Consider a desorption process in a spherical sample. Initially, the sphere has uniform species A mass

fraction $w_{A0}$ and radius $R$; for $t>0,$ the species A mass fraction at the surface of the sphere is

maintained at $w_{\text{A}\text{e}\text{q}}.$ Assuming $$ is constant and that $R$ is approximately constant, write in dimensionless

form the evolution equation for $w_A$ with the opportune initial and boundary conditions. Derive a

solution for the obtain partial differential equation using the change of variable $u=r{w}_A.$