Method Keeps Liquid Marbles Functional for Microfluidic Systems

JAKE SALTZMAN, NEWS EDITOR
jake.saltzman@photonics.com

A research team from Griffith University developed a technique that uses condensation to noninvasively refill liquid marbles with water. The method could improve the viability of applications such as drug delivery. According to the researchers, it could also establish improved opportunities for the droplet-size microreactors to see use in opto- and microfluidics.

“Liquid marbles are droplets of solution that wrap in a thin layer of microparticles that can be used for a large number of biological, chemical, and biochemical applications,” said Nam-Trung Nguyen, co-author of the study and director and professor from the Queensland Micro- and Nanotechnology Centre. “Liquid marbles are used as microreactors to house various chemical, biochemical, and biological purposes like growing cells and applications such as the common PCR [polymerase chain reaction test], a DNA amplification technique used to detect COVID-19.”

The benefit to refilling liquid marbles noninvasively stems from the tendency of liquid marbles to collapse due to evaporation. Liquid marbles are created when a droplet of reaction solution is rolled over a powder bed of hydrophobic (water-resistant) or oleophobic (oil-resistant) particles. The marbles create a barrier around the reaction solution that isolates the content from the surroundings.

Once the marbles are formed, however, evaporation can cause them to lose volume, buckle, and, ultimately, to collapse. This eliminates the advantages of their physical properties and their aptitude for use.

“The particle coating that forms around the droplet can contain liquid which varies in volume from a few nanoliters to a few microliters,” said lead author Sreejith Kamalalayam Rajan, from Queensland Micro- and Nanotechnology Centre.

Because the powder coating around the droplet is porous, liquid can evaporate through the coating. Over time, the liquid disappears, particularly when the outside temperature is higher or is constantly cycling between high and low temperatures.

This type of temperature fluctuation occurs in PCR reactions, Rajan said.

As the refilling mechanism relates to opto- and microfluidics, the researchers hypothesized that the mechanism could help to restore the fluorescence that is emitted by liquid marbles.

“The refilling process would help to recover the products of DNA amplification carried out in liquid marbles,” Rajan said. “The fluorescence emitted from the liquid marble diminishes toward the end of the process, as the liquid inside it is completely evaporated. We believe that the refilling process may help to restore the fluorescence to the maximum.”

The process developed by the researchers relies on condensation, similar to the way dew forms on a can filled with a cold liquid. In certain levels of humidity and temperature, water in the air condenses to form water droplets.

The researchers mimicked this process. They refilled liquid marbles by engineering the external conditions around an individual marble. This encouraged water in the air outside to condense on the marble as it does on a can of cold liquid. Subsequently, the water was collected inside the porous coating, allowing the marble to refill.

The concept of refilling a liquid marble (a). The schematic of experimental setup (b). The reaction chamber used in the work (c). Courtesy of Applied Physics Letters.

According to Rajan, by using the proposed method, the construction of the marble does not influence the ability to refill the marbles. At the same time, various hydrophobic powders may cover the liquid droplets with different porosities, which could influence the rate of refilling.

The researchers demonstrated the current refilling process in a specially engineered environment. They plan to optimize the technique for practical use in various microfluidics applications.

“The dynamics and characterization of the process must be studied, and a mathematical model should be developed,” Rajan said. “The model may help us to engineer and control the process of refiling according to our needs. This will allow us to develop specific systems to perform the process in an optimum manner.”

Rajan said the team’s method could also be tested for efficiency on liquid marbles for applications including soft photonic objects and structural coloration.

The research was published in Applied Physics Letters (www.doi.org/10.1063/5.0074887).