18 A. Longitudinal stripes In many situations, such as in microfluidic flow guiding and filamenting [2, 9], it is desirable to orient the growth of the contact line on a prescribed direction. For substrates with spatially varying wetting properties, the flow is expected to orient toward the more hydrophilic regions due to their lower flow resistance. Meanwhile, the front is expected to destabilize on the hydrophobic stripes if their width is of the order of the most unstable wavelength, Λ$_{max}$( θ$_{e}$ = 90 ◦ ) . Hence, our main interest will be to examine the effect of the In this section we explore the dynamics of the thin film on different hydrophilic- hydrophobic heterogeneous substrates. Our aim is to examine the effect of a hydrophilic pattern imposed on a hydrophobic substrate on the dynamics of the film. Furthermore, taking into account the periodicity of the front, we will focus on patterns of characteristic lengthscales which are comparable to Λ$_{max}$( θ$_{e}$ = 90 ◦ ). FIG. 6: Contact line growth on a pattern composed of hydrophilic-hydrophobic stripes oriented in the direction of flow. The dark and light stripes correspond to the hydrophilic (grey) and hydrophobic (white) substrates, respectively. Their width, w , equals the most unstable wavelength observed for the hydrophobic substrate, Λ$_{max}$( θ$_{e}$ = 90 ◦ ) /x$_{c}$ ≈ 8 . The first five panels show the contact line at time intervals of Δ t/t$_{c}$ = 33 . 5. The first panel shows the perturbed contact line at t/t$_{c}$ = 0. The last three panels show the effect of the longitudinal pattern by comparing the contact line on the patterned substrate to the heterogeneous and homogeneous substrates with the same initial condition at t/t$_{c}$ = 134. Wetting properties are fixed to θ$_{e}$ = 0 ◦ at the grey zones, while at the white zones θ$_{e}$ = 90 ◦ .