Jiale Yong

Bioinspired wetting surface via laser microfabrication.

Bioinspired special wettibilities including superhydrophobicity and tunable adhesive force have drawn considerable attention because of their significant potential for fundamental research and practical applications. This review summarizes recent progress in the development of bioinspired wetting surfaces via laser microfabrication, with a focus on controllable, biomimetic, and switchable wetting surfaces, as well as their applications in biology, microfluidic, and paper-based devices, all of which demonstrate the ability of laser microfabrication in producing various multiscale structures and its adaptation in a great variety of materials. In particular, compared to other techniques, laser microfabrication can realize special modulation ranging from superhydrophilic to superhydrophobic without the assistance of fluorination, allowing much more freedom to achieve complex multiple-wettability integration. The current challenges and future research prospects of this rapidly developing field are also being discussed. These approaches open the intriguing possibility of the development of advanced interfaces equipped with the integration of more functionalities.

A simple way to achieve superhydrophobicity, controllable water adhesion, anisotropic sliding, and anisotropic wetting based on femtosecond-laser-induced line-patterned surfaces

The superhydrophobicity, controllable water adhesion, anisotropic sliding, and anisotropic wetting, which are four typical aspects of the wettability of solid surfaces, have attracted much interest in fundamental research and practical applications. However, how to use a simple and effective method to realize all those properties is still a huge challenge. Here, we present a method to realize periodic line-patterned polydimethylsiloxane (PDMS) surfaces by a femtosecond laser simply and efficiently. By adjusting the period (D) or average distance of adjacent microgrooves, the as-prepared surfaces can exhibit superhydrophobicity, controllable water adhesion, anisotropic sliding, and anisotropic wetting. We believe that these multifunctional surfaces have enormous potential applications in novel microfluidic devices, microdroplet manipulation, liquid microdroplet directional transfer, and lab-on-chips.

Rapid Fabrication of Large-Area Concave Microlens Arrays on PDMS by a Femtosecond Laser

A fast and single-step process is developed for the fabrication of low-cost, high-quality, and large-area concave microlens arrays (MLAs) by the high-speed line-scanning of femtosecond laser pulses. Each concave microlens can be generated by a single laser pulse, and over 2.78 million microlenses were fabricated on a 2 × 2 cm(2) polydimethylsiloxane (PDMS) sheet within 50 min, which greatly enhances the processing efficiency compared to the classical laser direct writing method. The mechanical pressure induced by the expansion of the laser-induced plasmas as well as a long resolidifing time is the reason for the formation of smooth concave spherical microstructures. We show that uniform microlenses with different diameters and depths can be controlled by adjusting the power of laser pulses. Their high-quality optical performance is also demonstrated in this work.

Bioinspired underwater superoleophobic surface with ultralow oil-adhesion achieved by femtosecond laser microfabrication

Femtosecond laser microfabrication has been recently utilized in interface science to modify the liquid wettability of solid surfaces. In this paper, a silicon surface with hierarchical micro/nanostructure is fabricated by a femtosecond laser. Similar to fish scales, the laser-induced surface shows superhydrophilicity in air and superoleophobicity underwater. The oil contact angles can reach up to 159.4 ± 1° and 150.3 ± 2°, respectively, for 1,2-dichloroethane and chloroform droplets in water. In addition, the surface exhibits ultralow oil-adhesion. In the oil–water–solid three-phase system, water can be trapped in the hierarchical rough structure and form a repulsive oil layer according to the extended Cassies theory. The contact area between the as-prepared surface and oil droplets is significantly reduced, resulting in superoleophobicity and ultralow oil-adhesion in water. In addition, as a potential application, the working principle diagram of preventing blockage ability of underwater superoleophobic pipes is propounded.

