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Abstract The wetting effect and related applications of nanodroplets have attracted extensive research interest. In this work, we report molecular dynamics simulations of the confinement dynamics of nanodroplets at different droplet-surface interactions, surface separations, and ion concentrations under the electric field. The results show that in the case of a single droplet, there is a critical value of surface wettability for the shape transition of the confined droplet. Below this value, the droplet shrinks into a spherical shape and attaches to the surface. Above this value, the droplet splits into two smaller droplets which locate at the bottom and top surfaces, respectively. For the two-droplet cases, a liquid bridge forms and some ions migrate into the pure water droplet under the electric field. The results reveal that the ion concentration has a significant effect on the shape evolution of droplets. The stable morphology of droplets under the electric field depends on the droplet-surface interactions as well as the ion concentration. We explore the underlying mechanisms causing the morphological transition by analyzing the intermolecular interactions and interfacial characteristics. [ABSTRACT FROM AUTHOR]

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]

Small organic multiamine and multihydroxyl molecules have great potential for enhancing overall properties of poly(vinyl alcohol) (PVA) through the cross-linking effect of hydrogen bonds. However, experimentally there remains a remarkable lack of insightful understanding of the cross-linking effect on a molecular level. In the work, we report molecular dynamics simulations to reveal the cross-linking effect of hydrogen bonds of tetraaminopyrimidine (4N-2456) molecules on the structure, chain dynamics and mechanical properties of the PVA matrix. It was found that the addition of 4N-2456 leads to a nonlinear decrease of the free volume of PVA. A critical concentration of 4N-2456, about 5 wt%, was identified, resulting in the formation of 4N-2456 clusters. At this concentration, the PVA chains show the relatively slow mobility, the higher glass transition temperature and elastic modulus. Further increasing the 4N-2456 concentration enhances aggregation, and conversely weakens the interactions of hydrogen bonds between the PVA chains. Our work offers an understanding of how the 4N-2456 molecules influence the PVA chain dynamics and mechanical properties of the PVA matrix on molecular level. [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]

ABSTRACT We perform multiscale simulations based on the coupling of molecular dynamics and lattice-Boltzmann (LB) method to study the electrohydrodynamics of a polyampholyte-grafted spherical nanoparticle. The long-range hydrodynamic interactions are modeled by coupling the movement of particles to a LB fluid. Our results indicate that the net-neutral soft particle moves with a nonzero mobility under applied electric fields. We systematically explore the effects of different parameters, including the chain length, grafting density, electric field, and charge sequence, on the structures of the polymer layer and the electrophoretic mobility of the soft particle. It shows that the mobility of nanoparticles has remarkable dependence on these parameters. We find that the deformation of the polyampholyte chains and the ion distribution play dominant roles in modulating the electrokinetic behavior of the polyampholyte-grafted particle. The enhancement or reduction in the accumulation of counterions around monomers can be attributed to the polymer layer structure and the conformational transition of the chains in the electric field. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1435-1447 [ABSTRACT FROM AUTHOR]

[Display omitted] • Platelet margination and adhesion are two critical events during thrombus formation. • Margination and adhesion dynamics of platelets in microvascular blood flow are investigated. • Platelet margination and adhesion occur in shear-dependent manners. • Adherent platelets have negative impacts on margination dynamics of circulating ones. Platelet margination and adhesion are two critical and closely related steps in thrombus formation. Using dissipative particle dynamics (DPD) method that seamlessly models blood cells, blood plasma, and vessel walls with functionalized surfaces, we quantify the shear-induced margination and adhesion of platelets in microvascular blood flow. The results show that the occurrence of shear-induced RBC-platelet collisions has a remarkable influence on the degree of platelet margination. We characterize the lateral motion of individual platelets by a mean square displacement analysis of platelet trajectories, and find that the wall-induced lift force and the shear-induced displacement in wall-bounded flow cause the variation in near-wall platelet distribution. We then investigate the platelet adhesive dynamics under different flow conditions, by conducting DPD simulations of blood flow in a microtube with fibrinogen-coated wall surfaces. We find that the platelet adhesion is enhanced with the increase of fibrinogen concentration level but decreased with the increase of shear rate. These results are consistent with available experimental results. In addition, we demonstrate that the adherent platelets have a negative impact on the margination dynamics of the circulating platelets, which is mainly due to the climbing effect induced by the adherent ones. Taken together, these findings provide useful insights into the platelet margination and adhesion dynamics, which may facilitate the understanding of the predominant processes governing the initial stage of thrombus formation. [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]

Abstract: The B27K-DTrI insulin (human insulin with B28–30 removed and B27 Thr replaced by Lys) was reported to have superior monomeric property with 80% insulin activity in vivo. It has potential use as a new fast-acting analog of insulin. We cloned the monomeric insulin B27 DTrI precursor (MIP) into the pTWIN1 vector, and prepared by intein mediated expression in Escherichia coli. After tryptic digestion, the MIP was converted to B27K-DTrI insulin. The product was purified by HPLC. The mass spectrometry showed that the molecular mass of purified B27K-DTrI was consistent with the theoretical value. [Copyright &y& Elsevier]

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]

