Superhydrophobic-superhydrophilic patterned surfaces have attracted more and more attention due to their great potential applications in the fog harvest process. In this work, we developed a simple and universal electrochemical-etching method to fabricate the superhydrophobic-superhydrophilic patterned surface on metal superhydrophobic substrates. The anti-electrochemical corrosion property of superhydrophobic substrates and the dependence of electrochemical etching potential on the wettability of the fabricated dimples were investigated on Al samples. Results showed that high etching potential was beneficial for efficiently producing a uniform superhydrophilic dimple. Fabrication of long-term superhydrophilic dimples on the Al superhydrophobic substrate was achieved by combining the masked electrochemical etching and boiling-water immersion methods. A long-term wedge-shaped superhydrophilic dimple array was fabricated on a superhydrophobic surface. The fog harvest test showed that the surface with a wedge-shaped pattern array had high water collection efficiency. Condensing water on the pattern was easy to converge and depart due to the internal Laplace pressure gradient of the liquid and the contact angle hysteresis contrast on the surface. The Furmidge equation was applied to explain the droplet departing mechanism and to control the departing volume. The fabrication technique and research of the fog harvest process may guide the design of new water collection devices.
Superhydrophobic surfaces with hydrophilic patterns have wide applications in biomedical and chemical analysis domains. In this work, a rapid, simple and top-down micro-milling method was proposed for fabricating hydrophilic patterns such as micro dots, line and circle grooves on superhydrophobic surfaces. This method could be extended to construct various linear patterns on diverse substrates on account of its mechanical material-removal mechanism. Hydrophilic micro dots, line and circle grooves were milled on the superhydrophobic Al alloy surface. The milled micro dots demonstrate great adhesion towards water droplets without changing the contact angles whereas the pre-wetted line grooves exhibit strong anisotropic water adhesion, that is, the resistance force that restricts the droplet from detaching in directions parallel and perpendicular to the grooves is significantly different due to the different stress state of the droplet on the grooves. The adhesion force perpendicular to the groove, together with the sliding resistance that is generated by the milled dot, was investigated through experiments. The results show that the relationship between the adhesion force and the droplet-pattern interfacial widths could be well described using the classical Furmidge equation. This research could possibly be employed in such biomedical and chemical analysis domains as water harvesting, droplet storage and droplet transport.
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