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The electric field-induced shape transition of a nanodroplet confined in a silicon nanogroove with variable structure was investigated through molecular dynamics simulations. Our work provides insight into the relationship between the shape transition and the molecular interaction. We demonstrated that the geometric structure of the groove significantly influences the responsive behaviors of the droplet to the electric field. The simulations elucidate increasing the tilt angle of the groove reduces the critical field strength for the shape transition of the droplet under parallel electric field. Above the critical field, the droplet detaches from the groove and forms a liquid bridge across the groove, leading to weakened interfacial interactions. The detachment process of the droplet from the rectangular groove, which originates from the spreading of water molecules on perpendicular side walls, is different from the droplet initially confined in the groove with tilt angle higher than 90°. Under the perpendicular electric field, the droplet undergoes various shape transitions depending on the groove structure. In particular, the droplet in the groove with a moderate opening forms two asymmetrical liquid columns. The retraction behavior of the droplet in the groove with the small opening sheds light on the competition between interfacial interactions. The shape transition of droplets in the electric field due to the change of confinement nanostructures, such as liquid bridge, asymmetrical extension, branching and merging, is helpful to enrich the understanding of electrowetting and confinement dynamics of the droplets. [Display omitted] • A liquid bridge forms across the groove above critical parallel electric field. • The droplet in rectangular groove undergoes asymmetrical detachment process. • Geometric confinement influences orientational polarization of water molecules. • The droplet shows branching and merging shape under perpendicular electric field. [ABSTRACT FROM AUTHOR]

This study examines molecular interactions between two nanodroplets locating on the graphene monolayer and their morphological transition under the electric field parallel to the monolayer surface by using molecular dynamics simulations. The effects of the electric field and the salt concentration on the transmitted interactions, droplet shape and microscopic structure of interfacial water are elucidated in detail. It was found that the electric field can boost the electrostatic interaction between two pure water (PW) droplets, but weaken the short-range LJ interaction. The electric field with low strength mainly causes separation of two droplets rather than changes their shapes. There is a faster response of microscopic structures of water molecules to strong electric field than the morphological change of droplets, leading to a transition of the interaction energy from strong to weak. The salt concentration in the salty water (SW) droplets has negligible effect on the interaction energy in the absence of electric field. At strong electric field, the break-up behaviors of the SW droplets depend on the salt concentration. Simulation results reveal that the transmitted interactions between the PW and SW droplets as well as two SW droplets significantly affect the break-up behaviors and morphological transition of the droplets. • Electric field boosts electrostatic interaction between two pure water droplets. • Transmitted short-range interaction through the graphene monolayer is negligible in the presence of electric field. • Break-up behaviors of salty water droplets depend on salt concentration and transmitted interactions. [ABSTRACT FROM AUTHOR]

Coarse-grained molecular dynamics simulations are used to investigate physical deposition behavior of charged amphiphlic diblock copolymers. The effects of solvent selectivity, charge distribution in amphiphlic diblock copolymers, and electric field strength on deposition conformations are studied qualitatively. Flat amphiphilic bilayers, which consist of hydrophilic monolayer and hydrophobic brush, are formed by physical deposition of charged amphiphlic diblock copolymers in nonselective solvents. For physically deposited amphiphlic diblock copolymers in selective solvents, amphiphilic bilayers consist of disc-shaped hydrophilic monolayers and hydrophobic nanospheres are found. This study sheds light on the formation of various amphiphlic diblock copolymer deposition conformations in different solvents and interaction mechanism of different components. Furthermore, the evolution of physical deposition process of charged amphiphlic diblock copolymers layer offers new insight to the controlling of amphiphilic bilayer thickness, hydrophobic nanosphere size, and interface property of depositional amphiphlic diblock copolymers. [ABSTRACT FROM AUTHOR]

