Ice-dependent liquid-phase convective cells during the melting of frozen sessile droplets containing water and multiwall carbon nanotubes

The melting of frozen water droplets is a fundamental and ubiquitous process and the study of the transport processes occurring within the system during phase change is essential to understanding the forces that govern it. Multiwall carbon nanotubes (MWCNTs) can be added to liquid systems prior to crystallization in order to modify the properties of the phase change process and of the liquid or solid systems themselves. In this study, the melting behavior of frozen sessile droplets composed of water and 50 ppm of either functionalized or non-functionalized aqueous MWCNTs is investigated. Droplets are thawed from their base on a hydrophobic substrate set to temperatures between 1 and 30 °C. Tracking of MWCNT clusters during melting shows convective fluid motion occurring within the liquid melt at temperatures above 5 °C. This circulation is contingent on the presence of the ice phase above. The internal fluid dynamics are attributed predominantly to thermocapillary effects as a result of temperature-induced surface tension gradients along the air/liquid interface in the melt. Further, the melting times of MWCNT-containing systems were longer than pure water samples. These results highlight new and important mechanisms driving the melting processes within water droplet systems.