Bioinspired transparent underwater superoleophobic and anti-oil surfaces

Reported here is a bioinspired fabrication of transparent underwater superoleophobic and anti-oil surfaces using a femtosecond laser treatment. Rough nanoscale structures were readily created on silica glass surfaces by femtosecond laser-induced ablation. Underwater superoleophobicity and ultralow oil-adhesion were obtained by the rough nanostructures with a wide variation of processing parameters, and the as-prepared surfaces exhibited a high transparency in water. This phenomenon is attributed to the presence of the water environment because scattering and refraction are effectively weakened. As a maskless and cost-effective method, the femtosecond laser processing of transparent materials (glass) may provide a new method to create biomimetic transparent underwater surfaces, allowing for the development of novel underwater anti-oil optical devices.

Photoinduced switchable underwater superoleophobicity–superoleophilicity on laser modified titanium surfaces

Switchable underwater superoleophobicity–superoleophilicity on femtosecond laser-induced rough TiO2 surfaces by alternate UV irradiation and dark storage is achieved for the first time. Femtosecond laser ablation not only forms a micro/nanoscale hierarchical rough structure but also oxidizes the Ti materials, resulting in a rough TiO2 layer covering on the surface. The reversible switching of underwater oil wettability is caused by photoinduced switching between superhydrophobic and superhydrophilic states in air. These rough TiO2 surfaces can even respond to visible light. We believe this subtle switching method will be potentially applied in the biological and medical fields.

Fabrication of large-area concave microlens array on silicon by femtosecond laser micromachining

In this Letter, a novel fabrication of large-area concave microlens array (MLA) on silicon is demonstrated by combination of high-speed laser scanning, which would result in single femtosecond laser pulse ablation on surface of silicon, and subsequent wet etching. Microscale concave microlenses with tunable dimensions and accessional aspherical profile are readily obtained on the 1  cm × 1  cm silicon film, which are useful as optical elements for infrared (IR) applications. The aperture diameter and height of the microlens were characterized and the results reveal that they are both proportional to the laser scanning speed. Moreover, the optical property of high-performance silicon MLAs as a reflective homogenizer was investigated for the visible wavelength, and it can be easily extended to IR light.

Bioinspired superhydrophobic surfaces with directional Adhesion

Butterfly wings have the ability to directionally control the movement of water microdroplets. However, the realization of artificial directional sliding biosurfaces has remained challenging. Inspired by butterfly wings, a new kind of directional patterned surface is developed to achieve superhydrophobicity and anisotropic adhesive properties at the one-dimensional level. The surface is composed of a hydrophobic triangle array and surrounding superhydrophobic structure. On the as-prepared surface, a droplet rolls along one direction distinctly easier than its opposite direction. The maximum anisotropy of sliding angles along two opposite directions can reach 21°. This unique ability is ascribed to the direction-dependent arrangement of the two-dimensional (2D) triangle array patterns. The directional adhesive superhydrophobic surfaces could be potentially applied in novel microfluid-controllable devices and directional easy-cleaning coatings.

A bioinspired planar superhydrophobic microboat

In nature, a frog can easily rest on a lotus leaf even though the frog’s weight is several times the weight of the lotus leaf. Inspired by the lotus leaf, we fabricated a planar superhydrophobic microboat (SMB) with a superhydrophobic upper surface on a PDMS sheet which was irradiated by a focused femtosecond laser. The SMB can not only float effortlessly over the water surface but can also hold up some heavy objects, exhibiting an excellent loading capacity. The water surface is curved near the edge of the upper surface and the SMB’s upper edge is below the water level, greatly enhancing the displacement. Experimental results and theoretical analysis demonstrate that the superhydrophobicity on the edge of the upper surface is responsible for the SMB’s large loading capacity. Here, we call it the ‘superhydrophobic edge effect’.

Controllable underwater anisotropic oil-wetting

This Letter demonstrates a simple method to achieve underwater anisotropic oil-wetting using silicon surfaces with a microgroove array produced by femtosecond laser ablation. The oil contact angles along the direction perpendicular to the grooves are consistently larger than those parallel to the microgroove arrays in water because the oil droplet is restricted by the energy barrier that exists between the non-irradiated domain and the trapped water in the laser-ablated microgrooves. This underwater anisotropic oil-wetting is able to be controlled, and the anisotropy can be tuned from 0° to ∼20° by adjusting the period of the microgroove arrays.