Anisotropic diffusion in a particular cell thread
 

Hello everyone, I want to create a UDF to achieve anisotropic diffusion at first cells beneath a particular surface. What it does is simply just blocking horizontal diffusion in a 3D Cartesian coordinate, while only leaving the vertical direction to be "open". I read the UDF manual, but it seems like the anisotropic diffusion can only be hooked by creating a new material in the material panel. Is there any way to achieve the same effect while not creating a new material and new zone through UDF. Any help will be appreciated.