Acoustic droplet vaporization (ADV) is the physical process in which liquid undergoes phase transition to gas after exposure to a pressure amplitude above a certain threshold. In recent years, new techniques in ultrasound diagnostics and therapeutics have been developed which utilize microformulations with various physical and chemical properties. The purpose of this review is to give the reader a general idea on how ADV can be implemented for the existing biomedical applications of droplet vaporization. In this regard, the recent developments in ultrasound therapy which shed light on the ADV are considered. Modern designs of capsules and nanodroplets (NDs) are shown, and the material choices and their implications for function are discussed. The influence of the physical properties of the induced acoustic field, the surrounding medium, and thermophysical effects on the vaporization are presented. Lastly, current challenges and potential future applications towards the implementation of the therapeutic droplets are discussed.

The importance of the hydrodynamic cavitation phenomenon in small domains has been increasing during recent decades along with the global demand for microfluidic devices involving small-scale cavitation applications. Different characteristics of microscale hydrodynamic cavitation relative to the conventional size can be exploited in futuristic applications and improvements in the performances of new-generation microfluidic devices. Therefore, in-depth studies on the fundamentals of microscale hydrodynamic cavitation are required to reveal new physics of small-scale hydrodynamic cavitation. In this study, microfluidic devices with rough surfaces and micro restrictive elements are fabricated so that a basic study on ‘Hydrodynamic Cavitation on Chip’ is performed. Cavitating flows are investigated under transient and fully developed turbulent conditions within the Reynolds number range between 2962 and 8620 and cavitation number range between 2.025 and 0.72. The microfluidic devices have short restrictive elements with hydraulic diameters of 75, 66.6 and 50 µ m and lengths of 2 mm, which are connected to a bigger microchannel with a width of 900 µ m and length of 2 mm, called an ‘extended channel’. Different upstream pressures up to 900 Psi are applied at the inlet. The hydrodynamic cavitation inception is recorded and analyzed for each microfluidic device. Flow patterns are characterized inside the microfluidic devices from cavitation inception to chocked flow conditions. Accordingly, it is observed that the transition from inception to choked occurs slowly in contrast to microscale hydrodynamic cavitation results in the literature under laminar flow conditions. Moreover, the comparison between microfluidic devices with roughened and plane (smooth) surfaces reveals that the roughened surface results in more intense cavitating flows, especially at higher upstream pressures relative to the plane surface.

Coronavirus (COVID-19) is a highly infectious viral disease and first appeared in Wuhan, China. Within a short time, it has become a global health issue. The sudden emergence of COVID-19 has been accompanied by numerous uncertainties about its impact in many perspectives. One of major challenges is understanding the underlying mechanisms in the spread of this outbreak. COVID-19 is spread similar to the majority of infectious diseases through transmission via relatively large respiratory droplets. The awareness of the dispersal of these droplets is crucial in not only improving methods for controlling the dispersion of COVID-19 droplets, but also in discovering fundamental mechanisms of its transmission. In this study, a numerical model is developed to study the motion of droplets expelled through the respiratory system. Based on the source of these droplets, different sizes of droplets such as large ones and aerosols, which behave differently in the environment, can be generated. In this regard, diverse sources of droplets, namely breathing, coughing, and sneezing, are considered in this analysis. Besides, the time for a single droplet to fall from a height of 1.8 m is also obtained. The results reveal that the traditional distances suggested by different sources for keeping the social distance are not enough, which is linked to different nature of the droplet generation.

Cavitation is the sudden formation of vapor bubbles or voids in liquid media and occurs after rapid changes in pressure as a consequence of mechanical forces. It is mostly an undesirable phenomenon. Although the elimination of cavitation is a major topic in the study of fluid dynamics, its destructive nature could be exploited for therapeutic applications. Ultrasonic and hydrodynamic sources are two main origins for generating cavitation. The purpose of this review is to give the reader a general idea about the formation of cavitation phenomenon and existing biomedical applications of ultrasonic and hydrodynamic cavitation. Because of the high number of the studies on ultrasound cavitation in the literature, the main focus of this review is placed on the lithotripsy techniques, which have been widely used for the treatment of urinary stones. Accordingly, cavitation phenomenon and its basic concepts are presented in Section II. The significance of the ultrasound cavitation in the urinary stone treatment is discussed in Section III in detail and hydrodynamic cavitation as an important alternative for the ultrasound cavitation is included in Section IV. Finally, side effects of using both ultrasound and hydrodynamic cavitation in biomedical applications are presented in Section V.