A research team led by Lecturer Timothée Mouterde from the University of Tokyo’s School of Engineering has discovered how to prevent hot droplets friction, making easy manipulation of small volumes of hot liquid possible. Droplets smaller than a millimeter typically stick to surfaces due to dominant capillary and viscous forces, making them difficult to handle. Some insects overcome this by coating liquids with small hydrophobic particles that create an air layer between the liquid and their bodies, preventing adhesion. Inspired by this mechanism, droplets coated with particles—known as liquid marbles—have extremely low friction, allowing them to move easily. However, their ability to transport hot liquids had remained unclear.
The researchers found that placing hot liquid marbles on cooler surfaces causes condensation, which forms liquid bridges connecting the droplet to the surface. On hydrophilic surfaces, these bridges can rupture the marbles, causing the core liquid to spread. On hydrophobic surfaces, while the marbles remain stable, the liquid bridges still form which significantly increase adhesion, hindering their easy manipulation.
The research team demonstrated that this adhesion can be reduced by delaying the formation of liquid bridges either by increasing the particle size or by reducing the volatility of the liquid. Alternatively, they showed that using nanostructured hydrophobic surfaces suppresses the liquid bridge adhesion, offering another route to manipulate hot liquids effectively.
These findings expand the potential of liquid marbles for use in chemical microreactors involving exothermic reactions, as well as in cell cultures and biological analyses typically conducted above room temperature.Liquid marbles are droplets covered with fine hydrophobic particles (A). When placed on cooler substrates, hot liquid marbles may rupture on hydrophilic substrates (B) or have an increased friction due to liquid bridge formation in the air layer separating them from the substrate (C).
Papers
Journal: Proceedings of the National Academy of Sciences
Title: Hot liquid marbles
Authors: Pritam Kumar Roy, Yui Takai, Rui Matsubara, Mizuki Tenjimbayashi and Timothée Mouterde*
DOI: 10.1073/pnas.2500619122