• Transmission of molecular interactions through graphene can drive migration of droplets. • Enhancing electric field accelerates migration velocity of two droplets towards each other. • Dipole–dipole interactions of interfacial water enhance deflection of water dipoles. • Critical electric field to induce large deformation becomes lower for cases of two droplets. Understanding the physical mechanism of wetting transparency of monolayer graphene, or how atoms or molecules located on opposite sides of the atomically thin carbon monolayer transmit molecular interactions (such as van der Waals and electrostatic forces) is of significant importance for many extraordinary applications from scientific and practical perspectives. In the current work, we carried out molecular dynamics simulations to explore the interactions between two droplets (TD) separated by the monolayer graphene under applied electric field. The results indicate that in the absence of the electric field the transmission of molecular interactions can drive migration of two droplets towards each other, implying partial wetting transparency of the graphene. Compared to the cases of single droplet (SD), the critical electric field to induce large deformation is lower for the TD cases. However, the droplet–droplet interactions under strong electric fields suppress the reduction of the number of water molecules in the graphene/droplet interface and the elongation of droplets. To understand the enhancement of interactions between two droplets owing to the electric field, we analyze microscopic structures of interfacial water (dipole orientation distribution and hydrogen bonding). The dipole–dipole interaction through the graphene mainly contributes to the droplet–droplet interactions. It was found that significant deflection of water dipoles for the TD cases occurs as the electric field enhances, but the deflection for the SD cases is smaller and even the droplet is detached from the graphene surface. Our findings provide fundamental insights on the influence of the electric field on molecular interactions between two droplets through the monolayer graphene and clues for developing potential graphene-based nanofluidic applications controlled by external electric field. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were used to investigate the electric field-induced migration of nanoparticles in a nanochannel grafted with a polymer brush. The distribution of nanoparticles, brush monomer density and migration velocity are addressed at different electric field strengths and grafting densities. The increase of the grafting density leads to a decrease of the effective radius of the nanochannel. At high grafting density, the distribution of nanoparticles is shifted significantly towards the center of the channel. Enhancing the electric field leads to a very slight change of brush monomer density. The migration velocity increases with the electric field rapidly at weak electric fields, while a slow increase is observed at strong electric fields. Additionally, it is found that the migration velocity of nanoparticles exhibits a nonlinear dependence on the grafting density. [ABSTRACT FROM PUBLISHER]

Molecular dynamics simulations were performed for electro-osmotic flow (EOF) confined in a polyelectrolyte-grafted nanochannel under variable grafting density and normal electric field. With decreasing the value of the normal electric field, the brush undergoes a collapse transition, and the ion distribution is changed significantly. The brush thickness increases on increasing the grafting density at positive and weak negative electric fields, whereas a reduced brush thickness is observed at strong negative electric field. Our results further reveal that the flow velocity is not only dependent on conformational transition of the brush but also related to the cation and anion distributions. At low grafting density, the EOF is almost completely quenched at high electric field strength due to strong surface friction between ions and walls. For the case of very dense grafting, the flow velocity is influenced weakly within the brush when varying the grafting density. Additionally, a bidirectional flow occurs at an intermediate electric field. The investigation on fluid flux indicates that the fluid flux is insensitive to the grafting density, when the normal electric field is removed. For nonzero normal electric fields, a significant change in the fluid flux is observed at low grafting densities. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

We report a molecular dynamics study on non-equilibrium dynamics of polyelectrolyte brushes under external electric fields. In this work, the effects of chain stiffness and salt concentration on static and dynamic responses of the brushes are addressed in detail. Our simulations indicate that varying these parameters induce rich electro-responsive behavior of the brushes. The increase of salt concentration results in the enhancement of an opposite electric field formed by non-equilibrium distribution of cations and anions, which resists stretching or shrinkage of grafted chains. At strong positive electric fields, the flexible brushes are more sensitive to the change of salt concentration. When reversing the electric field, the stiff brushes undergo a conformational transition from collapse to complete stretching. At high salt concentrations, dynamic responsive magnitude of the brush thickness to added electric field is strongly reduced. It was found that the fall time for the stiff brush becomes much shorter than that for the flexible brush. Additionally, increasing ion concentration leads to an excess extension or shrinkage of flexible brushes. For strongly stiff brushes, such phenomenon occurs in the presence or absence of salt. [ABSTRACT FROM AUTHOR]

The mechanisms governing the electrocoalescence between droplets are intricate owing to the presence of many influential parameters. In this work, we conduct molecular dynamics simulations to explore the effects of electric field strength and ionic concentration on the electrocoalescence behaviors of one pure water droplet and one salty droplet. The dynamic evolution is difierent from that for two identical droplets. The electric field strengths are chosen near the critical value. When the electric field strength is less than the critical value, the coalescence process is similar regardless of the ionic concentration of the salty droplet. The analysis on the mobility of ions demonstrates that the diffusion time of ions is much longer compared to the characteristic time of deformation of the coalesced droplet. At a stronger electric field than the critical field, the coalescence behavior exhibits dependence on the ionic concentration. For cases of high ionic concentrations, a rebounding behavior between two droplets is identified due to aggregation of ions close to the contact region of the droplets. Further, we also investigate molecular interaction energy, hydrogen bonding and ejection of daughter droplets during the electrocoalescence. Our study reveals that the coupling effect between the electric field and the ionic concentration plays an important role in controlling the electrocoalescence behavior of droplets. • Mechanisms of electrocoalescence between pure water and salty droplets are explored. • Coupling effect between electric field and ionic concentration plays a critical role. • Remarkable inconsistency between ion diffusion and droplet deformation is identified. [ABSTRACT FROM AUTHOR]