Electrofreezing of confined water

Progress of the Impact of Terahertz Radiation on Ion Channel Kinetics in Neuronal Cells

In neurons and myocytes, selective ion channels in the plasma membrane play a pivotal role in transducing chemical or sensory stimuli into electrical signals, underpinning neural and cardiac functionality. Recent advancements in biomedical research have increasingly spotlighted the interaction between ion channels and electromagnetic fields, especially terahertz (THz) radiation. This review synthesizes current findings on the impact of THz radiation, known for its deep penetration and non-ionizing properties, on ion channel kinetics and membrane fluid dynamics. It is organized into three parts: the biophysical effects of THz exposure on cells, the specific modulation of ion channels by THz radiation, and the potential pathophysiological consequences of THz exposure. Understanding the biophysical mechanisms underlying these effects could lead to new therapeutic strategies for diseases.

Ice-Enabled Transfer of Graphene on Copper Substrates Enhanced by Electric Field and Cu2O

Graphene films grown by the chemical vapor deposition (CVD) method suffer from contamination and damage during transfer. Herein, an innovative ice-enabled transfer method under an applied electric field and in the presence of Cu2O (or Cu2O-Electric-field Ice Transfer, abbreviated as CEIT) is developed. Ice serves as a pollution-free transfer medium while water molecules under the electric field fully wet the graphene surface for a bolstered adhesion force between the ice and graphene. Cu2O is used to reduce the adhesion force between graphene and copper. The combined methodology in CEIT ensures complete separation and clean transfer of graphene, resulting in successfully transferred graphene to various substrates, including polydimethylsiloxane (PDMS), Teflon, and C4F8 without pollution. The graphene obtained via CEIT is utilized to fabricate field-effect transistors with electrical performances comparable to that of intrinsic graphene characterized by small Dirac points and high carrier mobility. The carrier mobility of the transferred graphene reaches 9090 cm2 V-1 s-1, demonstrating a superior carrier mobility over that from other dry transfer methods. In a nutshell, the proposed clean and efficient transfer method holds great potential for future applications of graphene.

Quantitative Analysis of Ice Crystal Growth During Freezing of Dimethyl Sulfoxide Solutions Under Alternating Current Electric Fields

During cryopreservation, the growth of ice crystals can cause mechanical damage to samples, which is one of the important factors limiting the quality of preserved samples. To enhance the preservation quality of biological samples, scholars have tried various engineering methods. Among them, an electric field is an essential factor affecting solution freezing. Dimethyl sulfoxide, as a commonly used cryoprotectant, can cause mechanical damage to cells due to ice crystals even when freezing at the optimal freezing rate. Water is a strongly polar dielectric material, and the applied alternating current (AC) electric field will affect the water freezing performance. Therefore, a quantitative study of ice crystal nucleation and growth during freezing of dimethyl sulfoxide solutions under different AC electric field conditions is needed to try to reduce ice crystal damage. We created a liquid-film device to approximate the ice crystal growth process as a two-dimensional image. The frequency of the AC voltage was set from 0 to 50 kHz. We measured the supercooling of the dimethyl sulfoxide solution under AC electric field conditions. As an objective and accurate quantitative analysis of the ice crystal growth process, we propose a Dilated Convolutional Segmentation Transformer for semantic segmentation of ice crystal images. It is concluded that the average area and the growth rate of single ice crystals decrease with increasing electric field frequency at a certain concentration of dimethyl sulfoxide solution. Lower concentrations of dimethyl sulfoxide solution in combination with an AC electric field can achieve similar ice suppression effects as when higher concentrations of dimethyl sulfoxide solution act alone. We believe that AC electric fields are expected to be an aid to cryopreservation and provide some theoretical basis and experimental foundation for its development.

