Surfactant transport with exchange relaxation
= Surfactant transport with exchange relaxation
{title2=$D_t C=-C\nabla_s\cdot\mathbf u_s-Cu_n\kappa+D_s\Delta_sC-k(C-C_0)$}
The <material derivative> of surface concentration combines tangential and normal area change, <surface diffusion>, and relaxation towards a reservoir concentration $C_0$. The rate $k$ represents exchange; setting $k=0$ recovers transport without exchange. The concentration is measured per interfacial area, so stretching dilutes it.