Understanding electrofreezing in water simulations

Molecular dynamics simulations of the effect of static electric field on progressive ice formation

Ice accumulation under static electric fields presents a significant hazard to transmission lines and power grids. Contemporary computational studies of electrofreezing predominantly probed excessive electric fields (109 V/m) that are significantly higher than those typically encountered in proximity to transmission lines. To elucidate the influence of realistic electric fields (105 V/m) on ice crystallization, we run extensive molecular dynamics (MD) simulations across dual ice-water coexistence systems. Three aspects of work were accordingly examined. First, we investigated the influence of the effect of static electric fields, with a strength of 105 V/m, along three orthogonal axes on the phase transition during the encountered freezing and melting processes. Second, we established the mechanism of how the direction of an electric field, the initial ice crystallography, and the adjacent crystal planes influence the solidification process. Third, the results of our MD simulations were further post-processed to determine the dipole moment, radial distribution, and angle distribution resulting from the static electric field. Our results indicate that while weak electric fields do not cause complete polarization of liquid water molecules, they can induce a transition to a more structured ice-like geometry of the water molecules at the ice-water interphase region, particularly when applied perpendicular to the ice-water interphase. Notably, the interface adjacent to cubic ice exhibits a greater response to the electric fields than that adjacent to hexagonal ice. This is attributable to the intrinsic differences in their original hydrogen bonding networks.

Confined liquid crystallization governed by electric field for API crystal polymorphism screening and massive preparation

Active pharmaceutical ingredients (APIs) crystal preparation is a significant issue for the pharmaceutical development attributed to the effect on anti-inflammatory, anti-bacteria, and anti-viral, etc. While, the massive preparation of API crystal with high polymorphism selectivity is still a pendent challenge. Here, we firstly proposed a criterion according to the molecular aggregation, molecular orientation, and hydrogen bond energy between INA molecules from molecular dynamics (MD) simulations, which predicted the hydrogen bond architecture in crystal under different electric fields, hinting the recognition of crystal polymorphism. Then, an electric field governing confined liquid crystallization was constructed to achieve the INA crystal polymorphism screening relying on the criterion. Further, magnifying confined liquid volume by 5000 times from 1.0 μL to 5.0 mL realized the massive preparation of INA crystal with high polymorphic purity (>98.4%), giving a unique pathway for crystal engineering and pharmaceutical industry on the development of innovative and generic API based drugs.

Reversible electrowetting transitions on superhydrophobic surfaces

An electric field applied across the interface has been shown to enable transitions from the Cassie to the Wenzel state on superhydrophobic surfaces with miniature corrugations. Molecular dynamics (MD) simulations manifest the possibility of reversible cycling between the two states when narrow surface wells support spontaneous expulsion of water in the absence of the field. With approximately 1 nm sized wells between the surface asperities, the response times to changes in the electric field are of O(0.1) ns, allowing up to GHz frequency of the cycle. Because of the orientation preferences of interfacial water in contact with the solid, the phenomenon depends on the polarity of the field normal to the interface. The threshold field strength for the Cassie-to-Wenzel transition is significantly lower for the field pointing from the aqueous phase to the surface; however, once in the Wenzel state, the opposite field direction secures tighter filling of the wells. Considerable hysteresis revealed by the delayed water retraction at decreasing field strength indicates the presence of moderate kinetic barriers to expulsion. Known to scale approximately with the square of the length scale of the corrugations, these barriers preclude the use of increased corrugation sizes while the reduction of the well diameter necessitates stronger electric fields. Field-controlled Cassie-to-Wenzel transitions are therefore optimized by using superhydrophobic surfaces with nanosized corrugations. Abrupt changes indicate a high degree of cooperativity reflecting the correlations between the wetting states of interconnected wells on the textured surface.

Ice nucleation forced by transient electric fields

Icing affects many technical systems, like aircraft or high-voltage power transmission and distribution in cold regions. Ice accretion is often initiated by ice nucleation in sessile supercooled water droplets and is influenced by several influencing factors, of which the impact of electric fields on ice nucleation is still not completely understood. Especially the influence of transient electric fields is rarely or not at all investigated, even though it is of great interest, e.g., for high-voltage transmission lines or for the food industry. In the present study the impact of transient electric fields on ice nucleation in supercooled sessile water droplets is experimentally investigated under well-defined conditions. A set of droplets is cooled down to a certain temperature and is subsequently exposed to electric fields generated from standard lightning or standard switching impulse voltages, which are commonly used for testing of high-voltage equipment. The nucleation behavior of individual droplets is captured using a high-speed camera and the effect of the transient electric field on ice nucleation is analyzed by considering both the singular and the stochastic nature of nucleation. While the singular nature of nucleation is referred to during analysis of the relative number of droplets remaining liquid long times after the impulse voltage, its stochastic nature is accounted for in the analysis of the temporal evolution of the relative number of frozen droplets. It is shown that low electric field strengths (E[over ̂]≤6.52kV/cm) only have a negligible impact on ice nucleation, independent of the supercooling. In contrast, high electric field strengths (E[over ̂]≥9.78kV/cm) promote significantly ice nucleation. It is also shown that depending on the supercooling, the freezing delay of the different droplets in the ensemble may vary over several magnitudes for the same conditions. It is demonstrated that the electric field appears to indirectly affect the nucleation rate by generating droplet oscillations, finally promoting ice nucleation. The experiments clearly demonstrate the possibility to actively force ice nucleation by applying transient electric fields. These results improve the understanding of ice accretion on high-voltage insulators and may also lend insight into freezing processes in food industry. We expect that these results will be a valuable contribution in formulating and/or validating new nucleation models.

