Microhole-Arrayed PDMS with Controllable Wettability Gradient by One-Step Femtosecond Laser Drilling for Ultrafast Underwater Bubble Unidirectional Self-Transport

Achieving the unidirectional transportation of bubbles in the liquid phase is of great importance for solving both academic and industrial issues. Here, Janus (hydrophobic and superhydrophobic surfaces) microhole-arrayed polydimethylsiloxane fabricated by one-step femtosecond laser drilling for ultrafast underwater bubble unidirectional transportation is reported. In aqueous solution, bubbles selectively penetrate from an aerophilic side to a superaerophilic one in the direction of both buoyancy and antibuoyancy, but are blocked in a reverse direction. More importantly, the bubbles readily penetrate through this Janus system within 81 ms, which is two orders of magnitude shorter than that of a previous Janus one because the aerophilic surface of current Janus system is more favorable for capturing and transporting the bubble than the superaerophobic surface of Janus system. Additionally, this "diode" presents a switchable property, which is dependent on the laser exposure dosage. According to X-ray photoelectron spectroscopy spectrum, the underlying mechanism is that the excessive laser exposure dosage is inclined to induce the graft of oxyhydryl group as the substitution of the original hydrocarbyl group. This work may provide an innovatory insight for designing advanced materials for ultrafast gas bubble directional transportation/collection in aqueous media, in addition to gas/liquid separation.