Chung-Ang University researchers are pioneering a new way to manipulate microdroplets

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Seoul, South Korea, June 22, 2022 /PRNewswire/ — Several types of microfluidic technology exist. 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 droplet, 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 means of controlling 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 Woo from Chung-Ang University, Korea, has 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 at 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.

Reference
Original Article Title: Driving Droplets on Liquid Repellent Surfaces via Light-Driven Marangoni Propulsion
Log: Advanced functional materials
DO I: https://doi.org/10.1002/adfm.202111311

About Chung-Ang University

Website: https://neweng.cau.ac.kr/index.do

Contact:
Seong Kee Shin
02-820-6614
[email protected]

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