Crystallization at the hexadecane/water interface observed under acoustic levitation

Alkanes are present in the atmosphere, commonly in the form of aerosols, and can thus interact with water droplets, leading to the formation of new interfaces. Yet, in the study of these interactions, traditional experimental methods often rely on the presence of sample containers, which can interfere with the observations. Acoustic levitation is a technique which allows the manipulation of samples in the microliter regime in a contact-free manner. Hence, interfacial phenomena can be studied without the presence of external surfaces, mimicking atmospheric conditions. Herein, we acoustically levitated a droplet of water in contact with a droplet of hexadecane and observed interfacial crystallization at the hexadecane/water interface that propagated to the entire droplet of hexadecane. It was found that the crystallization occurred up to 3 K above the melting temperature of hexadecane, and at a relative humidity below 30 %. Moreover, the volume ratio between water and hexadecane influenced the fraction of crystallized surface area; for a hexadecane:water ratio above 1:2 full surface crystallization occurred. Combining acoustic levitation with Raman spectroscopy allowed the characterization of the phase transition in real time, demonstrating the existence of a solid crystal of alkane upon water evaporation. This study provides insights into the process of interfacial crystallization of hexadecane and demonstrates the suitability of acoustic levitation to study contact-free interfacial phenomena between two immiscible liquids.