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 attractive colloidal and physicochemical properties of cellulose nanofibers (CNFs) at interfaces have recently been exploited in the facile production of a number of environmentally benign materials, e.g. foams, emulsions, and capsules. Herein, these unique properties are exploited in a new type of CNF-stabilized perfluoropentane droplets produced via a straightforward and simple mixing protocol. Droplets with a comparatively narrow size distribution (ca. 1–5 μm in diameter) were fabricated, and their potential in the acoustic droplet vaporization process was evaluated. For this, the particle-stabilized droplets were assessed in three independent experimental examinations, namely temperature, acoustic, and ultrasonic standing wave tests. During the acoustic droplet vaporization (ADV) process, droplets were converted to gas-filled microbubbles, offering enhanced visualization by ultrasound. The acoustic pressure threshold of about 0.62 MPa was identified for the cellulose-stabilized droplets. A phase transition temperature of about 22 °C was observed, at which a significant fraction of larger droplets (above ca. 3 μm in diameter) were converted into bubbles, whereas a large part of the population of smaller droplets were stable up to higher temperatures (temperatures up to 45 °C tested). Moreover, under ultrasound standing wave conditions, droplets were relocated to antinodes demonstrating the behavior associated with the negative contrast particles. The combined results make the CNF-stabilized droplets interesting in cell-droplet interaction experiments and ultrasound imaging.

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 µm and lengths of 2 mm, which are connected to a bigger microchannel with a width of 900 µm 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.