A drug is infused into a cylindrical blood vessel $($artery$).$ Drug concentration in the blood is constant at $C_b$ whereas drug concentration in the vessel is $C_0$ in the outer part of the artery. The inner radius of the artery is $R_b$ and the outer radius of the vessel is $R_0.$

Determine the concentration profile and flux of the drug into the artery at $R_b.$ Neglect chemical reaction.

Assumptions:

Steady$-$state, no chemical reaction, one$-$dimensional $($radial$),$ constant diffusion coefficient, stationary medium $($no convection$)$

Boundary conditions:

@ $r=R_b,c=c_b$

@$r=R_0,c=c_0$