The University of Science and Technology of China team creates a triboelectric s
Recently, Associate Researcher Yong Jiale and his team from the University of Science and Technology of China, with the assistance of an electric field and based on the ultra-slippery surface of liquid infusion, have enabled electrostatic tweezers to manipulate droplets under various conditions.
This technology is expected to be applied in microfluidic systems for controlling the direction of fluid movement, etc. It can also be used to manipulate liquids containing cells to achieve operations in cell engineering.
In space laboratories, this technology can also be used to manipulate droplets or other tiny objects, aiding in the conduct of space experiments.
Droplets: A Familiar Presence
In fact, droplets are not unfamiliar to us; they are one of the most common objects in human life.When you accidentally spill milk and beverages on the table, it is important to wipe them up promptly to prevent stains from forming. This is actually a simple process of droplet manipulation.
Droplet manipulation technology plays a crucial role in both basic research and practical applications and has been widely used in fields such as thermal management, bio-detection, digital microfluidics, chemical reactions, fog collection, and printing technology.
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In conventional droplet manipulation techniques, tools such as pipettes, glass rods, and syringe needles are used to release and move droplets. This manipulation technique is a contact-based method, which involves direct contact between the manipulation tool and the droplet.
However, direct contact with droplets can easily contaminate them. At the same time, due to the adhesion and residue of droplets, contact-based manipulation can also lead to a loss of droplet volume and affect the precision of the operation.
Therefore, in some applications with higher requirements, non-contact droplet manipulation techniques are often needed, which means using remote manipulation methods that do not allow the manipulation tool to come into direct contact with the droplets.Previously, in order to manipulate droplets in a non-contact manner, people began to use some methods based on magnetic fields and photothermal radiation.
However, for magnetic and optical fields, they cannot directly exert force on droplets.
Moreover, these methods usually require the addition of responsive dopants to the substrate material, and by changing the microstructure or other physicochemical properties of the prepared operation platform, to indirectly drive the movement of droplets.
These indirect operation methods are limited by the special material composition and microstructure of the operation platform, which makes it difficult to promote the application scenarios.
As we all know, triboelectricity can attract small objects such as hair and paper scraps. Can electrostatics attract droplets?The answer is affirmative; researchers have found that static electricity can directly exert electrostatic forces on droplets.
However, to manipulate or move droplets, merely applying force to them is insufficient.
For the manipulation platform, it must have very low adhesion to the droplets to prevent them from tightly adhering to the solid surface to the point where they cannot be moved.
To achieve this goal, the first thing that comes to mind is to use a superhydrophobic surface as the manipulation platform.
The reason is: superhydrophobic surfaces, like lotus leaves, have excellent water-repellent properties. Water droplets can easily roll on such surfaces without adhesion.By combining electrostatic forces and a superhydrophobic platform, people have achieved non-contact manipulation of droplets.
However, due to some limitations of superhydrophobic surfaces, even the method of electrostatic manipulation of droplets is powerless in some special scenarios.
For superhydrophobic surfaces, although they have extremely low adhesion to droplets, this property also leads to the easy detachment of droplets from the operation platform in weightless space or when the operation platform needs to be erected/inverted.
For example, for scientists or astronauts on a space station, droplets will detach from the superhydrophobic operation platform due to weightlessness, and therefore cannot use electrostatic forces to manipulate droplets.
In addition, although superhydrophobic surfaces have excellent water repellency, they are easily wetted when encountering organic liquids with low surface energy.Therefore, for superhydrophobic platforms, they are not suitable for the manipulation of organic droplets.
Inspired by the pitcher plant, fabricate liquid-infused ultra-slippery surfaces.
In response to the aforementioned limitations, Yong Jiale and others thought of liquid-infused ultra-slippery surfaces that mimic the surface of the pitcher plant.
When the porous structure of such surfaces is filled with lubricating liquid, a thin layer of lubricating liquid can form on the surface, thereby resisting the adhesion of liquids. At the same time, on these ultra-slippery surfaces, droplets can also slide easily.In the direction parallel to the surface, this ultra-slippery surface exhibits extremely low adhesion to droplets, while in the direction perpendicular to the surface, it exerts a significant binding force on the droplets.
That is to say, on such ultra-slippery surfaces, droplets can easily slide. Moreover, regardless of whether the surface is in a vertical state or an inverted state, the droplets will not fall off.
Based on this property, it can ensure that the droplets always move while adhering to the surface. Additionally, due to its unique anti-liquid principle, this surface can repel various liquids, including organic liquids with lower surface energy.
Based on this, the research team proposed the idea of manipulating droplets by combining friction electrostatic tweezers and ultra-slippery surfaces.
