Bubbles and bubbly flows are omnipresent in nature and technology, showing a multitude of phenomena, which can be either beneficial or a hindrance. In any case, for their control and their applications, it is crucial to understand their fundamentals and therefore from the very beginning of the International Journal of Multiphase Flow they have been central. In this synoptic review we give some examples for the fascinating fluid dynamics of bubbles and bubbly flows, starting from their nucleation and cavitation phenomena, then going to single bubble phenomena, and finally to bubbly flows, in which the collective effects of bubbles are key, and to mass transfer in such bubbly flows. The review ends with an outlook on future direction and open issues in the research on bubbles and bubbly flows. ... Read more

Bubble bursting and subsequent collapse of the open cavity at free surfaces of contaminated liquids can generate aerosol droplets, facilitating pathogen transport. After film rupture, capillary waves focus at the cavity base, potentially generating fast Worthington jets that are responsible for ejecting the droplets away from the source. While extensively studied for Newtonian fluids, the influence of non-Newtonian rheology on this process remains poorly understood. Here, we employ direct numerical simulations to investigate the bubble cavity collapse in viscoelastic media, such as polymeric liquids. We find that the jet and drop formations are dictated by two dimensionless parameters: the elastocapillary number (the ratio of the elastic modulus and the Laplace pressure) and the Deborah number (the ratio of the relaxation time and the inertio-capillary time scale). We show that, for low values of and, the viscoelastic liquid adopts a Newtonian-like behaviour, where the dynamics is governed by the solvent Ohnesorge number (the ratio of visco-capillary and inertio-capillary time scales). In contrast, for large values and, the enhanced elastic stresses completely suppress the formation of the jet. For some cases with intermediate values of and, smaller droplets are produced compared with Newtonian fluids, potentially enhancing aerosol dispersal. By mapping the phase space spanned by, and, we reveal three distinct flow regimes: (i) jets forming droplets, (ii) jets without droplet formation and (iii) absence of jet formation. Our results elucidate the mechanisms underlying aerosol suppression versus fine spray formation in polymeric liquids, with implications for pathogen transmission and industrial processes involving viscoelastic fluids. ... Read more

Micro-and nanodroplets have many important applications such as in drug delivery, liquid-liquid extraction, nanomaterial synthesis and cosmetics. A commonly used method to generate a large number of micro-or nanodroplets in one simple step is solvent exchange (also called nanoprecipitation), in which a good solvent of the droplet phase is displaced by a poor one, generating an oversaturation pulse that leads to droplet nucleation. Despite its crucial importance, the droplet growth resulting from the oversaturation pulse in this ternary system is still poorly understood. We experimentally and theoretically study this growth in Hele-Shaw-like channels by measuring the total volume of the oil droplets that nucleate out of it. In order to prevent the oversaturated oil from exiting the channel, we decorated some of the channels with a porous region in the middle. Solvent exchange is performed with various solution compositions, flow rates and channel geometries, and the measured droplets volume is found to increase with the Péclet number, with an approximate effective power law. A theoretical model is developed to account for this finding. With this model we can indeed explain the scaling, including the prefactor, which can collapse all data of the 'porous' channels onto one universal curve, irrespective of channel geometry and composition of the mixtures. Our work provides a macroscopic approach to this bottom-up method of droplet generation and may guide further studies on oversaturation and nucleation in ternary systems. ... Read more

The Brandaris 128 ultra-high-speed imaging facility has been updated over the last 10 years through modifications made to the camera’s hardware and software. At its introduction the camera was able to record 6 sequences of 128 images (500×292 pixels) at a maximum frame rate of 25 Mfps. The segmented mode of the camera was revised to allow for subdivision of the 128 image sensors intoarbitrary segments (1–128) with an inter-segment time of 17 μs. Furthermore, a region of interest can be selected to increase the number of recordings within a single run of the camera from 6 up to 125.By extending the imaging system with a laser-induced fluorescence setup, time-resolved ultra-highspeed fluorescence imaging of microscopic objects has been enabled. Minor updates to the systemare also reported here. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4758783] ... Read more