Unique shape changes during freezing of a nanofluid droplet

TL;DR

This study reveals unique shape changes in nanofluid droplets during freezing, including a flat plateau formation and particle ring patterns, driven by Marangoni flow, with implications for energy storage and microfabrication.

Contribution

It is the first detailed investigation of shape dynamics and particle distribution during nanofluid droplet freezing, highlighting new phenomena not observed in pure water.

Findings

Nanofluid droplets form a flat plateau at the top during freezing.

The plateau size increases with nanoparticle concentration.

A particle ring pattern similar to coffee stain forms during freezing.

Abstract

Understanding the dynamics during freezing of nanofluid droplets is of importance from a fundamental and practical viewpoint. While the freezing of a water droplet has been extensively studied, little information is available about the characteristics of freezing a nanofluid droplet. Here, we report unique shape changes observed during the freezing process of a nanofluid droplet. Instead of forming a pointy tip on the frozen deionized water droplet, we found that the top of a frozen nanofluid droplet exhibits a flat plateau shape and such plateau becomes larger with increasing nanoparticle concentration. We ascribe this characteristic shape change to an outward Marangoni flow which moves liquid from the droplet interior to its vertexes, thus engendering the volume redistribution and shape changes. Moreover, we also observed a particle ring pattern formed during freezing of a nanofluid droplet, which is similar to the well-known coffee stain phenomenon occurring during evaporation of a water droplet with dispersed particles. We believe our reported phenomena are not only of fundamental interest but have potential applications in the freezing of nanofluids for energy storage and developing nanofluid based new microfabrication methods.