Limin He

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.

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.

Several surfaces with special wettability: Influence on spreading and motion of W/O emulsion droplets

ABSTRACT Physical and chemical modifications were made on the surface of the aluminum sheet to change the surface properties and superhydrophobic–hydrophilic wettability gradient surface was made on the perspex surface by using microstructure-pattering technique and self-assembled-monolayer method. By using high-speed video camera system and optical tensiometer, this paper discusses the influence of special surfaces with different wettability on spreading and motion of water, oil, and W/O emulsion droplets both experimentally and theoretically. In addition, the paper also discusses the influence of the superhydrophobic–hydrophilic wettability gradient on fluidity of W/O emulsion droplets and the coalescence process of droplets. The results showed that the contact angle of W/O emulsion droplets on the modified surfaces was related to the water and oil distribution at the three-phase line. On the wettability gradient surface, the droplet moved spontaneously when the droplet was located at the junction of the gradient. A quasi-steady theoretical model was used to analyze the driving and resistant forces acting on a droplet to improve the understanding of the self-transport behavior of the droplets. GRAPHICAL ABSTRACT

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.