Hot liquid marbles

Abstract

In the insect realm, liquids become traps due to capillary and viscous forces dominant at their scale. Yet, aphids handle the highly viscous honeydew droplets they secrete by coating them with hydrophobic wax powder which maintains an air layer between their body and the liquid. These coated droplets, known as liquid marbles, exhibit low friction and high mobility, enabling manipulation of small liquid volumes which is useful for biomedical analysis where sample volumes are limited, chemistry to reduce chemical waste, or digital microfluidics for large-scale cell culturing and drug testing. For such applications—including exothermic reactions or biological studies typically conducted above room temperature—the ability to carry hot liquid is important but remains unexplored. This article investigates the stability and static friction of hot liquid marbles placed on a substrate cooler by ∆ T . We show that for large ∆ T , the core liquid evaporates and condenses within the air layer below the marbles creating liquid bridges resulting in marble rupture on hydrophilic substrates and increased static friction on hydrophobic ones. The temperature difference modifies the nature of static friction: solid friction dominates at small ∆ T , while at larger ∆ T , it is replaced by a liquid pinning force caused by the increased liquid bridge density resulting from condensation. Finally, our study provides ways to avoid the rupture and increased static friction of hot liquid marbles due to the bridge formation by increasing the particle size, decreasing the liquid volatility, or using nanostructured superhydrophobic substrates.