Researchers at Chung-Ang University are pioneering the new


image: (Top) Schematic showing near-infrared light-based control of a liquid droplet infused with polypyrrole (PPy) nanoparticles. Upon irradiation of one side of the PPy-infused droplet, the nanoparticles absorb light and heat up, inducing Marangoni flux caused by a temperature gradient. Real and infrared images of the aqueous dispersion of PPy (bottom left) and the deionized water droplet (bottom right) during manipulation of the droplets by NIR laser.
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Credit: Wiley Online Library

Microfluidics – a field of technology that deals with the manipulation of very small volumes of geometrically constrained fluids – has provided powerful laboratory tools for molecular and cellular biology and has found several applications, including laboratory devices on a chip, micromotors, and miniature reactors.

There are many types of microfluidic technology. One approach that is rapidly gaining traction is droplet-based microfluidics, which involves precise control of the movement, mixing, and splitting of small droplets on lubricant-impregnated surfaces.

One way to do this is to use heat to make a droplet move. This creates a temperature gradient inside the drop, inducing a phenomenon called the “Marangoni effect”. This is characterized by a flow from a region with lower surface tension towards a region with higher surface tension, the difference in surface tension being induced in this case by the temperature gradient. This “Marangoni flow”, in turn, provides a way to control the movement of the droplet. However, in previous studies, the temperature difference inside the drop was created by simply heating the substrate on which the drop rested. This makes it difficult to precisely control the direction of the drop’s movement. Additionally, heating the substrate requires a significant amount of energy and reduces the range of suitable substrates.

To solve these problems, a team of scientists led by Dr. Sanghyuk Wooh of Chung-Ang University, Korea, developed an innovative strategy. In their latest study Posted in Advanced functional materials, they presented a new way to induce Marangoni flow in droplets and control their movement using near-infrared (NIR) light, a non-contact approach and allowing much more precise control. Their article was available online on January 4, 2022 and was published in volume 32 number 15 of the journal on April 11, 2022.

The proposed method is significantly different from conventional thermal techniques. Instead of heating the substrate, the team heated the droplets directly and remotely. However, water and other commonly used fluids do not absorb much NIR light by themselves. To solve this problem, they added a small amount of polypyrrole nanoparticles to the droplets, which helped absorb NIR light and convert it into thermal energy. This, in turn, created a temperature gradient, moving the drop away from NIR light. The resulting Marangoni flux could be easily controlled by adjusting the laser power and position. It also allowed an equally simple control of the direction of movement of the droplets on the substrate.

The team also tested their approach using various types of water-repellent surfaces and mixtures of fluids, such as water and ethanol. Interestingly, they found that the composition of the droplet significantly affected the direction of Marangoni flow. Simply put, the composition and internal thermal gradient of a droplet dictated the direction in which it traveled. In fact, it was even possible to roll a drop back (toward the NIR light). Additionally, by using a superamphiphobic surface with a water contact angle greater than 160°, the spherical droplets demonstrated rolling motion instead of sliding.

Our approach opens up a general way to precisely manipulate droplet motion on various solid surfaces, with potential applications in microfluidics, microdroplet reactors, self-cleaning surfaces, and drug delivery.says Dr. Wooh.

The results of this study also have important implications for academic research, as Dr. Wooh points out: “The manipulation of droplets is at the heart of many phenomena in fundamental and applied physics, chemistry, materials science and engineering. From a more fundamental point of view, our work provides quantitative information on the mechanisms of droplet movement.




Author: Hyesun Hwang1Periklis Papadopoulos2.3Syuji Fujii4and Sanghyuk Wooh1


1School of Chemical Engineering and Materials Science, Chung-Ang University

2Department of Physics, University of Ioannina

3University Research Center of Ioannina (URCI), Institute of Materials Science and Informatics

4Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology

About Chung-Ang University

Chung-Ang University is a private comprehensive research university located in Seoul, South Korea. It was started as a kindergarten in 1918 and gained university status in 1953. It is fully accredited by the Ministry of Education of Korea. Chung-Ang University conducts research activities under the slogan “Justice and Truth.” Its new vision to end its 100 years is “The world leader in creation”. Chung-Ang University offers undergraduate, postgraduate, and doctoral programs, which encompass a law school, a management program, and a medical school; it has 16 undergraduate and graduate schools each. Chung-Ang University’s cultural and artistic programs are considered the best in Korea.


About Associate Professor Sanghyuk Wooh

Sanghyuk Wooh is an associate professor at the School of Chemical Engineering and Materials Science, Chung-Ang University (CAU), Korea. He received his Ph.D. from Seoul National University School of Chemical and Biological Engineering in 2013, and later conducted postdoctoral research at the Max Planck Institute for Polymer Research as a Humboldt Postdoctoral Fellow. His group focused primarily on small-scale surface and interface engineering. Wooh’s Interface & Surface Engineering group is one of the pioneers in superparticle synthesis based on surface models; they developed the method and published several important articles on the subject. The group currently shows remarkable performance in the surface modification of polymers and the manufacture of hydrophobic surfaces.

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