Ferroelectricity of ice nanotube forests grown in three-dimensional graphene: the electric field effect

Generating diverse ferroelectric ice nanotubes (NTs) efficiently has always been challenging, but matters in nanomaterial synthesis and processing technology. In the present work, we propose a method of growing ice NT forests in a single cooling process. A three-dimensional (3D) graphene structure was selected to behave as a representative container in which a batch of (5, 0) ice NTs was formed simultaneously under the cooling process from molecular dynamics simulation. Other similar 3D graphene structures but with different hole configurations, like uniform triangle or both triangle and pentagon, were also tested, revealing that ice NTs with different tube indices, i.e. both (3, 0) and (5, 0), could also be formed at the same time. Intriguingly, the orientations of the dipole moments of the water molecules of an ice NT formed were independent of each other, making the net ferroelectricity of the whole system weakened or even cancelled. An electric field could help change the orientation of the water molecules of the already obtained ice NTs and even twist the tube to be a spiral (5, 1) one if it was applied during the cooling process, such that the net ferroelectricity was greatly improved. The underlying physical mechanism of all phase transition phenomena, including the improvement of the ferroelectricity under an electric field, were explored in depth from the phase transition curves and structural point of view. The obtained results are of significant application value for improving the preparation efficiency of nano-ferroelectric materials, which are prosperous in nano-devices.

Various states of water species in an anion exchange membrane characterized by Raman spectroscopy under controlled temperature and humidity

Anion exchange membrane fuel cells (AEMFCs) hold the key to future mass commercialisation of fuel cell technology, even though currently, AEMFCs perform less optimally than proton exchange membrane fuel cells (PEMFCs). Unlike PEMFCs, AEMFCs have demonstrated the capability to operate independently of Pt group metal-based catalysts. Water characterization inside the membrane is one factor that significantly influences the performance of AEMFCs. In this paper, different water species inside an anion exchange membrane (AEM), QPAF-4, developed at the University of Yamanashi, were studied for the first time using micro-Raman spectroscopy. Spectra of pure water, alkaline solutions, and calculations based on density functional theory were used to identify the water species in the AEM. The OH stretching band was deconvoluted into nine unique Gaussian bands. All the hydrogen-bonded OH species increased steadily with increasing humidity, while the CH and non-H-bonded OH remained relatively constant. These results confirm the viability of micro-Raman spectroscopy in studying the various water-related species in AEMs. The availability of this technique is an essential prerequisite in improving the ionic conductivity and effectively solving the persisting durability challenge facing AEMFCs, thus hastening the possibility of mass commercialisation of fuel cells.

Germination of bean seeds (Vigna unguiculata L. Walp.) in strong electric fields

This paper presents a tutorial for the germination of bean seeds (Vigna unguiculata L. Walp.) in strong electrostatic fields up to 1240 V/cm. The seeds were allowed to germinate under different electric field strengths for 48 h. Although most of such germination experiments did not show any visible effect, the field strength of 945 V/cm strongly increased the seedling's vigor during the early growth stage. In the end, 30 % more yield was obtained from stimulated seeds when compared to the control group. This article postulates for the first time a hypothesis of the mechanism of action of the electric field during germination. In biological cells of any species, water confined between narrow surfaces can undergo a phase transition that shifts its melting point to higher temperatures when an external electric field is applied. This effect has already been known as electrofreezing, and has been confirmed by several experimental and molecular modeling studies. As a consequence, the transport kinetics of molecules across cell organelle membranes might be altered, which in turn leads to different plant properties. With emphasis on the presented method, this work reports: •An inexpensive electric circuit for the generation of strong electric fields•Instructions regarding the setup and operation of an adequate germination chamber.

Modeling the influence of the external electric fields on water viscosity inside carbon nanotubes

Equilibrium molecular dynamics simulations were performed to explore the effects of external electric fields and confinement on water properties inside various carbon nanotubes (CNTs). Using different GHz electric field frequencies as well as various constant electric field strengths, the radial distribution function and density profile were investigated, by which the impact of the electric fields and confinement on the water structure are revealed. The results indicated water molecules inside the CNT form layered structures due to topological confinement applying external electric fields can disturb this ordered water molecules structure and increase the viscosity of confined water, particularly in the case of CNTs with a radius less than 13.5 Å. Conversely, for CNTs with a radius greater than13.5 Å, the viscosity decreases under the influence of external oscillating or constant electric fields. How dose the synergism of confinement and external electric fields affect the water properties inside the CNTs?

