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Scientists develop a more precise and versatile platform for droplet-based microfluidics on solid surfaces.

By Chung-Ang University

Microfluidics, the manipulation of small volumes of fluid constrained to small scales, is a rapidly growing field with scientific and practical applications. In a recent study, Chung-Ang University scientists report a new light-based technique to precisely control the movement of minuscule droplets on a lubricated surface. Efficient and versatile, the new method paves the way for discovering new physics of droplet motion as well as better lab-on-chip devices, drug delivery applications, miniature reactors, and more.

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

There are many types of microfluidics 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 achieve this is by using 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 lower surface tension region to a higher surface tension region, the surface tension difference being induced by the temperature gradient in this case.

Caption: (Top) Schematic showing near-infrared light-based control of a liquid droplet infused with polypyrrole (PPy) nanoparticles. Upon irradiating one side of the PPy-infused droplet, the nanoparticles absorb the light and heat up, inducing Marangoni flow caused by a temperature gradient. Real and infrared images of PPy aqueous dispersion (bottom left) and deionized water (bottom right) droplet during droplet manipulation by NIR laser. Image credit: Wiley Online Library  License type: Copyright restricted

This “Marangoni flow,” in turn, provides a way to control the droplet’s motion. However, in previous studies, the temperature difference inside the droplet was created by simply heating the substrate on which the droplet was resting. This makes it difficult to precisely control the direction of the droplet’s movement. Moreover, heating the substrate requires a substantial amount of energy and narrows down the scope of suitable substrates.

To tackle these issues, a team of scientists led by Dr. Sanghyuk Wooh of Chung-Ang University, Korea, developed an innovative strategy. In their latest study published in Advanced Functional Materials, they presented a new way to induce Marangoni flow in droplets and control their motion using near-infrared (NIR) light, an approach that is contact-free and allows much more precise control.

The proposed method is substantially 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 on their own. To address this, they added a small amount of polypyrrole nanoparticles into the droplets, which helped absorb NIR light and convert it to thermal energy.

This, in turn, created a temperature gradient, making the droplet move away from the NIR light. The resulting Marangoni flow could be easily controlled by tuning the power and position of the laser. It also allowed an equally straightforward control of the direction of droplet motion on the substrate.

The team also tested their approach using various types of liquid repellent surfaces and fluid mixtures, such as water and ethanol. Interestingly, they found that the composition of the droplet significantly affected the direction of the Marangoni flow. Put simply, both the composition and internal thermal gradient of a droplet dictated the direction in which it moved.

In fact, it was even possible to make a droplet move backwards (towards the NIR light). Additionally, on using a superamphiphobic surface exhibiting a water contact angle over 160°, spherical droplets demonstrated a 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,” highlights Dr. Wooh.

The findings of this study have important implications for academic research as well, as Dr. Wooh points out: “Droplet manipulation is at the core of many phenomena in basic and applied physics, chemistry, materials science, and engineering. On a more fundamental side, our work provides quantitative insights into the mechanisms of droplet motion.

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Reference

Authors: Hyesun Hwang1, Periklis Papadopoulos2,3, Syuji Fujii4, and Sanghyuk Wooh1

Title of original paper: Driving Droplets on Liquid Repellent Surfaces via Light-Driven Marangoni Propulsion

Journal: Advanced Functional Materials

DOI: https://doi.org/10.1002/adfm.202111311