Witnessing a Discrete Microdroplet Freezing Event via Real-Time Electrochemical Monitoring of Solution Temperature

Abstract

Temperature monitoring has immediate relevance to many areas of research, from atmospheric environmental studies to biological sample and food preservation to chemical reactions. Here, we use a newly established, triple-barrel electrode to provide temperature readouts in bulk solution and microdroplets, as well as electrochemically monitor freezing events in a microdroplet. Using this method, we are able to identify distinct characteristics of a freezing aqueous droplet (supercooling, ice formation beginning and end, temperature change, and thawing) with greater temporal resolution than a standard thermocouple and without the use of microscopy. By correlating the amperometric signal change caused by alterations in the diffusion coefficient of the electrochemical system in response to temperature changes, we can calculate the instantaneous temperature at our electrode, as well as the physical behavior of ice formation and expansion. Our results suggest that these electrochemical techniques can provide real-time monitoring of the physical processes involved in aqueous temperature change and ice nucleation events. Here we present a novel method for monitoring freezing events in microenvironments using a triple-barrel, electrochemical probe. Because ice nucleation spans many research fields, it is important to have a variety of tools that can be used to better understand these frozen systems. Our data shows that electrochemistry can provide real-time information on the thermal properties of aqueous environments, and these types of measurements can be extended to microdroplets. The electrochemical signal details all of the significant moments in a droplet freezing event, from tracking the temperature drop to the supercooling event, to total crystallization of the droplet at the electrode surface, to droplet thawing. Finally, this data can be combined with Multiphysics finite element models to correlate ice growth kinetics, microdroplet viscosity, and electrochemical data. All together, these experiments show that electrochemistry can work as a stand-alone tool for monitoring freezing events with excellent temporal and spatial resolution.