Electro-Freezing of Supercooled Water Is Induced by Hydrated Al3+ and Mg2+ Ions: Experimental and Theoretical Studies

This work reports that the octahedral hydrated Al3+ and Mg2+ ions operate within electrolytic cells as kosmotropic (long-range order-making) "ice makers" of supercooled water (SCW). 10-5 M solutions of hydrated Al3+ and Mg2+ ions each trigger, near the cathode (-20 ± 5 V), electro-freezing of SCW at -4 °C. The hydrated Al3+ ions do so with 100% efficiency, whereas the Mg2+ ions induce icing with 40% efficiency. In contrast, hydrated Na+ ions, under the same experimental conditions, do not induce icing differently than pure water. As such, our study shows that the role played by Al3+ and Mg2+ ions in water electro-freezing is impacted by two synchronous effects: (1) a geometric effect due to the octahedral packing of the coordinated water molecules around the metallic ions, and (2) the degree of polarization which these two ions induce and thereby acidify the coordinated water molecules, which in turn imparts them with an ice-like structure. Long-duration molecular dynamics (MD) simulations of the Al3+ and Mg2+ indeed reveal the formation of "ice-like" hexagons in the vicinity of these ions. Furthermore, the MD shows that these hexagons and the electric fields of the coordinate water molecules give rise to ultimate icing. As such, the MD simulations provide a rational explanation for the order-making properties of these ions during electro-freezing.

Machine learning enabled quantification of the hydrogen bonds inside the polyelectrolyte brush layer probed using all-atom molecular dynamics simulations

The configuration of densely grafted charged polyelectrolyte (PE) brushes is strongly dictated by the properties and behavior of the counterions that screen the PE brush charges and the solvent molecules (typically water) that solvate the brush molecules and these screening counterions. Only recently, efforts have been made to study the PE brushes atomistically, thereby shedding light on the properties of brush-supported ions and water molecules. However, even for such efforts, there are limitations associated with using a generic definition to estimate certain properties of water and ions inside the brush layer. For example, water-water hydrogen bonds (HBs) will behave differently for locations outside and inside the brush layer, given the fact that the densely closely grafted PE brush molecules create a soft nanoconfinement where the water connectivity becomes highly disrupted: therefore, using the same definition to quantify the HBs inside and outside the brush layer will be unwise. In this paper, we address this limitation by employing an unsupervised machine learning (ML) approach to predict the water-water hydrogen bonding inside a cationic PE brush layer modeled using all-atom molecular dynamics (MD) simulations. The ML method, which relies on a clustering approach and uses the equilibrium coordinates of the water molecules (obtained from the all-atom MD simulations) as the input, is capable of identifying the structural modification of water-water HBs (revealed through appropriate clustering of the data) inside the PE brush layer induced soft nanoconfinement. Such capabilities would not have been possible by using a generic definition of the HBs. Our calculations lead to four key findings: (1) the clusters formed inside and outside the brush layer are structurally similar; (2) the margin of the cluster is shorter inside the PE brush layer confirming the possible disruption of the HBs inside the PE brush layer; (3) the average "hydrogen-acceptor-oxygen-donor-oxygen" angle that defines the HB is reduced for the HBs formed inside the brush layer; (4) the use of the generic definition (definition usable for characterizing the HBs in brush-free bulk) leads to an overprediction of the number of HBs formed inside the PE brush layer.

Effects of Electrostatic Interactions on Kapitza Resistance in Hexagonal Boron Nitride-Water Interfaces

Electrostatic interactions in nanoscale systems can influence the heat transfer mechanism and interfacial properties. This study uses molecular dynamics simulations to investigate the impact of various electrostatic interactions on the Kapitza resistance (R_k) on a hexagonal boron nitride-water system. The Kapitza resistance at hexagonal boron nitride nanotube (hBNNT)-water interface reduces with an increase in diameter of the nanotube due to more aggregation of water molecules per unit surface area. An increase in the partial charges on boron and nitride caused the reduction in R_k. With the increase in partial charge, a better hydrogen bonding between hBNNT and water was observed, whereas the structure and order of the water molecules remain the same. Nevertheless, the addition of NaCl salt into water does not have any influence on interfacial thermal transport. R_k remains unchanged with electrolyte concentration because the cumulative Coulombic interaction between the ions and the hBNNT is significantly less when compared with water molecules. Furthermore, the effect of electric field strength on interfacial heat transfer is also investigated by providing uniform positive and negative surface charges on the outermost hBN layers. R_k is nearly independent of the practical range of applied electric fields and decreases with an increasing electric field for extreme field strengths until the electrofreezing phenomenon occurs. The ordering of water molecules toward the charged surface leads to an increase in the layering effect, causing the reduction in R_k in the presence of an electric field.

