The reason is that the heat current which flows in response to \bnablaT as well as the momentum current which flows in response to \pzV\nsx/\pzz are due to the presence of collisions, whic...The reason is that the heat current which flows in response to \bnablaT as well as the momentum current which flows in response to \pzV\nsx/\pzz are due to the presence of collisions, which result in momentum and energy transfer between particles.