Deformation and breakup mechanism of water droplet in acidic crude oil emulsion under uniform electric field: A molecular dynamics study

Molecular insights into the adsorption and penetration of oil droplets on hydrophobic membrane in membrane distillation

Membrane fouling induced by oily substances significantly constrains membrane distillation performance in treating hypersaline oily wastewater. Overcoming this challenge necessitates a heightened fundamental understanding of the oil fouling phenomenon. Herein, the adsorption and penetration mechanism of oil droplets on hydrophobic membranes in membrane distillation process was investigated at the molecular level. Our results demonstrated that the adsorption and penetration of oil droplets were divided into four stages, including the free stage, contact stage, spreading stage, and equilibrium stage. Due to the extensive non-polar surface distribution of the polytetrafluoroethylene (PTFE) membrane (comprising 95.41 %), the interaction between oil molecules and PTFE was primarily governed by van der Waals interaction. Continuous oil droplet membrane fouling model revealed that the new oil droplet molecules preferred to penetrate into membrane pores where oil droplets already existed. The penetration of resin (a component of medium-quality oil droplets) onto PTFE membrane pores required the "pre-paving" of light crude oil. Finally, the ΔE quantitative structure-activity relationships (QSAR) models were developed to evaluate the penetration mechanism of pollutant molecules on the PTFE membrane. This research provides new insights for improving sustainable membrane distillation technologies in treating saline oily wastewater.

Unraveling Partial Coalescence Between Droplet and Oil-Water Interface in Water-in-Oil Emulsions under a Direct-Current Electric Field via Molecular Dynamics Simulation

When the electric field strength (E) surpasses a certain threshold, secondary droplets are generated during the coalescence between water droplets in oil and the oil-water interface (so-called the droplet-interface partial coalescence phenomenon), resulting in a lower efficiency of droplet electrocoalescence. This study employs molecular dynamics (MD) simulations to investigate the droplet-interface partial coalescence phenomenon under direct current (DC) electric fields. The results demonstrate that intermolecular interactions, particularly the formation of hydrogen bonds, play a crucial role in dipole-dipole coalescence. Droplet-interface partial coalescence is categorized into five regimes based on droplet morphology. During the contact and fusion of the droplet with the water layer, the dipole moment of the droplet exhibits alternating increases and decreases along the electric field direction. Electric field forces acting on sodium ions and the internal interactions within droplets promote the process of droplet-interface partial coalescence. High field strengths cause significant elongation of the droplet, leading to its fragmentation into multiple segments. The migration of hydrated ions has a dual impact on the droplet-interface partial coalescence, with both facilitative and suppressive effects. The time required for droplet-interface partial coalescence initially decreases and subsequently increases as the field strength increases, depending on the competitive relationship between the extent of droplet stretching and the electric field force. This work provides molecular insights into the droplet-interface coalescence mechanisms in water-in-oil emulsions under DC electric fields.

Taking a look accurately at the alteration of interfacial asphaltene film exposed to the ionic surfactants as demulsifiers

The water droplets surrounded by a rigid interfacial asphaltene (ASP) film is one of the major setbacks in the petroleum industry. In this study, the properties of the interfacial ASP films around water droplets exposed to ionic surfactants as demulsifier were investigated. According to molecular dynamics (MD) simulation, the anionic surfactants are more effective than the cationic surfactant in the demulsification process since the anionic surfactants have the exact desire to localize not only near the ASP molecules but also near the water molecules. It has been found that it is likely to cause film changes and ruptures. Also, the MD simulation results for the desired surfactant, anionic surfactant, demonstrated that an increase in the surfactant concentration had an adverse effect on the system by hindering the change in the interfacial film. The increase in the temperature along with the enhancement in the adsorption rate of the surfactant results in the better performance of the demulsifier. Taking the MD and quantum results into account, the film deformation is a decisive factor in demulsification. The quantum computation has indicated that the electrostatic interactions play a significant role in selecting the attraction position and adsorption energy of the surfactant molecules.

Molecular Dynamics Study on the Demulsification Mechanism of Water-In-Oil Emulsion with SDS Surfactant under a DC Electric Field

Application of an electric field is an effective demulsification method for water-in-oil (W/O) emulsions. For the W/O emulsions stabilized by anionic surfactants, the microscopic demulsification mechanism is still not very clear. In this work, the coalescence behavior of two droplets stabilized by the anionic surfactant sodium dodecyl sulfate (SDS) in the oil phase under a DC electric field is investigated by molecular dynamics simulation. The effects of electric field strength and oil type on the electrocoalescence of two water droplets are mainly considered. The trajectory snapshots and center of mass of the two water droplets suggest that there is almost no migratory coalescence. The movement of sodium ions and SDS, which is a combined effect of the electric field force and the resistance from the oil phase, is crucial for the deformation and connection of two water droplets. The results of mean square displacement, radial distribution function, hydration number, and interaction energies of Na⁺-H₂O and SDS-H₂O indicate that the sodium ion has a stronger ability to carry water molecules for movement than SDS. The stronger electric field strength will result in more severe deformation and shorter coalescence time. Under the higher electric field strength, the two droplets will be elongated into a slender water ribbon. By applying a pulsed DC electric field with suitable amplitude, frequency, and duty ratio, it is possible to achieve full coalescence for the ionic surfactant-stabilized W/O emulsions. The oil phase also plays an important role for the deformation of droplets and the migration of emulsion components. For the different oil phases, a longer time or stronger electric field strength would be needed for the electrocoalescence of droplets in the oil phase with higher density and viscosity. Our results are expected to be helpful for practical application in the petroleum industry and chemical engineering.

An Optimal Subspace Deconvolution Algorithm for Robust and High-Resolution Beamforming

Utilizing the difference in phase and power spectrum between signals and noise, the estimation of direction of arrival (DOA) can be transferred to a spatial sample classification problem. The power ratio, namely signal-to-noise ratio (SNR), is highly required in most high-resolution beamforming methods so that high resolution and robustness are incompatible in a noisy background. Therefore, this paper proposes a Subspaces Deconvolution Vector (SDV) beamforming method to improve the robustness of a high-resolution DOA estimation. In a noisy environment, to handle the difficulty in separating signals from noise, we intend to initial beamforming value presets by incoherent eigenvalue in the frequency domain. The high resolution in the frequency domain guarantees the stability of the beamforming. By combining the robustness of conventional beamforming, the proposed method makes use of the subspace deconvolution vector to build a high-resolution beamforming process. The SDV method is aimed to obtain unitary frequency matrixes more stably and improve the accuracy of signal subspaces. The results of simulations and experiments show that when the input SNR is less than -27 dB, signals of decomposition differ unremarkably in the subspace while the SDV method can still obtain clear angles. In a marine background, this method works well in separating the noise and recruiting the characteristics of the signal into the DOA for subsequent processing.

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

The microscopic mechanisms of electrocoalescence of O/W emulsions stabilized by surfactant were analyzed from the electric dipole moment of the surfactant, the interaction between surfactant and oil molecules and the deformation of the surfactant.