Electrically Suppressed Outflow of Confined Liquid in Hydrophobic Nanopores

Confining liquid in a hydrophobic nanoenvironment has enabled a broad spectrum of applications in biomedical sensors, mechanical actuators, and energy storage and converters, where the outflow of confined liquid is spontaneous and fast due to the intrinsic hydrophobic nature of nanopores with extremely low interfacial friction, challenging design capacity and control tolerance of structures and devices. Here, we present a facile approach of suppressing the outflow of water confined in hydrophobic nanopores with an electric field. Extensive molecular dynamics simulations show that the presence of an electric field could significantly strengthen hydrogen bonds and retard degradations of the associated networks during the outflow. The outflow deformation and strength are extracted to quantitatively characterize the electrical suppression to outflow and agree well with simulations. This study proposes a practical means of impeding the fast liquid outflow in hydrophobic nanopores, potentially useful for devising nanofluidics-based functional structures and devices with controllable performance.

Critical In-Plane Density of Polyelectrolyte Brush for the Ordered Hydrogen-Bonded Structure of Incorporated Water

A polymer electrolyte brush is a reasonable platform to confine water molecules within a nanoscopic area to study their role in the function of interacting media because of their adjustable nanospace and charge by changing the in-plane density and side chains of the brush. Here, we demonstrate how the in-plane spacing of cationic polymer brush chains, poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMTAC), affects the hydrogen bond configuration of incorporated water using soft X-ray emission spectroscopy. At the critical in-plane density σ = 0.30 chains/nm² of PMTAC, tetrahedrally coordinated water molecules started to melt into distorted or broken hydrogen-bonded configurations. Considering the charge on the quaternary ammonium cations, the electric field required to form a tetrahedrally coordinated hydrogen-bonded configuration was estimated as ∼500 kV cm^-1 and is effective up to ∼1 nm from the surface of the polymer chain. These findings are useful for designing specific interface properties and the resultant surface function of polyelectrolyte-based materials.

Molecular insights into the electrowetting behavior of aqueous ionic liquids

Molecular dynamics (MD) simulations were applied to investigate the wettability of aqueous hydrophilic and hydrophobic imidazolium-based ionic liquid (IL) nano-droplets on a graphite surface under a perpendicular electric field. Imminent transformation in the droplet configuration was observed at E = 0.08 V Å^-1 both for hydrophobic ILs 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][NTF₂] and SPC/E water droplets. However, for the hydrophilic IL, 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF₄], the droplet was entirely elongated to column-shaped at E = 0.09 V Å^-1 for lower weight percentages of ILs and at E = 0.15 V Å^-1 for a higher weight percentage of ILs (i.e., 50 wt%). We explored the impact of the electric field through various parameters such as mass and charge density distribution across the droplet, contact angle of the droplet, orientation of water dipoles, and hydrogen bond analysis. The external electric field was found to influence the orientation of water dipoles and the accumulation of charge at various interfaces was observed with an increase in an electric field, which finally leads to shape deformation and depletion of ions from the liquid-vapor interface of the droplet. However, this behavior strongly depends on the hydrophilicity or hydrophobicity of the ILs and thus, is critically examined for both the ILs.

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.

Dielectric Profile and Electromelting of a Monolayer of Water Confined in Graphene Slit Pore

A monolayer of water confined between two parallel graphene sheets exists in many different phases and exhibits fascinating dielectric properties that have been studied in experiments. In this work, we use molecular dynamics simulations to study how the dielectric properties of a confined monolayer of water is affected by its structure. We consider six of the popular nonpolarizable water models-SPC/E, SPC/Fw, TIP3P, TIP3P_M (modified), TIP4P-2005, and TIP4P-2005f-and find that the in-plane structure of the water molecules at ambient temperature and pressure is strongly dependent on the water model: all the 3-point water models considered here show square ice formation, whereas no such structural ordering is observed for the 4-point water models. This allows us to investigate the role of the in-plane structure of the water monolayer on its dielectric profile. Our simulations show an anomalous perpendicular dielectric constant compared to the bulk, and the models that do not exhibit ice formation show very different dielectric response along the channel width compared to models that exhibit square ice formation. We also demonstrate the occurrence of electromelting of the in-plane ordered water under the application of a perpendicular electric field and find that the critical field for electromelting strongly depends on the water model. Together, we have shown the dependence of confined water properties on the different water structures that it may take when sandwiched between bilayer graphene. These remarkable properties of confined water can be exploited in various nanofluidic devices, artificial ion channels, and molecular sieving.

