluid particles (black dots). The rectangular collision cell grid is shifted randomly in every time step.