Adjustable diffusion enhancement of water molecules in a nanoscale water bridge.

Capillary forces play an important role during the dewatering and drying of nanocellulosic materials. Traditional moisture removal techniques, such as heating, have been proved to be deterimental to the properties of these materials and hence, there is a need to develop novel dewatering techniques without affecting the desired properties of materials. It is, therefore, important to explore novel methods for dewatering these high-added-value materials without negatively influencing their properties. In this context, we explore the effect of electric field on the capillary forces developed by a liquid-water bridge between two cellulosic surfaces, which may be formed during the water removal process following its displacement from the interfibrillar spaces. All-atom molecular dynamics (MD) simulations have been used to study the influence of an externally applied electric field on the capillary force exerted by a water bridge. Our results suggest that the equilibrium contact angle of water and the capillary force exerted by the water bridge between two nanocellulosic surfaces depend on the magnitude and direction of the externally applied electric fields. Hence, an external electric field can be applied to manipulate the capillary forces between two particles. The close agreement between the capillary forces measured through MD simulations and those calculated through classical equations indicates that, within the range of the electric field applied in this study, Young-Laplace equations can be safely employed to predict the capillary forces between two particles. The present study provides insights into the use of electric fields for drying of nanocellulosic materials.

External electric field effects on asphaltene adsorption on Kaolinite-water interface: A molecular dynamics study

Understanding and controlling water molecules confined in water channels has great applications. Many artificial water channels have been designed to mimic water permeation across water channels. One kind of artificial water channels, which contains two disjoint carbon nanotubes that are not physically connected has attracted much attention. Water transfers through the disjoint nanochannel by a water bridge. However, the water bridge is affected by the nanogap between two disjoint carbon nanotubes. Here, we firstly report on the possibility of reinforcing a water bridge in a disjoint nanochannel by molecular dynamics simulations and find that the length of the nanogap, the electric field strength, the direction of the electric field and the effective pressure within water have an effect on the formation of a water bridge in the disjoint nanochannel. The analysis of the Lennard-Jones potential and the potential of mean force helps to reveal the mechanism. Our findings are beneficial for designing nanodevices with disjoint nanostructures and understanding the effects of the nanogap on the behaviors of water molecules in the disjoint nanochannel.

Random vapor nucleation leads to flooding condensation with degraded heat-transfer efficiency. Since an external electric field has a significant effect on manipulating droplets' motion, it is possible to be one of the effective methods to hinder flooding phenomena and improve the heat-transfer rate by applying the external electric field during condensation. However, the motion of nanodroplets is more sensitive to the electric field owing to the scale effect on the nanoscale. The effect of the electric field on growth has not explicitly been comprehended. This work studied the condensation processes on a nanodimpled surface under an electric field with various strengths and directions. The results showed that condensed droplets' growth under the electric field depends on the competition between the electric field force and solid-liquid interactions. Increased vertical electric field strength, the higher torsion by the electric field hindered the motion of vapor, decreased the collision frequency for water molecules with the cooled surface, and elongated the cluster when the electric field force dominates, thus deteriorating the condensation performance. While applying the horizontal electric field, the greater electric field strength leads to better condensation performance by the larger contacting area for heat exchange. A wetting transition induced by the electric field was observed when the electric field strength increased to a certain extent (E > 5.2 × 108 V/m in this study). When the V-shaped surface replaced the dimpled surface as the condensed substrate, the same wetting transition phenomena occurred under a more significant horizontal electric field strength, showing that this method is universal. Besides, different electric field frequencies influenced both the growth and the nucleation, thus exhibiting various condensation performances.

Probing the coalescence mechanism of water droplet and Oil/Water interface in demulsification process under DC electric field

Non-equilibrium molecular dynamics(MD) simulations were performed according to the electronic anti-fouling technology, and some structural parameters and dynamic parameters of CaCl2 aqueous solution were taken as indicators to compare the different effect on the anti-fouling performance by applying different electric fields. The results show that electric fields can effectively decrease the viscosity of CaCl2 aqueous solution and enhance the ionic activity by enlarging the self-diffusion coefficient. In addition, with the same electric field strength, the electrostatic field is more effective at decreasing the viscosity of CaCl2 aqueous solution and increasing the self-diffusion coefficient of water molecules, while the alternating electric field is more effective at increasing the self-diffusion coefficient of Ca2+. Furthermore, an alternating electric field with different frequencies was applied; the results show that an 800 kHz frequency is most effective to decrease the viscosity, and a 700 kHz frequency is most effective to enhance the self-diffusion coefficient of water molecule. Otherwise, 400 kHz is most effective to enhance the self-diffusion coefficient of Ca2+. Additionally, by studying the change of structure parameters, it was concluded that an external electric field can enhance the hydration between Ca2+ and coordinated water molecules, and the alternating electric field is more effective in this respect.

