When droplets are rolled over a powder bed of fine hydrophobic or oleophobic particles, they pick up a coating that can contain the liquid. These marbles hold a few nanoliters to a few microliters of liquid and could significantly reduce plastic waste.

Liquid marbles are widely used as microreactors in various biological, chemical, and biochemical applications. For example, the Polymerase Chain Reaction (PCR) DNA amplification technique, widely used for coronavirus identification, can employ the marbles.

However, liquid marbles face one major problem: evaporation. Because the powder coating surrounding the droplet is porous, the liquid can evaporate through the pores over time. This process causes the marble to buckle and collapse and is exacerbated by higher temperatures. Invasive refilling methods exist, but are very difficult, involving syringe pumps, flow sensing, and precise flow control.

Sreejith et al. developed a simple and noninvasive method for refilling liquid marbles. The team compares their refilling process to dew formation on the side of a cold soda can. The water in air condenses on the can to form water droplets when the can is placed in air with a given humidity.

“In the process of liquid marble refilling, we engineer the ambient condition of the liquid marble in such a way that water in the air outside a collapsed liquid marble would pass through the porous hydrophobic coating,” said corresponding author Nam-Trung Nguyen. “It will condense inside the coating, allowing the liquid marble to refill.”

The current refilling process was demonstrated in a specially engineered environment, but the researchers hope to optimize its use for practical use in various microfluidics applications.

Source: “Noninvasive refilling of liquid marbles with water for microfluidic applications,” by Kamalalayam Rajan Sreejith, Pradip Singha, Nhat-Khuong Nguyen, Chin Hong Ooi, Dzung Viet Dao, and Nam-Trung Nguyen, Applied Physics Letters (2022). The article can be accessed at https://doi.org/10.1063/5.0074887.