Shape variation and flat plateau formation of a frozen nanofluid droplet

Abstract

Droplet freezing is a phenomenon of great interest, not only from a fundamental perspective, but also due to its significant contributions to various industrial and ecological processes. Despite the extensive research on pure water droplet freezing, the freezing dynamics of nanofluid droplets have not been investigated to the same extent. It has been observed that frozen nanofluid droplets exhibit a distinctive flat plateau shape at their apex, in contrast to the pointed tip observed in pure water droplets. Our experimental investigation has revealed that when the vertical velocity of nanofluid droplet's freezing front is lower than a critical value, nanoparticles are expelled from the freezing liquid and aggregate at the freezing front to form a particle layer. This layer causes a deviation between the motion direction of the tri-junction line and the interface curve of the unfrozen liquid, resulting in an increased angle growth and the formation of a plateau at the top of the frozen droplet. The critical freezing velocity was found to be independent of the contact angle but showed a slight increase with subfreezing temperature. As the subfreezing temperature rises, the flat plateau of the frozen droplet transitions to a pointed tip. To further explore the freezing dynamics of nanofluid droplets, we conducted a statistical analysis of the cone angle at the apex of the frozen droplets. Our results indicates that in most cases, the cone angle is greater than the universal value observed for pure water droplets. And the cone angle is effected by the droplet volume size and subfreezing temperature.