by an axis which is oriented transversely to the entrance and exit edges 54 154 56 and 156 and extends from the entrance of the channel to the exit of the channel in connection with the second embodiment the substrate 100 is transported via rollers 80 and 90 through the above defined channel in a direction which is substantially parallel to the channel axis this transport occurs at a distance h1 above the surface 52 and at a distance h2 below the surface 152 the distances h1 and h2 need not be the same as shown in fig4 and 5 the second embodiment includes at least one row 60 and preferably two rows 60 and 70 of fluid jet injectors penetrating the surface 52 the fluid jet injectors of the rows 60 and 70 are of equal diameter d1 and the fluid jets which they emit exit the fluid jet injectors with fluid speed v1 the fluid in these fluid jets is characterized by a kinematic viscosity nu1 preferably the row 60 is positioned closer to the entrance edge 54 than to the exit edge 56 for the reason discussed above the second embodiment also includes at least one row 160 and preferably two rows 160 and 170 of fluid jet injectors penetrating the surface 152 the fluid jet injectors of the rows 160 and 170 are of equal diameter d2 and the fluid jets which they emit exit the fluid jet injectors with fluid speed v2 the fluid in these fluid jets is characterized by a kinematic viscosity nu2 the diameter d2 is not necessarily equal to the diameter d1 the kinematic viscosity nu2 is not necessarily equal to the kinematic viscosity nu1 and the fluid speed v2 is not necessarily equal to the fluid speed v1.