Casting pearls before marbles: contrasting mobility of non-wetting drops
Casting pearls before marbles: contrasting mobility of non-wetting drops lead image
Water sticks to most surfaces, but in cases where waterproof surfaces are needed, this adhesion can be detrimental. To prevent this, water can be made non-wetting through two methods. A hydrophobic coating can be applied to a given surface, causing the water to form into ‘pearls.’ Or the water droplets can be coated in hydrophobic particles, forming ‘marbles.’ These two methods are very similar, but their slight differences impact water mobility in unexpected ways.
To compare these differences, Jin et al. conducted races between the two types of non-wetting drops. They found that the static adhesion differs fundamentally between marbles and pearls, and that the faster species is not what they expected.
“Pearls are more mobile since they are heavier, while marbles have an adhesion that is not sensitive to their size,” said author David Quéré. “Marbles are always slower than pearls: their friction is higher, which seems to be due to their solid-like coating.”
The authors predicted the coated marbles would be faster, owing to their smaller interactions with the surface. However, the team suspects that type of interface — liquid/air for pearls, solid/air for marbles — plays a significant role and accounts for the relative speediness of pearls. The viscosity of the liquid also plays a role.
“Pearls are generally faster than marbles by a factor that depends on the liquid viscosity: this factor is three at small viscosity, and is closer to 1.5 at high viscosity,” said Quéré.
The team plans to continue their investigations by examining the effects of changing the surface inclinations, the sizes and shapes of the drops, and the concentrations of surface particles.
Source: “When marbles challenge pearls,” by Panlin Jin, Kexin Zhao, Zoé Blin, Malou Allais, Timothée Mouterde, and David Quéré, Journal of Chemical Physics (2023). The article can be accessed at https://doi.org/10.1063/5.0150082 .
This paper is part of the Chemical Physics of Controlled Wettability and Super Surfaces Collection, learn more here .
