How do I use the Boundary Conditions function in pdepe?

1) I'm using the below code to graph the heat equation at different times and I want DCdx(0) = 0 for my left boundary condition. However, when I graph it the graph only approaches zero at a faster rate for different times, it never actually crosses zero which is what I want it to do for small times. Am I entering the boundary conditions incorrectly and how do I fix this problem?

2) Also, if I want DCdx(Xc) = 0 for Xc = some critical X that I calculate separately, how do I incorporate that into my code?

function pdex1

m = 0; x = linspace(0,1,20); t = linspace(0,20,50); hold on

sol = pdepe(m,@pdex1pde,@pdex1ic,@pdex1bc,x,t); C = sol(:,:,1);

hold on plot(x,C(1,:)); % -------------------------------------------------------------- function [c,f,s] = pdex1pde(x,t,u,DCDx) L = 0.1; Dg = 5.2e-6; A = L^2/Dg; c = A; f = DCDx; s = 0; end % -------------------------------------------------------------- function C0 = pdex1ic(z) C0 = 0; end % -------------------------------------------------------------- function [pl,ql,pr,qr] = pdex1bc(xl,Cl,xr,Cr,t) pl = 0; ql = 1; pr = Cr - 1; qr = 0; end end