Ice crystallization is a ubiquitous phenomenon in nature and plays an important role in many fields of science (e.g., cloud formation, climate change, cryobiology) and industry (e.g., air transport, food preservation, operation of infrastructure). Despite extensive efforts in experimental and computational studies enriching our understanding of ice crystallization at a molecular level, the detailed mechanisms such as ice formation under a water flow, remain elusive and require further exploration. Aiming at offering unique insights into the microscopic aspects of ice crystallization, molecular dynamics (MD) simulations are employed in this thesis to study how shear flows affect homogeneous ice nucleation and ice growth. In addition, the microscopic mechanisms of ion rejection phenomena...[ Read more ]

Ice crystallization is a ubiquitous phenomenon in nature and plays an important role in many fields of science (e.g., cloud formation, climate change, cryobiology) and industry (e.g., air transport, food preservation, operation of infrastructure). Despite extensive efforts in experimental and computational studies enriching our understanding of ice crystallization at a molecular level, the detailed mechanisms such as ice formation under a water flow, remain elusive and require further exploration. Aiming at offering unique insights into the microscopic aspects of ice crystallization, molecular dynamics (MD) simulations are employed in this thesis to study how shear flows affect homogeneous ice nucleation and ice growth. In addition, the microscopic mechanisms of ion rejection phenomena during the freezing of aqueous solutions are also explored through MD simulations.

For the roles of shear rates in ice growth processes, it is found that under sufficient supercooling, the ice growth rate varies nonlinearly with the shear rate and reaches a maximum value at an intermediate shear rate. This is caused by two different roles of the shear. First, the shear can help disrupt the tetrahedral hydrogen bond of supercooled water and free some water molecules to reorganize at the water-ice interface to form ice. Second, the shear can also break the water-ice hydrogen bonds and hinder the formation of ice. The former effect plays an important role at relatively low shear rates, while the latter becomes dominant at higher shear rates. These two effects compete with each other and result in a maximum growth rate at an intermediate shear rate. Close to melting point, the relatively strong thermal fluctuation can easily break the hydrogen bond network, which weakens the role of the shear rate in promoting the ice growth rate, and the ice growth rate decreases monotonously with an increasing shear rate.

Homogeneous ice nucleation under shear is also studied. Similar to the non-monotonic behavior of the ice growth rate, the ice nucleation rate also assumes a maximum at an intermediate shear rate. The non-monotonic behavior of ice nucleation is caused by the competition between the kinetic pre-factor and the free energy barrier as the shear rate is varied. The former promotes while the latter hinders ice nucleation according to the classical nucleation theory. On one hand, shear enhances the diffusion of water molecules, which makes the kinetic pre-factor increase as the shear rate is increased. On the other hand, shear reduces the stability of small nuclei and hinders the formation of the critical nuclei, which leads to a relatively high free energy barrier for nucleation. At low shear rates, the kinetic pre-factor increases and dominates, and consequently increases the nucleation rate. By contrast, at relatively high shear rates, the free energy barrier becomes more sensitive to the shear rate than the kinetic pre-factor, leading to the decrease of the nucleation rate.

Finally, the mechanisms for ion rejection phenomena in the freezing of NaCl solutions are investigated. The hydration energy for the ion-water interaction is found to be stronger than that between ions and ice, which is the fundamental reason for ion rejection. Furthermore, the ion rejection rate increases as the temperature is increased. The probability of ion rejection is largely determined by the competition between the energy barrier near the ice-water interface and the thermal effect. The energy barrier attracts ions and may cause ion trapping in the ice, while an increase in temperature leads to an increase in the kinetic energy of ions, which assists them to overcome the energy barrier and consequently promotes the ion rejection rate.