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

Abstract

Underwater bubbles are significant fluid systems that have attracted great research interest because of their enormous potential applications in photovoltaic and environmental-remediation areas.[1–4] The presence of gas bubbles is sometimes contributive to elevating heat transfer in the ocean.[5,6] Nevertheless, the gas bubbles generated during the exploitation of crude oil including carbon dioxide, hydrogen sulfide, and oxygen can lead to the severe corrosion for the transporting pipes, which is inclined to shorten equipment lifetimes and increase resource consumption.[7,8] In consequence, realizing the controllable transportation of underwater bubbles is of great importance for solving practical issues. Inspired by the unidirectional fluid transportation,[9–17] researchers have revealed that the dominant factor playing the decisive role in achieving the unidirectional transportation of underwater gas bubbles should be assigned to the asymmetric wettability of a Janus system (an integration of hydrophobic and hydrophilic surface). On the basis of this strategy, great efforts have been paved to realize this target (Table 1). For the first time, Chen et al. employed a Janus copper mesh that had been decorated with a layer of hydrophilic TiO2 nanoparticles on the upper face and grafted a layer of hydrophobic dodecanethiol on the lower surface to fabricate an air “diode,” which endowed the mesh with an admired underwater bubble unidirectional transportation property.[18] Besides, Yong et al. utilized a Janus hierarchical through microhole-arrayed (MHA) polydimethylsiloxane (PDMS) sheet with an intrinsic hydrophobic upper surface and a hydrophilic lower surface that had been modified by oxygen plasma to realize a selective passage for underwater bubbles.[19] Recently, Pei et al. demonstrated an antibuoyancy unidirectional transportation of bubbles by using an integrated Janus copper mesh, which had been modified by chemical etching to induce a superaerophobic upper surface and n-tetradecyl mercaptan/ ethanol solution to obtain an aerophilic (AL) lower surface. 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.