Ion-Specific Effects on the Elongation Dynamics of a Nanosized Water Droplet in Applied Electric Fields

Electrocoalescence Behavior of Droplets Dispersed with Na2CO3 in Oil under the Electromagnetic Synergy Field

Electromagnetic synergy is a more effective physical method than a single AC electric field (ACEF) to enhance oil-water separation. However, the electrocoalescence behavior of droplets dispersed with salt ions in oil under the synergistic electromagnetic field (EMSF) still lacks research. Herein, the evolution coefficient of liquid bridge diameter (C₁) characterizes the growth rate of the liquid bridge diameter, a series of Na₂CO₃-dispersed droplets with different ionic strengths were prepared, and C₁ values of droplets under ACEF and EMSF were compared. Micro high-speed experiments revealed that C₁ under ACEF is larger than C₁ under EMSF. In particular, when σ = 100 μS·cm^-1and E = 629.73 kV·m^-1, C₁ under the ACEF is 15% larger than C₁ under EMSF. Additionally, the theory of ion enrichment is put forward, which explains the influence of salt ions on ζ potential and total surface potential in EMSF. This study provides guidance for designing high-performance devices by introducing electromagnetic synergy in water-in-oil emulsion treatment.

Confinement Dynamics of Nanodroplets between Two Surfaces: Effects of Wettability and Electric Field

The electrowetting effect and related applications of tiny droplets have aroused widespread research interest. In this work, we report molecular dynamics simulations of confinement dynamics of nanodroplets under different droplet-surface interactions and surface distances under an external electric field. So far, the effect of the surface-droplet interactions on electric field-induced dynamics behaviors of droplets in confined spaces has not been extensively studied. Our results show that in the absence of electric field there is a critical value of surface wettability for the shape transition of droplets. Above this value, the droplet is divided into small droplets adhered on the bottom and top surfaces; below this value, the droplets are detached from the surfaces. When an external electric field is applied parallel to the surfaces, the droplet spreads on the surface along the direction of the electric field. It was found that the surface separation significantly influences the transition of the droplet shape. The steady morphology of the droplets under the electric field depends on the surface-droplet interaction and surface separation. We explore the underlying mechanism causing the morphological transition through analyzing the molecular interactions, the number of interracial molecules and the interaction force between the droplets and surfaces. These results provide basic insights into the molecular interactions of nanodroplets under different confined environments, and clues for applications of confined nanodroplets under the control of electric field.

Effect of electric field on coalescence of an oil-in-water emulsion stabilized by surfactant: a molecular dynamics study

The microscopic understanding of electrocoalescence of oil-in-water (O/W) emulsions stabilized by surfactant is very important to improve the efficiency of electrical demulsification. The behaviors of the coalescence of O/W emulsion stabilized by surfactant in the presence of a direct electric field and a pulsed electric field were explored by nonequilibrium molecular dynamics simulations. According to the simulated results, an electrical method is feasible to demulsify an O/W emulsion stabilized by a surfactant. The configuration and movement of the sodium dodecyl sulfate (SDS) were determined by interactions between SDS molecules themselves and between SDS and oil/water molecules along with the force exerted by the applied electrical field. Two droplets will coalesce into one when the strength of the electric field exceeds 0.4 V nm-1. The SDS group can be broken up by an electric field larger than 0.6 V nm-1. The point when interaction energy between the hexadecane molecules of the two droplets begins to decrease from zero is consistent with the time when the two oil droplets came in contact. The coalescence process can be completed if the two droplets have begun to coalesce, even after the electric field was removed. Otherwise, the coalescence process cannot be completed. To enhance the efficiency of the electrocoalescence of O/W emulsions, strength, frequency and duty ratio of the electric field have to be optimized according to the properties of the emulsion. This research will help us to figure out how electric fields promote the efficiency of electrocoalescence of O/W emulsions with surfactant.

Molecular Investigation of Contact Line Movement in Electrowetted Nanodroplets

Molecular dynamics (MD) simulation of an electrowetted nanodroplet is performed to understand the fundamental origin of the involved parameters resulted from the molecular movement in the vicinity of the three-phase contact line (TPCL). During the spreading of the droplet, contact line friction (CLF) force is found to be the controlling one among all other resistive forces. Being molecular in nature, MD study is required to unveil the CLF, which is manifested by the TPCL friction coefficient ζ. The combined effect of temperature, electric field, and surface wettability, manifested by the solid-liquid Lennard-Jones interaction parameter, is studied to explore the droplet spreading. The entire droplet wetting dynamics is divided into two different regimes, namely, spreading regime and equilibrium regime. The molecular frequency during the TPCL movement in the equilibrium regime is affected by the presence of any external perturbation and results in an alteration of ζ. The predetermined knowledge of the alteration of CLF due to the coupling effect of electric field and temperature will have a potential application towards designing electric field-inspired droplet movement devices.

Orientational and Folding Thermodynamics via Electric Dipole Moment Restraining

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

Effects of Temperature and Ionic Concentration on Nanodroplet Electrocoalescence

Droplet electrocoalescence is of interest for various applications such as petroleum dehydration, electrospray ionization, and surface self-cleaning. Here, the effects of temperature and ionic concentration on nanodroplet electrocoalescence are investigated by molecular dynamics simulation. The results show that low ionic concentration rapidly drives ions towards water clusters and leads to dipole polarization of droplets. With an increase of ionic concentration, the particle-particle interaction is enhanced, but the mobility of free water molecules and salt ions is curbed by hydration and ion pairs, which then slows the electrocoalescence. Low temperature accelerates the rotation of water molecules but does not enhance the mobility of ions. Alternatively, high temperature not only breaks the self-assembly of water molecules along the electric field direction but also helps ions to overcome the electrostatic barrier between particles. The latter effect promotes dipole polarization to compensate for the shortcoming of less orientation polarization. The combined effects of ion concentration and temperature are investigated and unified by droplet conductivity from the microscopic point of view. The conductivity increases with the increase in temperatures and ionic concentrations. We confirm that the accurate control of droplet electrocoalescence can be achieved by a suitable combination of temperature and ionic concentration.

Electrowetting on 2D dielectrics: a quantum molecular dynamics investigation

Electrowetting on dielectrics (EWOD) is widely used to manipulate the spreading of a conductive liquid on a dielectric surface by applying an electric field. 2D hydrophobic dielectrics are promising candidates for EWOD applications. In this study, extensive quantum molecular dynamics (MD) simulations are performed to investigate the electrowetting behavior of salty water on hexagonal boron nitride (h-BN) monolayer. The proximal adsorption of salt ions and the associated realignment of the dipole moments of interfacial water with the applied electric field are found to be the physical origin of the electrowetting behavior. At low salt concentration and low electric fields, the proximal adsorption and the realignment follow the applied electric field, and the cosine of the water contact angle (WCA) follows a quadratic dependence on the applied electric field. At high salt concentration and high electric fields, the proximal adsorption saturates, which restricts further realignment and causes a saturation of the WCA. This case study provides physical insights into the much debated mechanism that underlies the contact angle saturation (CAS) found in macroscopic electrowetting phenomena and also provides an avenue for further studies of electrowetting at the atomic scale.