Directional liquid spreading on laser textured aluminum surface

The wettability property of a material surface is highly dependent on the topography of the surface micro/nanostructure. Femtosecond (fs) laser can be used to fabricate various kinds of surface micro/nanostructures. We present parallel microgroove arrays with unique cross-sectional profiles on the surface of an aluminum foil induced by varying the laser fluence and the scanning spacing. The laser textured aluminum surface shows directional spreading of water and inherently hydrophilic aluminum surface becomes superhydrophilic. A water droplet spreads highly anisotropically on the processed area and flows preferentially along the microgrooves. The maximum average spreading velocity of water droplet along the laser textured microgrooves is ~ 200 mm/s, while it is much slower in the direction perpendicular to the microgrooves, which is 41 mm/s. Moreover, the spreading distance of water and time t in both directions have a linear relation with t^1/2. The fast-self-spreading motion of water along the microgroove is due to its capillary pressure and a local energy barrier at the boundary between two parallel microgrooves. In addition, laser-modified surface roughness and laser-altered chemical composition on the aluminum surface play a role in enhancing the superhydrophilicity and the directional water spreading behavior. Our research can be applied in the fields of microfluidics, lab-on-chip technology, chemical and biological sensors and heat transfer devices.