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Highlights • Enhancement effect of electrostatic field on ultrasonic cavitation is explained. • Model for a nonspherical bubble in compressible liquid under the coupling effect of ultrasound and electrostatic field was developed. • Effect of different electric field strengths on the bubble cavitation was analyzed theoretically. • We compared gas pressure, temperature and cavitation threshold with and without electric stress., A model for a nonspherical bubble in a compressible liquid under the coupling effect of ultrasound and electrostatic field was developed in this study. The following assumptions are made: (1) the bubble undergoes adiabatic oscillation; (2) the gravity of the liquid is negligible; (3) the bubble is insulating. If the speed of sound approaches infinity (c→∞), the equation set is reduced to the equation set for an incompressible liquid. We found that, under ultrasonic irradiation coupled with electric stress, a nonspherical bubble cannot oscillate steadily in the liquid. The bubble is bound to collapse during several cycles. The presence of electric stress reduces the surface tension at the bubble wall, which produces a larger maximum bubble-radius during the rarefaction cycle and a smaller minimum bubble-radius during the compression cycle. Consequently, during the collapse, both the gas pressure and the temperature in the bubble center increase substantially, if the bubble is exposed to both ultrasound and electrostatic field instead of ultrasound alone. In addition, the cavitation threshold of the bubble within an electrostatic field decreases significantly, compared to the bubble without an electrostatic field. In general, bubble cavitation occurs more easily and violently in the liquid after the introduction of an electrostatic field.

A dynamic model for a double-bubble system in compressible liquid under the coupling effect of ultrasound and electrostatic field was developed here. In this study, we mainly discussed the effect of the interaction on the investigated bubble using the numerical solutions to the theoretic model. The variable parameters are the distance between bubble centers and the initial radius of the adjacent bubble. In addition, we applied approximate equations to analyse variations of the internal gas pressure and temperature of a bubble. We found that, the oscillation amplitude of a bubble with an adjacent bubble significantly reduces, compared to that of an isolated bubble.

Lycium barbarum fruit (Goji or wolfberry) is widely used as a functional food as well as an herb in Chinese medicine. The polysaccharides of L. barbarum (LBPs) are well-known for immunomodulation and antioxidant activities. This study evaluated the yields, properties and bioactivities of LBPs extracted from L. barbarum fruit by different methods including hot water (HWE 100 °C), ultrasonic water (UWE 30–40 °C), subcritical water (SWE 110 °C), and ultrasound-enhanced subcritical water extraction (USWE 110 °C). The total LBP yield (80 min extraction followed by ethanol precipitation) was the highest from USWE (14% w/w) and the lowest from HWE (7.6%). The LBP from USWE also showed the highest protein (27.9%) and phenolic content (5.3%, w/w), and the highest antioxidant activities (scavenging radicals by DPPH and trolox equivalent antioxidant capacity assays; ferric reducing power). It also showed the highest immunoactivities in activating the phagocytosis and NO production of RAW264.7 macrophages. USWE was proven the most efficient method for extraction of LBPs. Temperature and ultrasound were two chief factors affecting the extraction yield, chemical properties and bioactivities of LBP.

The total flavonoids from Hemerocallis citrina baroni are regarded as a green and natural health care product with many beneficial impacts on human health. In this study, ultrasound-synergized electrostatic field extraction (UEE) of the total flavonoids (TF) from H. citrina was investigated. Significant independent variables of the extraction, including the electrostatic field, ultrasonic power, ethanol concentration and extraction time, were optimized using the Box-Behnken (BB) method, and the optimal extraction conditions were obtained by response surface methodology (RSM). The extraction yield using UEE was compared with the yields obtained using only ultrasound extraction (UE) and water bath extraction (WE), using a UV-vis spectrophotometer. The best extraction yield of 1.536% using UEE was achieved under the following optimal conditions: electrostatic field of 7kV, ultrasonic power of 500W, ethanol concentration of 70% and extraction time of 20min. The optimal solid-liquid ratio (1:25g/mL) and extraction temperature (55°C) were determined by single factor experiments. Compared to other extraction methods, UEE not only increases the extraction yield of TF but also exhibits an excellent antioxidant activity in assays of the scavenging capacity for DPPH, hydroxyl and superoxide anion radicals. The availability of the UEE method can be supported by the ultrasonic cavitation effect, which plays the most important role in the UEE method. The electrostatic field can be regarded as a random disturbance for sonication, which can strengthen the cavitation effect and increase the cavitation yield. Moreover, the amount of iodine release in potassium iodide solution well validated the synergetic effect between the ultrasound and electrostatic field.