Now steepest gradient computed with central difference.

eff_Drop.jl

@@ -223,10 +223,14 @@ function solve_D(geometry,IC,AG,B,P,D_i,D_o,Δx,Δt,L,R,BP,x0,T,perc,BE,tE)
     #function m computes the steepest gradient
     if (D_o/Δxo)>(D_i/Δxi)
         Δxm=Δxi
-        m =(u,Δxm) -> abs(u[Nb-1]-u[Nb-2])/Δxm
+        m =(u,Δxm) -> abs(u[Nb-1]-u[Nb-3])/(2*Δxm)
+        #sets the index of the point with steepest derivative
+        Id=Nb-2
     else
         Δxm=Δxo
-        m =(u,Δxm) -> abs(u[Nb+3]-u[Nb+2])/Δxm
+        m =(u,Δxm) -> abs(u[Nb+4]-u[Nb+2])/(2*Δxm)
+        #sets the index of the point with steepest derivative
+        Id=Nb+3
     end
     m_mat=zeros(Nt)