We now turn to the specification of the wall model. As mentioned in the introduction, in our approach we treat this as a smooth, featureless substrate which interacts with the DPD beads through bead-specific wall potentials. For instance, consider a wall located at the z = 0 plane. We suppose generally there is a hard reflecting barrier at z = 0, so that DPD particles cannot penetrate to z < 0. This is implemented in the simulation using specular (mirror-like) reflection (rather than bounce-back reflections). For z > 0 we augment this with a soft, short-range potential of the form, U s,i