diff --git a/common/Domain.h b/common/Domain.h
index bb70d4dd..ea5f08e1 100755
--- a/common/Domain.h
+++ b/common/Domain.h
@@ -315,7 +315,13 @@ inline void GenerateResidual(char *ID, int Nx, int Ny, int Nz, double Saturation
 	//.......................................................................
 }
 
-
+inline void FlipID(char *ID, int N)
+{
+	for (int n=0; n<N; n++){
+		if  (ID[n] == 1)	ID[n] = 2;
+		else if  (ID[n] == 2)	ID[n] = 1;
+	}
+}
 
 inline void WriteLocalSolidID(char *FILENAME, char *ID, int N)
 {
diff --git a/tests/lb2_Color_wia_mpi.cpp b/tests/lb2_Color_wia_mpi.cpp
index 8a5add07..d604e76a 100644
--- a/tests/lb2_Color_wia_mpi.cpp
+++ b/tests/lb2_Color_wia_mpi.cpp
@@ -493,6 +493,14 @@ int main(int argc, char **argv)
 	// Generate the residual NWP 
 	if (rank==0) printf("Initializing with NWP saturation = %f \n",wp_saturation);
 	if (!pBC)	GenerateResidual(id,Nx,Ny,Nz,wp_saturation);
+	
+	// If negative phi_s is chosen, flip the ID for the wetting and non-wetting phase
+	if (phi_s < 0){
+		phi_s = -phi_s;
+	 	das = (phi_s+1.0)*0.5;
+		dbs = 1.0 - das;
+		FlipID(id,Nx*Ny*Nz);
+	}
 #endif
 
 	// Set up MPI communication structurese