Investigating the radial spreading behavior of superimposed liquid drops

The spreading behavior of a drop of liquid on the surface of a horizontal, spinning substrate is a well-understood phenomenon. It has importance in applications that require the precise coating of a solid surface, such as manufacturing integrated circuits or television screens, and production of encapsulated drops or microspheres.

However, the vast majority of studies have focused on a single liquid spreading over the rotating substrate. An article in Physics of Fluids reports on the spreading of a drop enclosed by another larger drop, which is also spreading simultaneously. The setup included a rotating aluminum disk and two liquids with the same viscosity and density, but very different surface tensions: glycerol and castor oil. A high-speed camera captured the spreading of both liquids on the disk.

The inner drop is found to spread out to a greater extent and break up into more fingers compared to its behavior without the enclosing drop, which can be attributed to lower interfacial tension of the inner drop compared to its surface tension without an enclosing liquid. The number of fingers formed are, however, fewer than those predicted by established theories which consider the tension at the contact line as one of the key parameters. The authors show that this observed discrepancy can be correctly accounted following a simple modification of the existing theories using linear stability analysis.

This work also showcases a newer phenomenon of the formation of minute drops from the fingers of the inner liquid, which is unique to the superposed drop configuration used in this work and never previously observed for a liquid spreading in air.

Source: “Spreading dynamics of superposed liquid drops on a spinning disk,” by Subhadarshinee Sahoo, Ashish V. Orpe, and Pankaj Doshi, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5002601 .