Monitoring Ice Nucleation in Pure and Salty Water via High-Speed Imaging and Computer Simulations

High-speed monitoring of the freezing process of freely suspended supercooled pure and salty water droplets is reported for the first time. Combined visual (VIS) and infrared (IR) imaging directly delivers three-dimensional and surface temperature information about the proceeding freezing front with up to 2000 frames per second. The freezing behavior changes gradually up to 1 M and dramatically above a 1 M NaCl concentration. To capture the initial stage of the nucleation molecular dynamics (MD), calculations with atomistic and femtosecond resolution have been performed, and homogeneous ice nucleation in a salt solution has been successfully simulated. A combination of experimental imaging and calculations allows one to unravel structural (e.g., preferred bulk or surface location of the ice nucleus and final ion distribution) and dynamical (time scales for nucleation and freezing) aspects of the freezing process in water and salt solutions. While the thermodynamic consequence of added salt, that is, lowering of the freezing point, is well-known, here, we elucidate the kinetic antifreeze effect of added salt and the molecular origin of the corresponding slow-down of ice nucleation and freezing.