Haipeng Yan

Investigation on Transient Oscillation of Droplet Deformation before Conical Breakup under Alternating Current Electric Field.

In this paper, the conical breakup of a water droplet suspended in oil under the alternating current (ac) electric field was experimentally studied with the help of a high-speed video camera. We observed three stages of transient oscillation of deformation characterized by deformation degree l* before the conical breakup that were described in detail. Then a theoretical model was developed to find out the dynamic mechanisms of that behavior. Despite a very small discrepancy, good agreement between model predictions and experimental observations of the evolution of the droplet deformation was observed, and the possible reasons for the discrepancy were discussed as well. Finally, the stresses on the interface were calculated with the theoretical model and their influence on the dynamic behavior before the breakup was obtained. The differences between the droplet breakup mode of ac and direct current electric field are also discussed in our paper.

An experimental study on the coalescence process of binary droplets in oil under ultrasonic standing waves

The coalescence process of binary droplets in oil under ultrasonic standing waves was investigated with high-speed photography. Three motion models of binary droplets in coalescence process were illustrated: (1) slight translational oscillation; (2) sinusoidal translational oscillation; (3) migration along with acoustic streaming. To reveal the droplets coalescence mechanisms, the influence of main factors (such as acoustic intensity, droplet size, viscosity and interfacial tension, etc) on the motion and coalescence of binary droplets was studied under ultrasonic standing waves. Results indicate that the shortest coalescence time is achieved when binary droplets show sinusoidal translational oscillation. The corresponding acoustic intensity in this case is the optimum acoustic intensity. Under the optimum acoustic intensity, drop size decrease will bring about coalescence time decrease by enhancing the binary droplets oscillation. Moreover, there is an optimum interfacial tension to achieve the shortest coalescence time.

An experimental study on the motion of water droplets in oil under ultrasonic irradiation.

The motion of a single water droplet in oil under ultrasonic irradiation is investigated with high-speed photography in this paper. First, we described the trajectory of water droplet in oil under ultrasonic irradiation. Results indicate that in acoustic field the motion of water droplet subjected to intermittent positive and negative ultrasonic pressure shows obvious quasi-sinusoidal oscillation. Afterwards, the influence of major parameters on the motion characteristics of water droplet was studied, such as acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension, and droplet diameter, etc. It is found that when the acoustic intensity and frequency are 4.89 W cm(-2) and 20 kHz respectively, which are the critical conditions, the droplet varying from 250 to 300 μm in lower viscous oil has the largest oscillation amplitude and highest oscillation frequency.

The study of deformation characteristics of polymer droplet under electric field

In this paper, the characteristic of polymer droplet deformation under high-voltage alternating current electric field is studied with high-speed photography, and the main factors are also researched which could have an influence on this behavior. The relationship between the concentration of polymer and the deformation of droplet is reported. Afterwards, the droplet deformation is further investigated when surfactant coexists with polymer. Experiment results show that the relationship is significantly changed in this binary system solution caused by an increasing elasticity. The achievements of this article are of great significance to enhance the separation of water in oil emulsions emulsions.

Suspension characteristics of water droplet in oil under ultrasonic standing waves

The suspension characteristics of water droplet in oil were investigated under ultrasonic standing waves with high-speed photography in this paper. Firstly, the suspension position of droplet was predicted by theoretical derivation. The motion trajectory of droplet was captured and a kinetic analysis was applied to characterize the suspension position of droplet. The effects of droplet size, acoustic pressure, frequency, as well as density ratio of water and oil on the suspension position of droplet were analyzed in details. It was proved that the droplet size had little effect on the suspension position at different frequencies. The suspension zone approached minimum at 39.4kHz, and the suspension position of droplet was insensitive to acoustic pressure amplitude and density ratio at this frequency. These would be advantageous to maintain the stability of droplet banding and shorten the width of banding. In addition, it was proved that the suspension position of droplet is approximately linear with the density ratio at different frequencies.

Droplets banding characteristics of water-in-oil emulsion under ultrasonic standing waves

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.

Phase separation technology based on ultrasonic standing waves: A review

The current understanding and developments of phase separation technology based on ultrasonic standing waves (USWs) are reviewed. Most previous reviews have focused on microscale applications of this technology in the fields of biological materials and food processing. This review covers different applications of ultrasonic separation technology, especially in petrochemical industry. The kinetic mechanism of ultrasonic, design of reactors, separation principles, and related applications are discussed in detail. We lay special stress on the motion characteristics of particles in USWs. According to the particle numbers, particle properties, and frequency characteristics, the separation principles are reasonably categorized as: (1) Bands effect; (2) Acoustophoretic coefficient; (3) Particle density; (4) Sweep frequency. Diverse separation principles improve the universality of ultrasonic separation technology. However, acoustic streaming and acoustic cavitation are two of the main challenges in the application of ultrasonic separation. Based on the current research, the future research can focus on the following aspects: (1) Explore the mechanism of ultrasonic demulsification; (2) Establish unified evaluation criteria for acoustic separation systems; (3) Develop the basis for determination of acoustic cavitation and non-cavitation.

Breakup Characteristics of Aqueous Droplet with Surfactant in Oil under Direct Current Electric Field

The breakup process of aqueous droplet with surfactant suspended in oil under direct current (DC) electric field is investigated in this paper. The characteristics of the breakup process, stretching, necking and breakup, are discussed quantitatively with the electric capillary number Ca and the dimensionless surfactant concentration C∗ which is the ratio of surfactant concentration to the critical micelle concentration. The results show that the presence of surfactant reduces the steady deformation of droplet and significantly decelerates the stretching process, resulting from the redistribution of surfactant molecules within the oil/water interface. The law of droplet stretching process when C∗≥1 indicates that the exchange of surfactant molecules between the bulk phase and the interface could not catch up with the increase of oil/water interfacial area. Ca and C∗ count a great deal to the necking position and the daughter droplet size. The daughter droplet size decreases with the increase of surfactant concentration. These results provide a mechanistic framework to promote the electrocoalescence efficiency of oil/water emulsion and to the application of electric emulsification.

An experimental investigation on the deformation of alkali-surfactant-polymer droplet under AC electric field

With the help of high-speed photography, the microscopic laws of alkali-surfactant-polymer (ASP) droplet deformation under high voltage alternating current (AC) electric field were studied. The influences of AC electric field intensity and frequency were investigated in detail. Moreover, the synergistic effect of alkali-surfactant-polymer on drop deformation was reported. Experimental results demonstrate that there is a power law relationship between the deformation of alkali-surfactant-polymer droplet and the electric field strength. And it could be inferred that ASP interactions should be responsible for the complex relationship between ASP droplet deformation and the AC electric field frequency.