Droplet deformation and mechanical behavior under external electric field are fundamental in electrohydrodynamic research. Based on the volume-of-fluid (VOF) method and adding electric force to Navier-Stokes equations as an additional source term, a simulation method considering fluid flow and electric field in two-way coupling is proposed. Using this method, the deformation/motion of leaky dielectric droplet and charged droplet under external uniform and non-uniform electric field is investigated respectively. The shape deformation coefficient calculated by the proposed method is close to the theory value of leaky dielectric droplet under uniform electric field in small deformation condition; thus the validity of the numerical method proposed in this study is verified. The results show that the charge redistribution modes of leaky dielectric droplet depend on the property difference between droplet and the around fluid, and the droplet may deform into either prolate or oblate shapes. The electric field strength only affects the droplet shape deformation coefficient. The external uniform electric field induces strong circulation motion inside the leaky dielectric droplet without moving the droplet. Because of the effect of Coulomb force, the charged droplet may deform into prolate and move along the electric field direction. However both leaky dielectric droplet and charged droplet may move along the electric field direction, resulting in different shapes simultaneously under the non-uniform electric field. The numerical method coupling fluid flow with electric field proposed in this paper could provide an effective approach to electrostatic spray, electrophoresis and other complex engineering electrohydrodynamic researches.

Molecular dynamics simulations are conducted to study the flow behavior of CsF solutions in nanochannels under external electric fields E. It is found that the channel surface energy greatly affects the flow behavior. In channels of high surface energy, water molecules, on average, move in the same direction as that of the electric field regardless of the strength of E. In low surface energy channels, however, water transports in the opposite direction to the electric field at weak E and the flow direction is changed when E becomes sufficiently large. The direction change of water flow is attributed to the coupled effects of different water-ion interactions, inhomogeneous water viscosity, and ion distribution changes caused by the electric field. The flow direction change observed in this work may be employed for flow control in complex micro- or nanofluidic systems.

Bridge water mediates nevirapine binding to wild type and Y181C HIV-1 reverse transcriptase—Evidence from molecular dynamics simulations and MM-PBSA calculations

We investigate the influence of an external electric field on the dewetting behavior of nitrogen-water systems between two hydrophobic plates using molecular dynamics simulations. It is found that the critical distance of dewetting increases obviously with the electric field strength, indicating that the effective range of hydrophobic attraction is extended. The mechanism behind this interesting phenomenon is related to the rearrangement of hydrogen bond networks between water molecules induced by the external electric field. Changes in the hydrogen bond networks and in the dipole orientation of the water molecules result in the redistribution of the neutral nitrogen molecules, especially in the region close to the hydrophobic plates. Our findings may be helpful for understanding the effects of the electric field on the long-range hydrophobic interactions.

Summary Micro/nano structures formed via self-assembly of biological macromolecules, such as DNA and RNA, have potential applications as building units in micro/nano/biodevices. The self-assembled structures of biological macromolecules can be manipulated by applying electric field. Here we demonstrate such manipulation by using coarse-grained molecular dynamics simulations. Starting from randomly distributed CG DOPE lipids and polarizable water molecules, we studied the selfassembled structures in different hydration levels of lipid (the number of water molecules per lipid) and different strength of external electric fields. When the hydration level of lipid was 24 water per lipid (w/l) or 36 w/l, the self-assembled structures were lamellar bilayers. When the external electric field of 0.5 V/nm was applied on systems with different hydration levels of 24, 32 36 or 40 w/l, the self-assembled structures were inverted cylinders for the system with the hydration level of 24, 32 or 36 w/l and bilayer with pleats for the system with the hydration level of 40 w/l. When the hydration level was fixed at 36 w/l and different external electric field of 0.0, 0.1, 0.3, 0.5 or 0.7 V/nm was applied, bilayer structures could form only when the external electric field was 0.0 and 0.1 V/nm. Interestingly, the bilayer structure was curved and mirror symmetry when the external electric field was 0.1 V/nm. The curved membrane was stable during the following 150 ns after its formation at about 40 ns. Longer simulations were not performed because of the computational power limitation. The imperfect mirror-symmetric membrane also formed in the simulation with the external electric field of 0.3 V/nm, but quickly it changed to a inverted cylinder structure.

