"Isothermal" simply means that its one temperature from top to bottom.
Settling is what happens to heavy particles in the liquid. Not surprisingly, big heavy particles drop like a rock and end up at the bottom (H=0). Others take more time and fall onto the accumulation of bigger particles.
Okay, so would you consider the cell to be 'isothermal'? I don't, but apparently it is according to this Paul Todd paper (See attached).
Overall the cell may be 'isothermal' to maintain a constant 37 oC... but within the molecular environment there endothermic/exothermic reactions where heat is being transferred and local temperature variations occur due to increases in kinetic energy.
He mentions this isothermal settling is a process to be considered inside cells. What do you think about that? Isothermal settling is a static state of equilibrium of no net mass movement, with more particles concentrating towards the bottom. I wouldn't think cells would be able to survive under these conditions.