Structure, dynamics, and morphology of nanostructured water confined between parallel graphene surfaces and in carbon nanotubes by applying magnetic and electric fields

Water molecules experience certain changes in their properties when they feel an external magnetic or electric field. These changes are significant in different applications, such as biological and biotechnological processes, nano-pumping, and water treatment. In this work, we have performed molecular dynamics (MD) simulations to investigate the different thermodynamics, structure, and dynamics of water molecules confined between two parallel surfaces and also confined in carbon nanotubes (CNTs). We have also applied different electric and magnetic fields in different directions to the confined molecules. In the graphene system, no polygonal shape was formed in either low or high electric fields, whereas rhombic and pentagonal shapes were formed in low and high magnetic fields. In the CNT system, applying electric fields in all three dimensions made the pentagonal shape disappear and the confined water molecules formed a ring shape when the electric field was applied in the axial direction. Applying the electric field perpendicular to the graphene surfaces increases the self-diffusion of the confined molecules, whereas applying the electric and magnetic fields along the CNT axis decreases the self-diffusion of the confined water molecules. In the graphene system, applying the electric field perpendicular to the graphene surfaces decreases the average number of hydrogen bonds (〈HB〉) whereas the magnetic field has little effect on the 〈HB〉. In the CNT system, applying E_x also leads to a smaller number of HBs. Also, applying the magnetic field along the x-direction (along the CNT direction) leads to a greater number of HBs than the other fields.

Heterogeneous Electrofreezing Triggered by CO2 on Pyroelectric Crystals: Qualitatively Different Icing on Hydrophilic and Hydrophobic Surfaces

By performing icing experiments on hydrophilic and hydrophobic surfaces of pyroelectric amino acids and on the x-cut faces of LiTaO₃ , we discovered that the effect of electrofreezing of super cooled water is triggered by ions of carbonic acid. During the cooling of the hydrophilic pyroelectric crystals, a continuous water layer is created between the charged hemihedral faces, as confirmed by impedance measurements. As a result, a current of carbonic acid ions, produced by dissolved environmental CO₂ , flows through the wetted layer towards the hemihedral faces and elevates the icing temperature. This proposed mechanism is based on the following: (i) on hydrophilic surfaces, water with dissolved CO₂ (pH 4) freezes at higher temperatures than pure water of pH 7. (ii) In the absence of the ionic current, achieved by linking the two hemihedral faces of hydrophilic crystals by a conductive paint, water of the two pH levels freeze at the same temperature. (iii) On hydrophobic crystals with similar pyroelectric coefficients, where there is no continuous wetted layer, no electrofreezing effect is observed.

Kinetic study on electro-nucleation of water in a heterogeneous propane nano-bubble system to form polycrystalline ice Ic

Elucidating the underlying mechanisms of water solidification in heterogeneous systems is crucially important for a panoply of applications; gaining such an understanding has also proven to be very challenging to the community. Indeed, one such example lies in clarifying the thermodynamics and kinetics of electro-crystallization in heterogeneous systems, such as micro- and nano-bubble systems. Here, we employ non-equilibrium molecular dynamics of water in heterogeneous environments experiencing direct contact with a propane gas phase at various temperatures in externally applied static electric fields, elucidating significant external-field effects in inducing poly-crystalline cubic-ice formation. This is in stark contrast with recent work on homogeneous cubic-ice electro-nucleation to produce largely fault-free single crystals. We explore the kinetics of heterogeneous cubic-ice electro-nucleation under different field intensities and thermal conditions and provide an overview of time-dependent dynamics of evolution of polycrystallinity.