The presence of water molecules can significantly determine the macroscopic mechanical property and microscopic chain dynamics of poly(vinyl alcohol) (PVA). However, to date it has remained fully understood the molecular mechanism behind experimentally and theoretically. Herein, we systematically examine the effect of water contents on the mechanical property, glass transition, free volume, and intermolecular interactions by combining experimental and MD simulation. Our results show that the presence of water significantly reduces the mechanical strength of PVA, and only 1.8 wt% of water reduces the tensile strength by ~32% but notably increases the plasticity of PVA by ~2.5 times. Meanwhile, the inclusion of water remarkably lowers the glass transition temperature, increases free volume, and promotes the relaxation and mobility of PVA chains. This is mainly because the presence of water gives rise to both the plasticization and even lubricating effect on the PVA chains. This work unravels how water governs the mechanical performances of the PVA. Image 1 • The presence of water significantly reduces the mechanical strength of PVA. • 1.8 wt% of water reduces tensile strength by ~32% but increases the ductility of PVA by ~2.5 times. • The inclusion of water remarkably lowers the glass transition temperature of PVA. • The addition of water increases the free volume and promotes the relaxation of PVA chains. • The presence of water giving rise to the plasticization and lubricating effect on the PVA chains. [ABSTRACT FROM AUTHOR]

We study the conformational behavior of spherical polyelectrolyte brushes in the presence of monovalent and trivalent counterions in a confined environment. The confinement is exerted by two parallel walls on the brushes. The enhancement of the confinement induces the extension of grafted chains. For the monovalent case, the increase of the charge fraction leads to extended brush conformation for different slit width (distance between two walls) but collapsed brush in the presence of trivalent counterions is observed. The confinement does not affect electrostatic correlation between trivalent counterions and charged monomers. However, it was found that narrow slit width contributes to stronger electrostatic correlation for the monovalent case. This is because more monovalent counterions are inside the brush at strong confinement, but almost all trivalent counterions are trapped into the brush independently of the slit width. The diffusion of counterions under the confinement is related to the electrostatic correlation. Our simulations also reveal that the brush thickness depends on the slit width nonlinearly. [ABSTRACT FROM AUTHOR]

A molecular dynamics study of the electrophoresis of bottle-brush polyelectrolytes (BPEs) through nanochannels is reported. The BPE molecules consist of a neutral backbone with charged side chains. For strong attractive interactions between the BPE and the wall, the BPE is being trapped on the channel surface. A stretching-shrinking migration of the BPE in a channel of radius 6 σ is observed at relatively strong electric fields. The stretching-shrinking transition is periodic for intermediate electric fields but not for stronger electric fields. The BPE also shows a transverse migration toward the wall at weak electric fields, while toward the center with further enhancing the electric field. For a channel with larger radius 12 σ, the BPE does not migrate in the stretching-shrinking manner. [ABSTRACT FROM AUTHOR]

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]

A hybrid particle–continuum method is used to study the shear flow confined between two opposing walls, one of which is coated with polymer chains. Molecular dynamics (MD) is used in the particle region near the brush and Navier–Stokes (NS) equations are applied in the remaining region where the continuum assumption holds. The information exchange from the continuum region to the particle region is implemented using the constrained particle dynamics. Both Couette shear flow and oscillatory flow are considered in the present work. The effect of the shear flow on the conformational characteristics of polymer brushes is analyzed. In the overlap region, the velocities obtained from MD simulations are smoothly connected with those from NS equations. Our investigations demonstrate that the hybrid particle–continuum model is valid in exploring the shear behavior of polymer brushes. [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]

A basic understanding of the transport mechanisms of nanostructures in a polymer brush-modified nanochannel as well as the brush-nanostructure interactions at molecular level is important to design and fabricate emerging smart nano/microfluidic channels. In this work, we report coarse-grained molecular dynamics simulations of the translocation of nanoparticles through a cylindrical nanochannel coated with the polymer brush. The effects of the interparticle interaction and grafting density on the distribution and electrokinetic transport of nanoparticles are addressed in detail. Analysis of the distribution and velocity profiles of nanoparticles from the simulations indicate that the location of nanoparticles along the radial direction and their migration velocity are very sensitive to the change of interparticle interaction. We find complicated transport dynamics of nanoparticles under the influence of various grafting densities. The nanoparticles show markedly different translocation behavior upon increasing the grafting density, which depends on the counterion distribution, free room within the brush, nanoparticle-polymer friction, and brush configuration. Our results may serve as a useful starting point for the transport of nanostructures in polymer-modified channels and help to guide the design of novel smart nanofluidic channels for controlling the migration behavior of nanostructures. [ABSTRACT FROM AUTHOR]

We report molecular dynamics simulations on bottle-brush polyelectrolytes end-grafted to a planar surface. For each bottle-brush polyelectrolyte, flexible charged side chains are anchored to one neutral main chain. The effects of the counterion valence and the grafting density on the density profiles and the structural characteristics of the brush were studied in this work. It is found that the electrostatic repulsion between charged monomers in the side chains leads an extended conformation of the brush in a solution containing monovalent counterions, while strong electrostatic binding of multivalent counterions to the side chains has a significant contribution to the collapse of the brush. For the trivalent case, the distribution of end monomers in the main chains becomes broader upon decreasing the grafting density, as compared with the monovalent case. However, the position of the distribution for the monovalent case is relatively insensitive to the change of the grafting density. Additionally, with increased counterion valence, enhanced electrostatic correlation between counterions and charged side chains also weakens the diffusive ability of counterions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 [ABSTRACT FROM AUTHOR]