Evaporation Model Shows Mechanistic Behaviors of Liquid Marbles

Researchers from King Abdullah University of Science and Technology (KAUST) have introduced a framework for the evaporation behavior of liquid marbles — nonstick and hydrophobic particle-coated droplet structures. Liquid marbles’ physical properties give the biological structures cross-disciplinary use, in soft matter photonics and as components of biochemical reactors and biosensors.

Additional applications for liquid marbles include detection of water pollution, monitoring of environmental gases and interfacial reactions, blood typing, and polymerase chain reaction assay.

“Even though the water surface of a liquid marble is covered by hydrophobic (water-repellent) particles, they can still evaporate faster than bare water droplets. This counterintuitive fact stoked our curiosity,” said Adair Gallo Jr., a Ph.D. student who worked on the study alongside Himanshu Mishra and colleagues.

In the work, the KAUST team studied marbles formed from particles with different hydrophobic natures and surface roughness and sizes, varying from nano to micro. It investigated the evaporation of liquid marbles formed with particles of varied sizes — from over 7 nm to 300 μm, and chemical compositions ranging from hydrophilic, or wetting, to superhydrophobic. Gallo used high-speed imaging to determine that liquid-particle and particle-particle interactions significantly influenced evaporation behavior. He then grouped the outcomes into three cases.

First, marbles formed from particles with high liquid-particle adhesion and moderate interparticle friction kept their total surface area intact as they deflated, leading to faster evaporation and flattened shapes. Gallo grouped most marble examples examined as part of the study in this category.

Fabricating a liquid marble. (A) Silanization process. (B) Various hydrophobic surface compositions obtained using silanes. (C) Snapshots from a published movie, where the hydrophobic particles are poured onto a water droplet to create a liquid marble. The particles slide along the air -water interface and cover the liquid surface bottom-up, thus covering the droplet. DOI: 10.1039/D1SM00750E.

For the second case, Gallo experimented with microscale silica particles coated with nanoscale particles that exhibited ultra-water-repellence. “As these liquid marbles evaporated, they ejected particles from their surface and remained spherical; we had not expected to see this,” Gallo said. “This happens because of very low liquid-particle and interparticle forces.

Gallo said the liquid marbles in this case curiously showed the same evaporation rates as bare water droplets.

The third case involved sticky nanoparticles that interacted closely with each other but not with the liquid inside. As the liquid evaporated, the particles were pushed out from the water surface to form a multilayered coating. The marbles retained a spherical shape but evaporated at much slower rates because of the thicker particle layers.

The team finally used the data to build a mathematical model to accurately predict the evaporation behavior of all the liquid marbles studied in this work, as well as in other published reports.

The research was published in Soft Matter (www.doi.org/10.1039/D1SM00750E).