Which diffusion coefficient to use in evaporation of moisture from a slurry droplet?
 

I am doing multiphase modelling in which slurry droplet moisture is evaporated to the air. Which eventually results in solid particle formation. I am using Lagrangian particle tracking. In the particle properties, I have a value of diffusivity. I dont know whether I have to put diffusivity of water into air or do I have to put diffusivity of moisture from the bulk of the droplet/particle to the surface. These two diffusivities differ in several orders of magnitude.

Can anyone help me?