Electric field triggered release of gas from a quasi-one-dimensional hydrate in the carbon nanotube

We systematically investigate the effects of an axial electric field on the formation and decomposition of quasi-one-dimensional nitrogen gas hydrates within a single-walled carbon nanotube (SWNT) by using molecular dynamics (MD) simulations. We find that the nitrogen hydrate in the SWNT undergoes a series of structure phase transitions with increasing electric field. Corresponding to the structure transition, the nitrogen gas releases from the carbon nanotube in the electric field range of 1 V nm^-1 to 2 V nm^-1. However, nitrogen molecules are trapped as guest molecules, forming a molecule wire, in the ice nanotube when the electric field is less than 1 V nm^-1 or larger than 2 V nm^-1. Our simulations indicate that the nanotube is an excellent tiny gas tank that can be used to trap gas molecules and control their release triggered sensitively by electric signals. The key to this phenomenon is the change in orientations of water dipoles induced by the electric field, which leads to the structural change in the hydrogen-bonding network and the change in the diffusion coefficient of the water molecules. Our findings here may help understanding the mechanism of the electrorelease of gas from hydrates confined in the nanoscale space.

Electric Field Induced Dewetting of Hydrophobic Nanocavities at Ambient Temperature

The understanding of water dewetting in nanoporous materials is of great importance in various fields of science and technology. Herein, we report molecular dynamics simulation results of dewetting of water droplet in hydrophobic nanocavities between graphene walls under the influence of electric field. At ambient temperature, the rate of dewetting induced by electric field is significantly large. Whereas, it is a very low rate of dewetting induced by high temperature (423 K) due to the strong interaction of the hydrogen-bonding networks of water droplets in nanocavities. In addition, the electric filed induced formation of a water column has been found in a vacuum chamber. When the electric field is turned off, the water column will transform into a water droplet. Importantly, the results demonstrate that the rate of electric field-induced dewetting increases with growth of the electric field. Overall, our results suggest that electric field may have a great potential application for nanomaterial dewetting.

A primary model of THz and far-infrared signal generation and conduction in neuron systems based on the hypothesis of the ordered phase of water molecules on the neuron surface I: signal characteristics

In this paper, we use the theory of quantum optics and electrodynamics to study the electromagnetic field problem in the nervous system based on the assumption of an ordered arrangement of water molecules on the neuronal surface. Using the Lagrangian of the water molecule-field ion, the dynamic equations for neural signal generation and transmission are derived. Perturbation theory and the numerical method are used to solve the dynamic equations, and the characteristics of high-frequency signals (the dispersion relation, the time domain of the field, the frequency domain waveform, etc.) are discussed. This model predicts some intrinsic vibration modes of electromagnetic radiation on the neuronal surface. The frequency range of these vibration modes is in the THz and far-infrared ranges.

Replica exchange MD simulations of two-dimensional water in graphene nanocapillaries: rhombic versus square structures, proton ordering, and phase transitions

The hydrogen bond patterns, proton ordering, and phase transitions of monolayer ice in two-dimensional hydrophobic confinement are fundamentally different from those found for bulk ice. To investigate the behavior of quasi-2D ice, we perform molecular dynamics simulations of water confined between fixed graphene plates at a distance of 0.65 nm. While experimental results are still limited and theoretical investigations are often based on a single, often empirically based force field model, this work presents a systematic study modeling the water-graphene interaction by effective Lennard-Jones potentials previously derived from high-level ab initio CCSD(T) calculations of water adsorbed on graphene [Phys. Chem. Chem. Phys., 2013, 15, 4995]. For the water-water interaction different water force fields, i.e. SPCE, TIP3P, TIP4P, TIP4P/ICE, and TIP5P, are used. The water occupancy of the graphene capillary at a pressure of 1000 MPa is determined to be between 13.5 and 13.9 water molecules per square nanometer, depending on the choice of the water force field. Based on these densities, we explore the structure and dynamics of quasi-2D water for temperatures ranging from 200 K to about 600 K for each of the five force fields. To ensure complete sampling of the configurational space and to overcome the barriers separating metastable structures, these simulations are based on the replica exchange molecular dynamics technique. We report different tetragonal hydrogen bond patterns, which are classified as nearly square or as rhombic. While many of these arrangements are essentially flat, in some cases puckered arrangements are found, too. Also the proton ordering of the quasi-2D water structures is considered, allowing us to identify them as ferroelectric, ferrielectric or antiferroelectric. For temperatures between 200 K and 400 K we find several second-order phase transitions from one ice structure to another, changing in many cases both the arrangements of the oxygen atoms and the proton ordering. For temperatures between 400 K and 600 K there are melting-like transitions from a monolayer of ice to a monolayer of liquid water. These first-order phase transitions have a latent heat between 3.4 and 4.0 kJ mol^-1. Both the values of the transition temperatures and of the latent heats display considerable model dependence for the five different water models investigated here.

