Fundamental interfacial mechanisms underlying electrofreezing

The Contribution of Pyroelectricity of AgI Crystals to Ice Nucleation

“…[1] Silver iodide (AgI) is one of the most explored compounds for glaciogenic cloud seeding. [4] We have demonstrated, in previous studies,t hat the pyroelectricity of polar amino acids and LiTaO 3 crystals,s trongly affects ice nucleation. Thehexagonal AgI crystals are polar, yet those studies ignored the possible role that might be played by the electric charge created at the two hemihedral faces of apolar crystal upon cooling,known as the pyroelectric effect.…”

“…Yet the icing temperature can be controlled heterogeneously by using auxiliary additives. [4] We have demonstrated, in previous studies,t hat the pyroelectricity of polar amino acids and LiTaO 3 crystals,s trongly affects ice nucleation. [1a, 2] Despite the numerous investigations performed ever since its discovery,the mechanism of how AgI promotes ice nucleation is still far from being understood, as emphasized in the recent comprehensive review by Marcolly et al, [2c] stating "Theoretical and experimental studies over the last 60 years provide ac omplex picture of silver iodide as an ice-nucleating agent with conflicting and inconsistent results".…”

“…[3] It has been reported that surface charge on dielectric materials can affect the icing temperature of super-cooled water (SCW). [4] We have demonstrated, in previous studies,t hat the pyroelectricity of polar amino acids and LiTaO 3 crystals,s trongly affects ice nucleation. [1b,5] As imilar role that might be played by the pyroelectric charge of AgI crystals has not been considered in previous investigations.However,invariance to the above crystals,AgI is as emiconductor [6] and it must be considered that internal currents through the sample may reduce and even eliminate the pyroelectric charge.T hus,i ti sc rucial to determine whether the pyroelectricity of AgI has any effect on the icing temperature.Thep yroelectric properties of AgI have not been measured so far,w hich is presumably due to challenges involved in growing single crystals of the required dimensions for such measurements.T oovercome this hurdle,wedesigned experiments with large poled and non-poled crystalline ceramic pellets of AgI.…”

Electrofreezing of the phase-change material CaCl2•6H2O and its impact on supercooling and the nucleation time

“…In general, the nucleator can impact the change in thermophysical parameters, such as the supercooling temperature, supercooling degree, specific heat, and the latent heat enthalpy. Recently, Sutjahja et al [14] reported the effectiveness of 1 wt% CuO to reduce the supercooling degree and further modify the thermal parameters of CaCl 2 ·6H 2 O by graphite and CuO dopants, although logical explanations for their effectiveness apparent do not exist.The effect of electrical energy on the behaviour of supercooled fluids has been studied since two decades ago [15][16] and is commonly called electrofreezing or electronucleation; see [17][18][19] for the review and references therein. In most of the past experiments water droplets were mainly exposed to high-voltage electric fields, with the success expressed by higher nucleation temperatures, reduction in the supercooling degree, lowering the induction times, and increased nucleation probability.…”

“…The success of electrofreezing experiments were determined by certain factors, such as the type of the solution material, salt concentration, electrode material [21][22][30][31][32], surface properties of the electrode, electrode configuration [30][31][32][33][34], static (DC) or a fluctuating (AC) electric field [19,32,[34][35][36], magnitude of the voltage or electric field [37][38][39][40][41][42], temperature at the applied field [21,25], and duration of application of the electric field [21]. Although it is commonly observed that an increase in the electric field affects reduction in the supercooling degree, despite the variations in the experimental setups and procedures across these studies, the compiled data from many experimental results show that the magnitude of the supercooling degree is strongly dependent on the field and tends to increase by increasing the applied field [17].The electrofreezing phenomenon is based on the impact of a strong electric field to induce crystallisation of a supercooled liquid. From the thermodynamics perspective, the process of crystallisation is initiated by the formation of nuclei and is closely related to the change in Gibbs free energy, with two main contributions from the surface free energy (G s ) and volume free energy (G v ).…”