In recent years, bionic functional surfaces have played an important role in the manipulation of microfluidic dynamics. Among these, droplets and bubbles have been extensively studied as common interfacial fluids owing to their important application in fields such as droplet microreactors and cell screening . Therefore, it is highly meaningful to achieve a targeted transport and effective collection of droplets and bubbles. Researchers have achieved stable surface-wetting properties, as well as the ability to drive droplets and underwater bubbles simply and efficiently by preparing statically anisotropic structures. However, these static anisotropic structures are prepared by composite structures composed of multiple materials or by electrochemical etching. The preparation process is complex, susceptible to environmental pollution, and only a single target is considered, which significantly limits the application range. In this study, a static anisotropic superhydrophilic asymmetric double rail (SHADR) structure is prepared on the surface of a titanium sheet via nanosecond laser ablation. Based on the Laplace pressure gradient generated by the asymmetric structure, the SHADR structure achieves a spontaneous unidirectional transport of liquid droplets and underwater bubbles. This functional surface provides new ideas for microfluidic manipulation, biomedicine, and other fields.

The titanium sheet is ultrasonically cleaned in deionized water for 10 min to remove surface impurities before processing. A nanosecond fiber laser is used to vertically scan the surface of the titanium sheet to increase the surface roughness. Subsequently, the samples are ultrasonically cleaned once again and modified with a commercial superhydrophobic reagent to enhance the surface hydrophobicity and reduce the surface adhesion. Finally, the modified superhydrophobic surface is scanned using a nanosecond fiber laser-marking machine, and the SHADR structure with a length of 40 mm and an end spacing of 1 mm is processed. The contact angles of 5 μL water droplets on the surface of the titanium sheet are measured using a contact angle measurement system to characterize the surface wettability of the titanium sheet with different structures. The optical images of the droplets and motion of bubbles are captured by a high-speed camera.

Inspired by the conical spines of a cactus and the long-billed beak of a bird, a simple and effective droplet and underwater bubble manipulation platform, that is, a SHADR struture, is prepared on the surface of a titanium sheet via nanosecond laser ablation (Fig.1). Using the Laplace pressure gradient generated by the static anisotropic structure, a SHADR structure achieves the spontaneous unidirectional transport of both droplets (Fig.2) and underwater bubbles (Fig.4). In this study, the effects of the structural parameters, such as the single-rail width (w) and branching angle (α), on the transport performances of the droplets and underwater bubbles are quantitatively investigated (Fig.3) and a simplified mechanical model is analyzed.

In this study, a simple and effective droplet and underwater bubble manipulation platform is prepared on the surface of a titanium sheet using nanosecond laser ablation.The SHADR struture enables the anisotropic spontaneous transport of droplets and bubbles based on the Laplace pressure difference. The droplet transport is recorded by a high-speed charge-coupled device camera, systematically quantifying the droplet transport performances in the horizontal SHADR struture under different parameters (branching angle, single rail width, and bubble volume). This functional surface is not only simple to operate but will also have wide applicability in the fields of microfluidics and interface science.