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Deformation and coalescence behaviors of water droplets in viscous fluid under a direct current electric field are investigated with experiments and numerical simulations. An electro-hydrodynamic model is developed based on fluid dynamics equations and electrostatic equations. A coupled level set and volume of fluid method is proposed to capture the dynamic interface. The numerical results are validated to be in good accordance with classic results in the literature. The effects of electric capillary number Ca and viscosity ratio λ on droplet deformation and coalescence are systematically analyzed. Two different droplet response modes are obtained, namely overdamping response and underdamping response. The conversion of response modes is discussed with the (Ca, λ) phase diagram. Moreover, the distributions of electric field and flow field before and after the droplets contact are compared and analyzed. Numerical results show the time evolution of liquid bridge is dominated by the interfacial tension and viscosity of continuous phase. The electric stress is extremely weak at the liquid bridge and has a slight influence on evolution of liquid bridge. The correlation formula between dimensionless liquid bridge radius and conical angle during the growth of liquid bridge is regressed. These results are helpful for the optimization of electro-coalescence process.

This paper focuses on the application range and optimal parameters for separating water-in-oil (W/O) emulsions using ultrasonic standing waves (USWs). Firstly, the effect of droplet bandings on the demulsification process is theoretically analyzed. The formation of droplet bandings coincides with droplet sedimentation when horizontal irradiation is applied, which improves the separation efficiency. On this basis, the effects of acoustic parameters and oil-water properties on demulsification are analyzed in detail. It is shown that ultrasonic treatment is an efficient demulsification method, and that maximum demulsification efficiency can be achieved by optimizing the acoustic intensity and treatment time. Excessive acoustic intensity causes acoustic cavitation in low-frequency USWs and acoustic streaming in high-frequency USWs, resulting in a low separation efficiency. Acoustic cavitation also causes secondary emulsification, dispersing tiny oil droplets into separated water. In addition, ultrasonic separation is significantly affected by oil-water properties. The demulsification efficiency of W/O emulsions decreases with increasing oil viscosity and decreasing oil-water interfacial tension. Ultrasonic treatment has significant advantages for separation of emulsions with low water content and interfacial tension. This study presents an in-depth analysis of ultrasonic separation.

Coalescence of conducting droplets dispersed in an immiscible medium can be facilitated by an electric field. However, droplets recoil promptly after contact in sufficiently high electric fields if the cone angle between droplets exceeds a critical value. To elucidate the critical condition for droplet coalescence, the behavior of two suspended droplets after contact with a direct current electric field is studied. It is shown that the critical angle is determined not only by the droplet geometry but also conductivity, surfactant concentration, and size. As the droplet conductivity increases, more identical ions accumulate on the adjacent interfaces of two droplets due to the faster ionic migration, which results in Coulombic repulsion between droplets and a reduced critical angle. For surfactant-laden droplets, film drainage induces a surfactant concentration gradient on the leading edges of droplets, and then Marangoni stress is formed to reduce the critical angle. In the case of large droplets, the bri...

To enhance the detection precision of samples with scattering characteristics by near infrared spectroscopy (NIRS), this study developed a heteromorphic sample pool and established the related 2D light intensity acquisition system, which can simultaneously acquire multi-path exit light adsorption and scattering information of the samples under test. The Intralipid-20% phantom solutions in 34 samples with different concentrations were detected, while one-dimensional (1D) exit light intensity distributions and two-dimensional (2D) exit light intensity distributions on the surface of the samples were analyzed and modeled using partial least squares. In contrast with the prediction results based on the modeling method of 1D exit light intensity distribution, the modeling method of 2D exit light intensity distribution exhibits more favorable results; specifically, correlation coefficient enhanced by 2.48%, while root mean square error reduced by 6.89%. The experimental results demonstrate that using heteromorphic sample pool can effectively achieve NIRS-based detection precision and speed of chemical components in the solutions with scattering characteristics, which can provide important references for high-throughput and high-precision detection of turbid media in analytical chemistry.

