Surface Reaction on Streamline and Velocity Vectors

Why is there a surface reaction on the streamline and velocity vectors of the wall?

It is possible to see a surface reaction at the end of the streamlines of the wall, with the direction of the velocity vector towards the wall. This is physically correct. By the decomposition reaction of gaseous species, the rate on the wall will not occur. This speed is dependent on the deposition rate of the gas species deposited on the wall (Deposition Rate). By checking the mass flux balance in all of the border, including a reaction wall, mass flux flowing into the reaction wall will be clear.