Heterogeneous Electrofreezing of Super-Cooled Water on Surfaces of Pyroelectric Crystals is Triggered by Trigonal Planar Ions

Electrofreezing experiments of super-cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO₃ and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO₃ ^- ) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H₃ O⁺ ) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO₃ ^- ions self-assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice-like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO₃ ^- and guanidinium (Gdm⁺ ), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl^- and SO₄ ^2- ions of different configurations reduce the icing temperature.

Orientational and Folding Thermodynamics via Electric Dipole Moment Restraining

The projection of molecular processes onto a small set of relevant descriptors, the so-called reaction coordinates or collective variables (CVs), is a technique nowadays routinely employed by the biomolecular simulation community. In this work, we implemented two CVs to manipulate the orientation (i.e., angle) (μ⃗_a) and magnitude (|μ⃗|) of the electric dipole moment. In doing so, we studied the thermodynamics of water orientation under the application of external voltages and the folding of two polypeptides at zero-field conditions. The projection of the free-energy [potential of mean force (PMF)] along water orientation defined an upper limit of around 0.3 V for irrelevant thermodynamic effects. On the other hand, sufficiently strong μ⃗_a restraints applied on 12-alanine (Ala₁₂) triggered structural effects because of the alignment of local dipoles; for lower restraints, a full-body rotation is achieved. The manipulation of |μ⃗| produced strong perturbations on the secondary structure of Ala₁₂, promoting an enhanced sampling to its configurational space. Rigorous free-energy calculations in the form of 2-D PMFs for deca-alanine showed the utility of |μ⃗| as a reaction coordinate to study folding in small α helices. As a whole, we propose that the manipulation of both components of the dipole moment, μ⃗_a and |μ⃗|, provides thermodynamics insights into the structural conformation and stability of biomolecules. These new CVs are implemented in the Colvars module, available for NAMD and LAMMPS.

Viscous field-aligned water exhibits cubic-ice-like structural motifs

Strong electric fields are known to greatly accelerate the freezing of water in molecular dynamics simulations, and have also been shown to affect the thermodynamics of the phase transition. In this work, a mechanistic explanation for field-induced crystallization of water is presented. Due to the coupling between the rotational and the translational degrees of freedom of individual water molecules, an applied field can directly drive the formation of cubic-ice like local motifs in water. Analysis of the angular distributions of water molecules in TIP4P-2005 water at field strengths between 0.0 and 0.32 V Å-1 demonstrates the existence of such motifs in the field-aligned liquid phase that is observed prior to the onset of the freezing transition. The dynamic properties of this field-aligned liquid phase are also studied, and its viscosity is shown to be within a factor of two of that of regular liquid water using the Green-Kubo method as well as mean squared displacements. The choice between the NPT and the NVT ensembles is shown to have a strong impact on the evolution of molecular dynamics trajectories at field strengths close to the threshold for the freezing transition, and the importance of properly accounting for the electric field terms in the pressure virial is emphasized.

Phase boundaries, nucleation rates and speed of crystal growth of the water-to-ice transition under an electric field: a simulation study

We investigate with computer simulations the effect of applying an electric field on the water-to-ice transition. We use a combination of state-of-the-art simulation techniques to obtain phase boundaries and crystal growth rates (direct coexistence), nucleation rates (seeding) and interfacial free energies (seeding and mold integration). First, we consider ice Ih, the most stable polymorph in the absence of a field. Its normal melting temperature, speed of crystal growth and nucleation rate (for a given supercooling) diminish as the intensity of the field goes up. Then, we study polarised cubic ice, or ice Icf, the most stable solid phase under a strong electric field. Its normal melting point goes up with the field and, for a given supercooling, under the studied field (0.3 V nm^-1) ice Icf nucleates and grows at a similar rate as Ih with no field. The net effect of the field would then be that ice nucleates at warmer temperatures, but in the form of ice Icf. The main conclusion of this work is that reasonable electric fields (not strong enough to break water molecules apart) are not relevant in the context of homogeneous ice nucleation at 1 bar.