Electric dehydration method is widely used in oil industry to separate water from oil. Droplets resting on the oil–water interface discharge their liquids into the bulk phase, and the process can be altered by the change of electric field strength (E) or properties of water. Phenomenon of silica (SiO2) nanoparticle-laden droplets coalescing with a planar interface under direct current electric field was observed using a high-speed digital video camera. The effects of initial droplet diameter (D), E, and weight percent of nanoparticles in deionized water (C) were studied experimentally. The results showed that increasing D or E contributed to the formation of secondary droplets. Addition of SiO2 nanoparticles to water produced two competing effects: reduction in oil–water interfacial tension and increase in water conductivity, and a shift in the dominant effect occurred with the increase of E. Furthermore, formation of a ring by the separation of vortices was visualized by movement of nanoparticles in experiments, and its downward moving velocity decreased with time, which is small compared to that reported in literature. Finally, it was confirmed that the dimensionless WO number is not suitable for describing the volume fraction of secondary droplets for droplet–interface coalescence containing nanoparticles.

The electrocoalescence behavior of surfactant-laden droplet pairs in oil under a DC electric field was investigated in this paper. The characteristic parameters such as coalescence time, liquid bridge diameter and contact angle were analyzed for varying surfactant concentrations and electric capillary numbers. The results show that the horizontal electrocoalescence of two identical droplets with the same physical properties is symmetrical. Moreover, the surfactant has a significant influence on the pressure difference at the liquid bridge, which results in a variation of the liquid bridge growth and coalescence time. The pressure difference between the droplet and the liquid bridge is significantly dependent on the initial contact angle, which is determined by the electric capillary number Ca and surfactant concentration. The theoretical analysis and experimental results show that the large initial contact angle restrains the growth of the liquid bridge in the initial coalescence stage. Considering the effect of the initial contact angle, a characteristic time is proposed to describe the evolution of the liquid bridge.

In this study, the electrocoalescence characteristic of two drops with different surfactant concentrations suspended in oil is revealed. Different from the symmetrical coalescence of identical drops, the asymmetrical liquid bridge is induced by the capillary force, which involves the interfacial tension gradient between the drops. In addition, a mushroom-like jet is formed during asymmetrical coalescence, causing mixing of drops. A characteristic time considering the interfacial tension gradient is proposed to describe the asymmetrical liquid bridge evolution. The mushroom-like jet is found to be driven by the pressure difference between drops, which has a positive correlation with the interfacial tension gradient. Moreover, it is found that the electrocoalescence time is shortened as the interfacial tension increases. This study indicates that increasing the interfacial tension gradient will enhance the jet flow rate and reduce the coalescence time, which is of significance to improve the mixing efficien...

The separation characteristics of W/O emulsion were investigated under ultrasonic standing wave field in this paper. The effects of the irradiation time, acoustic intensity, frequency, oil viscosity, and oil-water interfacial tension on the separation characteristics of emulsion were studied in details. It was proved that there existed an optimal irradiation time, acoustic intensity and oil-water interfacial tension to achieve the maximum separation efficiency in ultrasonic treatment process. Excessive long irradiation time had little effect on improving the dehydration rate of emulsion but rather increased unnecessary processing time. Excessive high acoustic intensity caused the occurrence of acoustic cavitation phenomenon. The acoustic cavitation would cause the aggregated droplets to be dispersed by oscillating bubbles or even emulsified again by high speed micro jets. The acoustic cavitation threshold is effectively and accurately obtained by a spectral analysis method. The micro acoustic streaming which resulted from the attenuation of acoustic energy occurred with the increase of frequency. It sheared the aggregated droplets, destroyed the stable droplets banding and reduced the separation efficiency of emulsion. The excess surfactant adsorbed on the oil-water interface resulted in the increase of the interfacial film strength, which hindered the coalescence process of droplets and reduced the separation efficiency.

Ultrasound is an emerging and promising method for demulsification, which is highly affected by acoustic parameters and emulsion properties. Herein, a series of microscopic and dehydration experiments are carried out to investigate the parameter optimization of ultrasonic separation. The results show that the optimal acoustic parameters highly depend on the emulsion properties. For low frequency ultrasonic standing waves (USWs), mechanical vibrations not only facilitate droplet collision and coalescence, but also disperse the surfactant absorbed on the interface to decrease the interfacial strength. Therefore, low frequency ultrasound is suitable for separating emulsions with high viscosity and high interfacial strength. Increasing the energy density to produce moderate cavitation can increase demulsification efficiency. However, excessive cavitation results in secondary emulsification. In high frequency USWs, the droplets migrate directionally and form bandings, thereby promoting droplet coalescence. Therefore, high frequency ultrasound is favorable for separating emulsions with low dispersed phase content and small droplet size. Increasing the energy density can accelerate the aggregation of droplets, however, excessive energy density causes acoustic streaming that disturbs the aggregated droplets, resulting in reduced demulsification efficiency. This work presents rules for acoustic parameter optimization, further advancing industrial applications of ultrasonic separation.

