Under-oil self-driven and directional transport of water on a femtosecond laser-processed superhydrophilic geometry-gradient structure

Self-driven and continuous directional transport of water droplets in an oil environment has great potential applications in microfluidics, oil-water separation, etc. Nevertheless, most current studies exploit water behaviors occurring in air, and the directional regulation of water in a viscous oil medium remains a challenge. In this work, a superhydrophilic geometry-gradient stainless steel platform with nanoparticle-covered nanoripple structures is proposed using femtosecond laser direct writing technology. The as-prepared platform spontaneously and directionally transported water droplets in the oil environment from the minor side to the large side of the trapezoidal platform surface, but not in the opposite direction. The transport velocity of water droplets as a function of trapezoid angle and tilt angle of the as-prepared platform was investigated in detail. In addition, a pumpless under-oil water transport platform was successfully prepared on other substrates including Ti and Ni sheets, polyimide film, and C cloth, and exhibited transport capabilities when the platform was flexed and combined into various shapes. This work offers insight into the simple fabrication of a flexible and substrate-independent pumpless under-oil directional transport device for water.