It is also reported that for ultra-slippery surfaces, the preparation process is as follows: by using femtosecond laser to create porous micro-nano structures on the surface of polydimethylsiloxane material, and then infusing silicone oil to complete the preparation.Based on the attractive force of triboelectricity, they found that on the prepared super-slippery surfaces, it is easy to manipulate droplets.
Especially when the super-slippery surface is placed vertically or turned upside down, the droplets hanging on the super-slippery surface can also be well moved.
Moreover, this super-slippery surface can repel various liquids. With the help of electrostatic action, it is even possible to control the alcohol droplets with a surface tension as low as 22.3mN/m.
Under certain specific needs, compared with manipulating droplets on super-hydrophobic platforms, using triboelectricity to control droplets on super-slippery platforms has more advantages.Create "Electrostatic Tweezers" with Triboelectricity
After setting this topic, Yong Jiale and others verified the feasibility of the aforementioned idea.
Then, using femtosecond laser micro-machining technology, they prepared the required super-slippery surface. At this point, they released a droplet on the super-slippery surface and gradually brought the triboelectric rod close to the liquid.
When the distance is reduced to a certain range, they found that the droplet would slide towards the electrostatic rod. This indicates that the idea of this time has a certain feasibility, that is, the electrostatic rod can exert an attractive force on the droplet, allowing the droplet to slide on the super-slippery surface.
Next, they systematically studied various factors affecting the manipulation of the droplet and gradually mastered the key technology for controlling the movement of the droplet.At the same time, under different conditions, the operation process for different liquids was explored.
By doing so, it was found that even when the super-slippery platform is placed vertically or inverted, the electrostatic tweezers can operate on the droplets suspended on the surface without worrying about the droplets falling off.
Even corrosive strong acid droplets, strong alkali droplets, and concentrated salt solution droplets can be well moved.
As for organic droplets, even alcohol droplets with a surface tension as low as 22.3mN/m, can be moved to some extent on the super-slippery platform.
In contrast, the above functions cannot be realized on the super-hydrophobic operation platform.After comparing the operation of droplets on superhydrophobic surfaces, the research team also found that using triboelectricity to manipulate droplets on super slippery surfaces has unique advantages.
By using simulation software, they further analyzed the physical mechanism of droplet manipulation by triboelectricity.
The results showed that an electrostatic field is generated in the space around the triboelectric rod. Influenced by the electrostatic field, the positive and negative charges inside the droplet will redistribute.
The non-uniform distribution of charges causes the droplet to be affected by the electrostatic force, and thus can be moved by the electrostatic rod.
To more accurately control the height and position of the electrostatic rod, they installed the triboelectric tweezers on a three-dimensional mechanical translation stage.With the assistance of mechanical systems, the position of the triboelectric electrostatic tweezers can be adjusted, thereby achieving more precise movement or manipulation of droplets.
Furthermore, they have explored some application scenarios based on triboelectric electrostatic tweezers, and have preliminarily realized a series of liquid-related applications such as surface defogging, surface cleaning, motion switches, liquid sorting, cell staining, droplet micro-reactions, and droplet motion guidance.
Recently, the related paper titled "Triboelectric 'electrostatic tweezers' for manipulating droplets on lubricated slippery surfaces prepared by femtosecond laser processing" was published in the International Journal of Extreme Manufacturing (IF 16.1).
Associate Researcher Yong Jiale is the first author, and Associate Researcher Yong Jiale and Associate Researcher Wang Chaowei from the University of Science and Technology of China are the co-corresponding authors[1]. The research was also guided by Professor Wu Dong with great care.
In fact, around 2017, the idea of using triboelectric manipulation to control droplets had already emerged in the field.At that time, Yong Jiale arranged for a master's student to try out his idea, but later the student reported that the experimental results were not very good.
Yong Jiale then temporarily shelved this idea. Now, when he personally re-conducted the experiment of electrostatic manipulation of droplets, he found that the effect was so good and the operation was so flexible.
"I do have a lot of regrets in my heart, because there was an opportunity to achieve results a few years earlier. In the past few years, other research groups at home and abroad have also developed various methods for manipulating droplets based on electrostatics, including electrostatic tweezers technology," said Yong Jiale.
However, as the saying goes, "Good food is not afraid of being late." He and his team have also achieved better results this time.
Subsequently, Yong Jiale will conduct a more systematic study on the technology of electrostatic manipulation of droplets. This includes studying the impact of different substrate materials on the operation effect, studying the impact of droplet size and type on the operation process, so that this technology can be used in more application scenarios.Additionally, he will further explore the practical applications of this technology, such as using it to control the movement of liquids in microfluidic systems and preparing digital microfluidic systems.