Electro-nucleation of water nano-droplets in No Man's Land to fault-free ice Ic

Elucidating water-to-ice freezing, especially in "No Man's Land" (150 K < T < 235 K), is fundamentally important (e.g., predicting upper-troposphere cirrus-cloud formation) - and elusive. An oft-neglected aspect of tropospheric ice-crystallite formation lies in inevitably-present electric fields' role. Exploring nucleation in No Man's Land is technically demanding, owing to rapid nucleation rates, to mention nothing of difficulties of applying relevant electric fields thereto. Here, we tackle these intriguing open questions, via non-equilibrium molecular-dynamics simulations of sub-microsecond formation of rhombus-shaped ice I_c nano-crystallites from aggressively-quenched supercooled water nano-droplets in the gas phase, in external static electric fields. We explore droplets' nano-confined geometries and the entropic-ordering agent of external electric fields as a means of realising cubic-ice formation, especially with very few stacking faults and defects.

Fundamental interfacial mechanisms underlying electrofreezing

This article reviews the fundamental interfacial mechanisms underlying electrofreezing (promotion of ice nucleation via the application of an electric field). Electrofreezing has been an active research topic for many decades, with applications in food preservation, cryopreservation, cryogenics and ice formation. There is substantial literature detailing experimental and simulations-based studies, which aim to understand the complex mechanisms underlying accelerated ice nucleation in the presence of electric fields and electrical charge. This work provides a critical review of all such studies. It is noted that application-focused studies of electrofreezing are excluded from this review; such studies have been previously reviewed in literature. This review focuses only on fundamental studies, which analyze the physical mechanisms underlying electrofreezing. Topics reviewed include experimental studies on electrofreezing (DC and AC electric fields), pyroelectricity-based control of freezing, molecular dynamics simulations of electrofreezing, and thermodynamics-based explanations of electrofreezing. Overall, it is seen that electrofreezing can enable disruptive advancements in the control of liquid-to-solid phase change, and that our current understanding of the underlying mechanisms can be significantly improved through further studies of various interfacial effects coming into play.

The accretion of the new ice layer on the surface of hexagonal ice crystal and the influence of the local electric field on this process

The process of creation of a new layer of ice on the basal plane and on the prism plane of a hexagonal ice crystal is analyzed. It is demonstrated that the ordering of water molecules in the already existing crystal affects the freezing. On the basal plane, when the orientations of water molecules in the ice block are random, the arrangement of the new layer in a cubic manner is observed more frequently-approximately 1.7 times more often than in a hexagonal manner. When the water molecules in the ice block are more ordered, it results in the predominance of the oxygen atoms or the hydrogen atoms on the most outer part of the surface of the ice block. In this case, the hexagonal structure is formed more frequently when the supercooling of water exceeds 10 K. This phenomenon is explained by the influence of the oriented electric field, present as a consequence of the ordering of the dipoles of water molecules in the ice block. This field modifies the structure of solvation water (i.e., the layer of water in the immediate vicinity of the ice surface). We showed that the structure of solvation water predetermines the kind of the newly created layer of ice. This effect is temperature-dependent: when the temperature draws nearer to the melting point, the cubic structure becomes the prevailing form. The temperature at which the cubic and the hexagonal structures are formed with the same probabilities is equal to about 260 K. In the case of the prism plane, the new layer that is formed is always the hexagonal one, which is independent of the arrangement of water molecules in the ice block and is in agreement with previous literature data. For the basal plane, as well as for the prism plane, no evident dependence on the ordering of water molecules that constitute the ice block on the rate of crystallization can be observed.

Source of Electrofreezing of Supercooled Water by Polar Crystals

Polar crystals, which display pyroelectricity, have a propensity to elevate, in a heterogeneous nucleation, without epitaxy, the freezing temperature of supercooled water (SCW). Upon cooling, such crystals accumulate an electric charge at their surfaces, which creates weak electric fields, <kV·cm(-1), that are thousands of times lower than necessary for inducing homogeneous ice nucleation. By performing comparative freezing experiments of SCW on the same surfaces of three different polar crystals of amino acids, we demonstrate that preventing the formation of charge at these surfaces, by linking the two hemihedral faces of the polar crystals with a conducting paint, reduces the temperature of freezing by 2-5 °C. The temperature of ice nucleation was found to be correlated with the amount of the surface charge, thus implying that the surface-charge-induced interactions affect the interfacial water molecules that trigger freezing at a higher temperature. This finding is in contrast to previous hypotheses, which attribute the enhanced SCW freezing to the effect of the electric field or capture of external ions or particles. Possible implications of this mechanism of freezing are presented.

A theoretical study of the dissociation of the sI methane hydrate induced by an external electric field

Molecular dynamics simulations in the equilibrium isobaric-isothermal (NPT) ensemble were used to examine the strength of an external electric field required to dissociate the methane hydrate sI structure. The water molecules were modeled using the four-site TIP4P/Ice analytical potential and methane was described as a simple Lennard-Jones interaction site. A series of simulations were performed at T = 260 K with P = 80 bars and at T = 285 K with P = 400 bars with an applied electric field ranging from 1.0 V nm(-1) to 5.0 V nm(-1). For both (T,P) conditions, applying a field greater than 1.5 V nm(-1) resulted in the orientation of the water molecules such that an ice Ih-type structure was formed, from which the methane was segregated. When the simulations were continued without the external field, the ice-like structures became disordered, resulting in two separate phases: gas methane and liquid water.