Liquids have a fixed volume, but the others can expand into the maximum space available.
In biological applications, these particles are usually either molecules or ions. Diffusion is not involved in mixing due to currents caused by pressure differences, or convection, e.g. draughts, winds, pumped liquids, etc. In diffusion, we always think that particles will move from high to low concentration , i.e. down along a concentration gradient . If there is no such difference, no net movement occurs, but we say a dynamic equilibrium exists. Look at this picture of a cell alongside a blood vessel.