Multifunctional droplet manipulation technology combining femtosecond laser-designed slippery surface and electrostatic interactions

Controllable droplet manipulation is essential in a variety of applications, such as biological detection, digital microfluidics, chemical reactions, heat management, water harvesting, and printing technology. The contact operation will carry the risk of contaminating the droplet and can easily cause volume loss due to liquid adhesion.

Therefore, noncontact droplet manipulation is required in high-standard application scenarios. The existing methods of contactless droplet manipulation are usually limited by short transport distances, poor flexibility, and specially designed substrates. Despite significant progress, developing an on-demand, noncontact, and substrate-independent method of manipulating droplets still remains challenging.

In a new paper published in the International Journal of Extreme Manufacturing, a team of researchers, led by Prof. Dong Wu and Dr. Jiale Yong from the Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, China, have proposed an innovative strategy for achieving noncontact and multifunctional droplet manipulation by combining femtosecond laser-designed lubricated slippery surfaces and electrostatic interactions.

Under the influence of the electrostatic field generated by the charged rods, the positive and negative charges inside the droplet are redistributed, making the droplet move under the action of electrostatic force. This technology is named as “triboelectric electrostatic tweezer (TET)”.

A Nepenthes-inspired lubricated slippery surface was prepared by femtosecond laser processing, which has ultralow adhesion to various droplets and was used as the operation platform. Femtosecond (10⁻¹⁵ s) laser has become an essential tool for modern extreme and ultra-precision manufacturing because of its two key features: extremely high peak intensity and ultrashort pulse width. Hierarchical porous micro/nanostructures were easily produced on the surface of the inherently hydrophobic polydimethylsiloxane (PDMS) sheet by femtosecond laser processing.

After the infusion of lubricant silicone oil into the laser-induced porous microstructures, a smooth and ultrathin lubricant layer is formed on the PDMS surface. The slippery surface shows excellent liquid repellence because the trapped lubricant layer prevents the droplet from making effective contact with the solid substrate, so that droplets can easily move on such a slippery surface.

The features of slippery surfaces endow TET with diverse working conditions and functions. For example, TET can drive a droplet to climb an inclined surface and even move suspended droplets on an inverted/upright surface. Various droplets with different volumes (from 100 nL to 0.5 mL), chemical compositions, and surface tensions can be manipulated on the slippery surface. Even the corrosive HCl acid, NaOH alkali, NaCl salt solutions, and ethanol with a surface tension of only 22.3 mN/m can be manipulated by electrostatic interaction.

The manipulation is also suitable for small bubbles and solid balls. The flexibility of liquids allows droplets to be driven to pass through a narrow slit. The electrostatic force can also attract droplets to slide over damaged areas because of the inherent self-repairing ability of slippery surfaces. TET can support a wide range of droplet-related applications, such as motion guidance, motion switching, droplet-based microreaction, surface cleaning, surface defogging, liquid sorting, and cell labeling. The combination of TET with slippery surfaces will lead to exciting applications ranging from biological analysis to chemical manufacturing and from microfluidics to intelligent printing.

Associate Professor Jiale Yong has identified the significance of the research and the potential applications of this technology (triboelectric electrostatic tweezer) as follows:

“How to think of using tribostatic electricity to manipulate droplets?”

“Tribostatic electricity is one of the most common physical phenomena in daily life. Static electricity can attract some of the little things, such as hair, scraps of paper, etc. When we hold an electrostatic glass rod close to free-falling droplets, we find that the droplets can either be attracted or repelled away. These phenomena suggest that tribostatic electricity can exert an electrostatic force on the droplet and thus can potentially be used to remotely manipulate droplets.”

“Why use lubricated slippery surfaces prepared by femtosecond laser processing as a platform for droplet manipulation?”

“In order to transport the droplet, it is also required that the droplet cannot firmly adhere to the operating platform in addition to applying a driving force to the droplet; that is, the droplet must be able to slide easily on the platform. Nepenthes-inspired lubricated slippery surfaces meet this need. The slippery surface has ultralow adhesion to different droplets in the horizontal direction, allowing the droplet slide on such surface easily. However, there is a binding force to the droplet in the vertical surface direction, ensuring that the droplet will not break away from the surface. Femtosecond laser is one of the advanced micro/nano-manufacturing technologies, which can directly prepare porous microstructures on the surface of different materials (which is the most critical structure for preparing slippery surfaces), and is one of the most common means for preparing lubricated slippery surfaces. Therefore, the slippery surface prepared by the femtosecond laser is used as a platform for droplet manipulation.”

“Besides water droplets, what else can triboelectric electrostatic tweezers operate?”

“Experiments have shown that small bubbles and plastic balls can also be moved by triboelectric electrostatic tweezers. Of course, other small objects that can respond to static electricity can also be manipulated.”

“What are the advantages and the potential applications of the triboelectric electrostatic tweezer in manipulating droplets?”

“The droplet control technology based on triboelectric electrostatic tweezers has the characteristics of simple operation, high operation precision, and strong operation flexibility. We believe that the combination of triboelectric electrostatic tweezer with slippery surfaces will lead to exciting applications ranging from biological analysis to chemical manufacturing and from microfluidics to intelligent printing.”

Researchers also point out that this technology for multifunctional droplet manipulation based on the triboelectric electrostatic tweezer, while validated, is still in its infancy. The influence of various factors on the performance of droplet manipulation needs further research. The practical applications based on triboelectric electrostatic tweezer also need to be developed.

About IJEM:

International Journal of Extreme Manufacturing (IF: 14.7, 1st in the Engineering, Manufacturing category in JCR 2023) is a new multidisciplinary and open-access after double-anonymous peer reivew journal uniquely covering the full spectrum of extreme manufacturing.

The journal is devoted to publishing original articles and reviews of the highest quality and impact in the areas related to the science and technology of manufacturing functional devices and systems with extreme dimensions (extremely large or small) and/or extreme functionalities, ranging from fundamental science to cutting-edge technologies that support the manufacturing of high-performance products involving emerging techniques and breaking the limits of currently known theories, methods, scales, environments, and performance.

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