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The physics of electrospray has been subject to an intense debate for three decades regarding the ultimate electrokinetics that determines the electric current and the size of the emitted droplets in the steady Taylor cone-jet mode (TCJ). In order to solve with a high degree of accuracy the complete electrokinetic structure of the TCJ, in this work, we have used the full Poisson–Nernst–Planck model electrokinetic equations, which have been solved using a high accuracy numerical scheme. We consider a formulation with no interfacial adsorption of ions, as in Mori & Young (J. Fluid Mech. , vol. 855, 2018, pp. 67–130). Our simulations corroborate Mori and Young's conclusion that the classical leaky dielectric model (LDM) recovers the electrodiffusion theory for weak electrolytes when disregarding ion adsorption at the interface. However, for strong electrolytes, our results differ drastically from those provided by the LDM. In this case, we observe that the ion distribution, and consequently the conductivity in the bulk, can be strongly non-homogeneous. Given the rather universal validity of the LDM experimentally observed so far, we postulate that ion interfacial adsorption must be considered in the case of strong, highly dissociated electrolytes to retrieve the LDM limit, mostly for a cone jet operating in the vicinity of the minimum flow rate. [ABSTRACT FROM AUTHOR]

Abstract: In this review, we aim at establishing a relationship between the fundamentals of the microfluidics technologies used in the Pharmacy field, and the achievements accomplished by those technologies. We describe the main methods for manufacturing micrometer drops, bubbles, and capsules, as well as the corresponding underlying physical mechanisms. In this regard, the review is intended to show non-specialist readers the dynamical processes which determine the success of microfluidics techniques. Flow focusing (FF) is a droplet-based method widely used to produce different types of fluid entities on a continuous basis by applying an extensional co-flow. We take this technique as an example to illustrate how microfluidics technologies for drug delivery are progressing from a deep understanding of the physics of fluids involved. Specifically, we describe the limitations of FF, and review novel methods which enhance its stability and robustness. In the last part of this paper, we review some of the accomplishments of microfluidics when it comes to drug manufacturing and delivery. Special attention is paid to the production of the microencapsulated form because this fluidic structure gathers the main functionalities sought for in Pharmacy. We also show how FF has been adapted to satisfy an ample variety of pharmaceutical requirements to date. [Copyright &y& Elsevier]

Flow focusing is providing new tools in Analytical Chemistry for its unique combination of simplicity, droplet size control just above the micrometer scale, and aerosol beam collimation. However, the most interesting droplet size range lies at the stability limit of the standard flow focusing operating mode. Nevertheless, that stability limit can be forced down to produce jets on the submicrometer scale by underpinning the cone-like meniscus at the tip of a hypodermic needle. Using a nebulizer design based on that concept, we show that the flow focusing steady jetting regime extends down to the limit imposed by the instability of jet itself, even for Reynolds numbers as low as 10 −2 . This stability enhancement leads to droplet sizes well below those previously obtained with flow focusing. In this work, we experimentally determine the influence on the jet's radius of the liquid flow rate and viscosity, as well as the focusing orifice. We analyze the instability mechanisms that impose the minimum values of the issued flow rate and jet's size. Finally, the jet's breakup and the size of the resulting micron-sized droplets are examined from high-speed optical imaging. [ABSTRACT FROM AUTHOR]