Conservation of insoluble surfactant on a moving interface
= Conservation of insoluble surfactant on a moving interface
For concentration $C$ per unit <surface area>, conservation on a moving material interface gives
$$
\frac{DC}{Dt}=-C\left[\nabla_s\cdot\mathbf u_s+u_n\nabla_s\cdot\mathbf n\right]+D_s\Delta_sC.
$$
The <material derivative> follows the surface particles. The two dilution terms are tangential expansion and curvature-induced area change from normal velocity $u_n$; the last term is <surface diffusion>.