An apparatus comprising: a micro channel comprising a specular wall portion, an inflow opening, and an outflow opening; and a fluid comprising constituent particles, the fluid being induced to flow through the micro channel through operation of a pressure differential between a first pressure of the fluid proximal to the inflow opening and a second pressure of the fluid proximal to the outflow opening; wherein the transfer of energy from the wall portion to the fluid occurs predominantly through the average change in the speed of said constituent particles associated with the change in the velocity component perpendicular to the plane of said wall portion during the collisions between the constituent particles and the wall portion; and wherein either the micro channel has a generally continuously increasing cross section along the direction of flow of the fluid, or the variation in the cross section of the micro channel as a function of a length in the first direction between the inflow opening and the outflow opening is substantially abrupt in a region proximal to the inflow opening, is substantially constant between the region proximal to the inflow opening and the outflow opening, and the cross section between the region proximal to the inflow opening and the outflow opening is greater than the cross section in the region proximal to the inflow opening. a micro channel comprising a specular wall portion, an inflow opening, and an outflow opening; and a fluid comprising constituent particles, the fluid being induced to flow through the micro channel through operation of a pressure differential between a first pressure of the fluid proximal to the inflow opening and a second pressure of the fluid proximal to the outflow opening; wherein the transfer of energy from the wall portion to the fluid occurs predominantly through the average change in the speed of said constituent particles associated with the change in the velocity component perpendicular to the plane of said wall portion during the collisions between the constituent particles and the wall portion; and wherein either the micro channel has a generally continuously increasing cross section along the direction of flow of the fluid, or the variation in the cross section of the micro channel as a function of a length in the first direction between the inflow opening and the outflow opening is substantially abrupt in a region proximal to the inflow opening, is substantially constant between the region proximal to the inflow opening and the outflow opening, and the cross section between the region proximal to the inflow opening and the outflow opening is greater than the cross section in the region proximal to the inflow opening. the micro channel has a generally continuously increasing cross section along the direction of flow of the fluid, or the variation in the cross section of the micro channel as a function of a length in the first direction between the inflow opening and the outflow opening is substantially abrupt in a region proximal to the inflow opening, is substantially constant between the region proximal to the inflow opening and the outflow opening, and the cross section between the region proximal to the inflow opening and the outflow opening is greater than the cross section in the region proximal to the inflow opening.