Supercooling of aqueous NaCl and KCl solutions under acoustic levitation

Capturing the Freeze-Drying Dynamics of NaCl Nanoparticles Using THz Spectroscopy

Sodium chloride (NaCl) aqueous solution becomes NaCl hydrate, NaCl·2H₂O, at low temperature, which is different from potassium chloride and is a typical complex model for studying the freeze-drying process in foods and pharmaceuticals. Here, we detected unit-cell-sized NaCl particles in ice as precursor substances of NaCl·2H₂O during freezing of NaCl solution by using terahertz (THz) spectroscopy. In the freezing process, Na⁺ and Cl^- ions form two types of metastable unit-cell-sized NaCl particles on the pathway to the well-known NaCl·2H₂O crystal production, which are not listed in the phase diagram of freezing of NaCl solution but have absorption peaks in THz spectra. This finding of single unit-cell-sized particles signifies the importance of studying the freeze-drying process in-depth and offers a new possibility for the development of freeze-drying technology for the manufacture of nanometer-sized particles such as ultrafine pharmaceutical powders, which more readily dissolve in water.

A wall-free climate unit for acoustic levitators

Acoustic levitation represents the physical background of trapping a sample in a standing acoustic wave with no contact to the wave generating device. For the last three decades, sample holders based on this effect have been commonly used for contact free handling of samples coupled with a number of analytical techniques. In this study, a wall-free climate unit is presented, which allows the control of the environmental conditions of suspended samples. The insulation is based on a continuous cold/hot gas flow around the sample and thus does not require any additional isolation material. This provides a direct access to the levitated sample and circumvents any influence of the climate unit material to the running analyses.