Enhancement of the local asymmetry in the hydrogen bond network of liquid water by an ultrafast electric field pulse

Condensed phase electron decomposition analysis based on density functional theory has recently revealed an asymmetry in the hydrogen-bond network in liquid water, in the sense that a significant population of water molecules are simultaneously donating and accepting one strong hydrogen-bond and another substantially weaker one. Here we investigate this asymmetry, as well as broader structural and energetic features of water's hydrogen-bond network, following the application of an intense electric field square pulse that invokes the ultrafast reorientation of water molecules. We find that the necessary field-strength required to invoke an ultrafast alignment in a picosecond time window is on the order of 108 Vm-1. The resulting orientational anisotropy imposes an experimentally measurable signature on the structure and dynamics of the hydrogen-bond network, including its asymmetry, which is strongly enhanced. The dependence of the molecular reorientation dynamics on the field-strength can be understood by relating the magnitude of the water dipole-field interaction to the rotational kinetic energy, as well as the hydrogen-bond energy.

Effect of Electric Field on Gas Hydrate Nucleation Kinetics: Evidence for the Enhanced Kinetics of Hydrate Nucleation by Negatively Charged Clay Surfaces

Natural gas hydrates are found widely in oceanic clay-rich sediments, where clay-water interactions have a profound effect on the formation behavior of gas hydrates. However, it remains unclear why and how natural gas hydrates are formed in clay-rich sediments in spite of factors that limit gas hydrate formation, such as small pore size and high salinity. Herein, we show that polarized water molecules on clay surfaces clearly promote gas hydrate nucleation kinetics. When water molecules were polarized with an electric field of 10⁴ V/m, gas hydrate nucleation occurred significantly faster with an induction time reduced by 5.8 times. Further, the presence of strongly polarized water layers at the water-gas interface hindered gas uptake and thus hydrate formation, when the electric field was applied prior to gas dissolution. Our findings expand our understanding of the formation habits of naturally occurring gas hydrates in clay-rich sedimentary deposits and provide insights into gas production from natural hydrate deposits.

Control of ice nucleation: freezing and antifreeze strategies

Water freezing remains a perennial topic of great relevance to many important aspects of our lives; from the climate to human society and from economics to medicine, frozen water profoundly influences our living environment and life activities.

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.

Nonexponential kinetics of ion pair dissociation in electrofreezing water

Temporally- or spatially-heterogeneous environments can participate in many kinetic processes, from chemical reactions and self-assembly to the forced dissociation of biomolecules. Here, we simulate the molecular dynamics of a model ion pair forced to dissociate in an explicit, aqueous solution. Triggering dissociation with an external electric field causes the surrounding water to electrofreeze and the ion pair population to decay nonexponentially. To further probe the role of the aqueous environment in the kinetics, we also simulate dissociation events under a purely mechanical force on the ion pair. In this case, regardless of whether the surrounding water is a liquid or already electrofrozen, the ion pair population decays exponentially with a well-defined rate constant that is specific to the medium and applied force. These simulation data, and the rate parameters we extract, suggest the disordered kinetics in an electrofreezing medium are a result of the comparable time scales of two concurrent processes, electrofreezing and dissociation.

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates that the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the "buckling failure" of the square-ice-nanotube columns, which is dominated by the lateral pressure.

