The no flux boundary conditions are making trouble. How to handle them? The problem I’m trying to solve is as follows: PDE: 0.036*D[C[x,y],x]==2.4*10^-10*(D[C[x,y],{x,2}]+D[C[x,y],{y,2}]) Boundary conditions: 1. C[0,y]==Piecewise[{{1,H/21)==0 3. (D[C[x,y],y]/.y->0)==0 4. (D[C[x,y],y]/.y->0.005)==0

I wrote the following: sol2=NDSolve[{0.036*D[C[x,y],x]==2.4*10^-10*(D[C[x,y],{x,2}]+D[C[x,y],{y,2}]),C[0,y]==Piecewise[{{1,H/21)==0,(D[C[x,y],y]/.y->0)==0,(D[C[x,y],y]/.y->0.005)==0},C,{x,0,1},{y,0,0.005}]

The error message shows-The dependent variable in the boundary condition Dirichlet condition need to be linear. But I noticed that a similar no flux boundary condition works just fine (for a different problem though). sol3=NDSolve[{D[u[t, x], t] == D[u[t, x], x, x],u[0, x] == 0,u[t, 0] == Sin[t],(D[u[t, x], x] /. x -> 0)== 0},u, {t, 0, 10}, {x, 0, 5}]

What is the issue in the previous case and how to resolve it?