The orientation of water molecules within nanochannels is pivotal in influencing water transport, particularly under the influence of electric fields. This study delves into the effects of electric field direction on water transport through disjoint nanochannels, a structure which is of emerging significance. Molecular dynamics simulations are conducted to study the properties of water in complete nanochannel and disjoint nanochannels with gap sizes of 0.2 nm and 0.4 nm, respectively, such as occupancy, transport, water bridge formation, and dipole orientation, by systematically varying the electric field direction from 0 to 180 degrees. The simulation results disclose that the electric field direction has little influence on water flow through complete nanochannels. However, as the size of the nanogap expands, the declining trend of water transfer rate through disjoint nanochannels becomes more distinctive when the electric field direction is shifted from 0 to 90 degrees under an electric field with a strength of 1 V/nm. Notably, results also reveal distinct behaviors at 90 degrees under an electric field with a strength of 1 V/nm, where the stable water chains, unstable water bridges, and no water bridges are observed in complete nanochannels, disjoint nanochannels with 0.2 nm gap, and 0.4 nm gap, respectively. Moreover, simulations indicate that increasing the electric field strength in a polarization direction perpendicular to the tube axis facilitates water bridge breakdown in disjoint nanochannels. This research sheds light on the intricate interplay between electric field direction and water transport dynamics in disjoint nanochannels, presenting valuable insights into various applications.

Using molecular dynamics simulations, we investigated the effect of external electric field on ice formation with the present of a substrate surface. It turns out that the electric field can affect the ice formation on substrate surface by altering the dipole orientation of interfacial water molecules (IWs): a crossover from inhibiting to promoting ice formation with the increase of electric field strength. According to the influence of the electric field on ice formation, the electric field strength range of 0.0 V nm−1–7.0 V nm−1 can be divided into three regions. In the region I and region III, there are both ice formation on the substrate surface. While, the behavior of IWs in the region I and region III are distinguished, including the arrangements of oxygen atoms and the dipole orientation distribution. In region II, ice formation does not occur in the system within 5 × 200 ns simulations. The IWs show a disorder structure, preventing the ice formation process on substrate. The interfacial water molecular orientation distribution and two-dimensional free energy landscape reveals that the electric field can alter the dipole orientation of the interfacial water and lead a free energy barrier, making the ice formation process harder. Our result demonstrates the external electric field can regulate the behavior of IWs, and further affect the ice formation process. The external electric field act as a crystallization switch of ice formation on substrate, shedding light into the studies on the control of ice crystallization.

Electrodialysis based direct air dehumidification: A molecular dynamics study on moisture diffusion and separation through graphene oxide membrane

Formation of a water bridge across the lipid bilayer is the first stage of pore formation in molecular dynamic (MD) simulations of electroporation, suggesting that the intrusion of individual water molecules into the membrane interior is the initiation event in a sequence that leads to the formation of a conductive membrane pore. To delineate more clearly the role of water in membrane permeabilization, we conducted extensive MD simulations of water bridge formation, stabilization, and collapse in palmitoyloleoylphosphatidylcholine bilayers and in water-vacuum-water systems, in which two groups of water molecules are separated by a 2.8 nm vacuum gap, a simple analog of a phospholipid bilayer. Certain features, such as the exponential decrease in water bridge initiation time with increased external electric field, are similar in both systems. Other features, such as the relationship between water bridge lifetime and the diameter of the water bridge, are quite different between the two systems. Data such as these contribute to a better and more quantitative understanding of the relative roles of water and lipid in membrane electropore creation and annihilation, facilitating a mechanism-driven development of electroporation protocols. These methods can be extended to more complex, heterogeneous systems that include membrane proteins and intracellular and extracellular membrane attachments, leading to more accurate models of living cells in electric fields.