A Theoretical Study of the Hydration of Methane, from the Aqueous Solution to the sI Hydrate-Liquid Water-Gas Coexistence

Monte Carlo and molecular dynamics simulations were done with three recent water models TIP4P/2005 (Transferable Intermolecular Potential with 4 Points/2005), TIP4P/Ice (Transferable Intermolecular Potential with 4 Points/ Ice) and TIP4Q (Transferable Intermolecular Potential with 4 charges) combined with two models for methane: an all-atom one OPLS-AA (Optimal Parametrization for the Liquid State) and a united-atom one (UA); a correction for the C–O interaction was applied to the latter and used in a third set of simulations. The models were validated by comparison to experimental values of the free energy of hydration at 280, 300, 330 and 370 K, all under a pressure of 1 bar, and to the experimental radial distribution functions at 277, 283 and 291 K, under a pressure of 145 bar. Regardless of the combination rules used for σ_C,O, good agreement was found, except when the correction to the UA model was applied. Thus, further simulations of the sI hydrate were performed with the united-atom model to compare the thermal expansivity to the experiment. A final set of simulations was done with the UA methane model and the three water models, to study the sI hydrate-liquid water-gas coexistence at 80, 230 and 400 bar. The melting temperatures were compared to the experimental values. The results show the need to perform simulations with various different models to attain a reliable and robust molecular image of the systems of interest.

Commensurability Effects in Viscosity of Nanoconfined Water

The rate of water flow through hydrophobic nanocapillaries is greatly enhanced as compared to that expected from macroscopic hydrodynamics. This phenomenon is usually described in terms of a relatively large slip length, which is in turn defined by such microscopic properties as the friction between water and capillary surfaces and the viscosity of water. We show that the viscosity of water and, therefore, its flow rate are profoundly affected by the layered structure of confined water if the capillary size becomes less than 2 nm. To this end, we study the structure and dynamics of water confined between two parallel graphene layers using equilibrium molecular dynamics simulations. We find that the shear viscosity is not only greatly enhanced for subnanometer capillaries, but also exhibits large oscillations that originate from commensurability between the capillary size and the size of water molecules. Such oscillating behavior of viscosity and, consequently, the slip length should be taken into account in designing and studying graphene-based and similar membranes for desalination and filtration.

Perspectives on external electric fields in molecular simulation: progress, prospects and challenges

In this review, the application of a wide variety of external electric fields in molecular simulation shall be discussed, including time-varying and electromagnetic, as well as the utility and potential impact and prospects for exploitation of such simulations for real-world and industrial end use. In particular, non-equilibrium molecular dynamics will be discussed, as well as challenges in addressing adequate thermostatting and scaling field amplitudes to more experimentally relevant levels. Attention shall be devoted to recent progress and advances in external fields in ab initio molecular simulation and dynamics, as well as elusive challenges thereof (and, to some extent, for molecular dynamics from empirical potentials), such as timescales required to observe low-frequency and intensity field effects. The challenge of deterministic molecular dynamics in external fields in sampling phase space shall be discussed, along with prospects for application of fields in enhanced-sampling simulations. Finally, the application of external electric fields to a wide variety of aqueous, nanoscale and biological systems will be discussed, often motivated by the possibility of exploitation in real-world applications, which serve to underpin our molecular-level understanding of field effects in terms of microscopic mechanisms, and possibly with a view to control thereof.

Interfacial structure and wetting properties of water droplets on graphene under a static electric field

The equilibrium water droplets present a hemispherical, a conical and an ordered cylindrical shape with the increase of external E-field intensity.

Phase transitions of a water overlayer on charged graphene: from electromelting to electrofreezing

We show by using molecular dynamics simulations that a water overlayer on charged graphene experiences first-order ice-to-liquid (electromelting), and then liquid-to-ice (electrofreezing) phase transitions with the increase of the charge value. Corresponding to the ice-liquid-ice transition, the variations of the order parameters indicate an order-disorder-order transition. The key to this novel phenomenon is the surface charge induced change of the orientations of water dipoles, which leads to the change of the water-water interactions from being attractive to repulsive at a critical charge value qc. To further uncover how the orientations of water dipoles influence the interaction strength between water molecules, a theoretical model considering both the Coulomb and van der Waals interactions is established. The results show that with the increase of the charge value, the interaction strength between water molecules decreases below qc, then increases above qc. These two inverse processes lead to electromelting and electrofreezing, respectively. Combining this model with the Eyring equation, the diffusion coefficient is obtained, the variation of which is in qualitative agreement with the simulation results. Our findings not only expand our knowledge of the graphene-water interface, but related analyses could also help recognize the controversial role of the surface charge or electric field in promoting phase transitions of water.

Ferroelectric hexagonal and rhombic monolayer ice phases

Two new monolayer ice phases are predicted from molecular dynamics simulations, both proven to be ferroelectric.