The electrocoalescence of droplets in a viscous fluid is resisted by viscous and inertial forces. In this paper, the effects of these forces on the droplet approaching and liquid bridge expansion are investigated. The results show that the approaching of droplets can be described by the equation Re = 60Ca/Oh2. In addition, we propose a modified equation for the approaching of droplets, which is in better agreement with experimental data. In the initial stage of liquid bridge expansion, the dimensionless number Oh is used to identify different regimes, which is collapsed into a unified expression by using a characteristic time. In this way, evolution of the liquid bridge in different regimes is described. Moreover, it is found that larger liquid bridge width in the viscous regime can result in a slower expansion in the later stage. This study deepens the understanding of the kinetics of droplet electrocoalescence.

The mixing of binary droplets is of paramount importance in microfluidic systems. In order to reveal the mixing mechanism of two free droplets suspended in the immiscible phase, we have developed a novel experimental setup to study the internal mixing in coalescing droplets with varying interfacial tension differences and droplet sizes. It is confirmed that the interfacial energy of droplets supports the jet flow and liquid bridge expansion during the coalescence of droplets. The increase of interfacial tension difference can increase the intensity of jet flow accompanied with slower liquid bridge expansion, which enhances the mixing of droplets. The decrease of droplet size can increase the initial velocity of jet flow. However, the intensity of jet flow decreases due to the rapid expansion of the liquid bridge, which results in weaker internal mixing. On this basis, a Reynolds number incorporating the jet velocity and droplet size is proposed to characterize the vortex size, which represents the degree of droplet mixing. This study presents an effective approach for enhancing the mixing of droplets.

Droplets banding is critical to emulsion separation under ultrasonic irradiation as it can greatly improve the separation efficiency. In this paper, the formation process of droplets banding under ultrasonic standing waves was precisely captured by high-speed microscopic photography; by processing the images, the droplets banding characteristics, including the banding formation time and banding interval, were extracted. Then the effects of acoustic intensity, frequency, droplet size, and physical properties of oil and water on the droplets banding characteristics were discussed in details. The results show that the range of acoustic intensities, within which the droplets banding can form, increases with the increase of the frequency; a maximum allowable acoustic intensity exists for banding formation, which also increases with the frequency. The banding formation time, which increases with increasing oil viscosity but decreases with droplet size, is found to be hardly affected by the oil-water interfacial tension. In addition, the banding interval is only related to the frequency, which closely corresponds to the half wavelength.

In order to improve the accuracy of the detection of complex solution components in wedge-shaped sample pool, this paper obtained multi-dimensional spectroscopy (MDS) of multi-path non-linear features including samples by multi-position and multi-wavelength detection methods. And this research constructed a visualized light intensity distribution image by multi-dimensional spectroscopy fusion (MDSF) method. The detection of 46 samples of India Ink and Intralipd-20% mixed phantoms was carried out, and the visualized light intensity distribution image was analyzed and trained by partial least squares (PLS) method, and compared to training method that the raw data without the MDSF (Non-MDSF). In this study, the multi-dimensional spectral fusion modeling method is used to increase the correlation coefficient of the model by 1.29%, and the mean square error is reduced by 3.07%. The results show that the use of MDSF method to analyze the composition of complex solutions, the composition of the solution with scattering characteristics is fast and accurate, which is of great significance for the field of complex solution detection and analysis.

The accuracy of component detection of turbid media can be difficult to improve due to the mutual influence of scattering and absorption in light attenuation. In this study, a heteromorphic sample pool was introduced containing turbid media with India Ink and Intralipid-20% fat emulsion, which increases the scattering information of the non-circumferential symmetric hyperspectral image of the turbid media. A gray level co-occurrence matrix (GLCM) was used to extract textural features from the hyperspectral images. Subsequently, the textural features were correlated with the concentrations of Intralipid-20% by means of partial least squares regression, and it was compared with the frequently used analysis of two-dimensional exit light intensity. Experimental results show that textural feature modeling is superior to conventional light intensity modeling with a correlation coefficient of prediction (Rp) = 0.9831 and a root-mean-square error of prediction (RMSEP) = 0.0631% in the prediction set. This study provides a potentially viable method for detecting the components of turbid media quantitatively in analytical chemistry.