Freezing and freeze-drying processes are ubiquitous in applications ranging from food and pharmaceutical production to basic research in atmospheric sciences and cryobiology. For example, freeze-drying pharmaceutical products can greatly extend the shelf life and thus allow longer times for storage and transport. Similarly, understanding freezing processes in the upper atmosphere can enable better climate modelling and weather forecasts. Unfortunately, conventional techniques, such as differential scanning calorimetry and freeze-dry microscopy, are incapable of measuring nucleation and crystal growth rates. In a recent study, a multidisciplinary team from ETH Zurich set out to use a droplet-based microfluidic platform to systematically quantify the freezing process in solution.
The authors used the Microfluidic Ice Nuclei Counter Zurich (MINCZ) to generate and manipulate well-defined populations of sucrose droplets. The authors monitored droplet brightness as a function of temperature to quantify the crystal growth process. This enabled them to map out the liquid-solid phase boundaries for a sucrose-water system and allowed them to study freezing at temperatures as low as -55º C. Importantly, the authors calculated the concentration of a maximally freeze-concentrated solution, which is essential in many pharmaceutical applications. Contrary to common assumptions, they also found evidence of multiple nucleation sites forming within elongated droplets. Understanding crystal growth process with this technique paves the way for efficient freezing protocols that ultimately enhance product quality. Finally, being independent of the chosen mixture, the presented microfluidic technique can be applied to any industrially relevant liquid solution.
Written by Prerit Mathur
The full paper can be read here.