The surface of the PDMS film was scanned under an optical profilometer (Zegage optical profilometer procured from Zygo), through an area: 1.7 mm x 1.7 mm in non-contact mode. The profile along a line normal to the channel axis was measured to obtain the deflection, ) (x ψ and the maximum deflection, δ at the location of minimum thickness, 0 t of the channel wall.