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Coaxial electrospray is a well established procedure to generate coaxial micro- and nano-structures (microcapsules and coaxial fibers). In this work we show the capabilities of a high-precision numerical code using dynamic mapping, analytic Jacobian formulation and the Taylor–Melcher leaky dielectric model (LDM) to deal with real coaxial electrospray configurations. Since the number of parameters involved in the problem is large, we focus on an experimentally tested liquid pair configuration where the thickness, relative position, and end sharpness of the coaxial feeding tubes are geometrical parameters of interest. The effects of those parameters and the voltage applied at the tubes on the coaxial liquid menisci, liquid streamlines, size of jet and core, and electric current are discussed.

Atomization of polymer solutions has important technological implications across many fields. Here, we investigated the atomization dynamics of diluted, polymer solutions using Flow Blurring (FB) technology. Aqueous solutions of poly(ethylene oxide) [PEO] of viscosity-averaged molecular weight in the range 100000 g/mol – 4000000 g/mol and varying concentrations were sprayed with a FB atomizer having an orifice diameter (D) of 700μm and a liquid feed-tube-to-orifice separation (H) of 100μm. The solutions belong to the dilute regime, where polymer coil overlap does not occur, that is φ= [Formula presented] <φcrit (Modesto-López, Pérez-Arjona, & Gañán-Calvo, 2019). Shear viscosity measurements indicated that the solutions had viscosities of the order of that of the solvent and exhibited a Newtonian-like behavior. However, during the atomization, and due to the relatively high shear stress induced in the atomizer, the solutions exhibit extensional rheology, which most likely arises from the stretching of the polymer chains in-flight. Although initially the atomization resulted in formation of filaments, these broke up into droplets at relatively short distances from the atomizer discharge orifice as elucidated by images from ultra-high speed videos. The phenomenon is in contrast with that observed in FB-based atomization of semi-diluted polymer solutions with concentrations larger than the polymer coil overlap concentration, c∗. FB atomization of the diluted solutions resulted in a decrease in droplet size with increasing the gas-to-liquid mass ratio (GLR). The approach herein aims at understanding the droplet formation dynamics of viscoelastic, polymer solutions with FB, for applications in large-scale synthesis of materials

The production of viscoelastic capillary jets with gaseous flow focusing is studied experimentally. In this technique, the liquid is injected at a constant flow rate through a feeding capillary located in front of the discharge orifice. A gas stream coflows with the jet across the orifice driven by a constant pressure drop. The gas stream sucks and drags the liquid, reducing the jet's diameter well below the orifice diameter. Because of the rheological nature of the liquid, this focusing phenomenon differs from the Newtonian one in several regards. For given values of the polymer concentration, the injected flow rate, and the applied pressure drop, there is an interval of the capillary-to-orifice distance for which the jetting regime is reached. Outside that interval, the jet either suffers from the pull-out instability or breaks up before reaching the discharge orifice. Significant free surface oscillations can be observed in most of the jetting realizations. This oscillatory behavior is caused by a transient die swell effect which continuously appears right at the capillary exit. Ejection interrupts because the jet bulges to such an extent that the free surface pinches. Because of the stabilizing effect of the polymeric contribution to the axial stress, micrometer filaments with lengths up to 1 cm and Weber numbers on the order of 10-4 can be produced in front of the discharge orifice. The shear viscous stresses exerted on the emitted jet by the gas stream beyond the discharge orifice prevent the macromolecule recoiling. The resulting extensional viscosity inhibits the break-up process, and thus very long jets are produced.

The breakup of a charged liquid column is studied numerically using Volume-OF-Fluid (VOF) for a range of timescales where electrokinetic phenomena may become significant, i.e when the time to breakup becomes comparable or shorter than the diffusion and the electroosmotic migration times of charged species. Here we propose a conservative method to deal with the diffusion of a tracer in VOF schemes when the diffusion is limited to one of the phases. The method consists in weighing the diffusivity with the value of the volume fraction computed from the analytically reconstructed interface. In this way, the interface is made impermeable to the tracer, which is conservatively kept within one of the phases. The performance of this method is first tested by comparing simple configurations with existing analytical solutions. In the cases when the diffusion, electroosmotic motion and hydrodynamic singularities com pete, the results indicate that, after breakup, charges distribute between droplets differently from models assuming homogeneous and constant electrical conductivities (i.e. no electrokinetic effects). However, such departure does not alter the main hydrodynamic balances leading to well-established scaling laws of breakup.

The absolute and convective instability of an electrically charged viscoelastic liquid jet is studied. The liquid is assumed to be (i) a dilute polymer solution described by the Oldroyd-B viscoelastic model, and also and (ii) a leaky dielectric defined by the Taylor-Melcher leaky dielectric theory. A generalized eigenvalue equation is obtained and solved numerically. Two different viscoelastic liquids, i.e. a PEO aqueous solution and a PIB Boger fluid, are taken as examples to study the effect of electric field and elasticity on the absolute and convective instability characteristic of the axisymmetric and first non-axisymmetric modes of a viscoelastic jet. The analysis shows that normal electric field may induce absolute instability of both axisymmetric and non-axisymmetric modes, being the effect of electric field larger on the latter. Elasticity has a profound destabilizing effect on the absolute and convective instability of the axisymmetric mode while its effect on the non-axisymmetric mode is quite limited. Strategies for suppressing absolute instability of an electrically charged viscoelastic jet are explored. The result indicates that increasing jet velocity or decreasing jet radius may effectively avoid the occurrence of absolute instability

In this paper we present a new method for the production of bubble-liquid suspensions (from now on BLS) composed of micron-sized bubbles and with gas to liquid volume ratios larger than unity. We show that the BLS gas fraction ʎ=Qg and Q1 , being Qg and Q1 the flow rates of gas and liquid, respectively, is controlled by a dimensionless parameter which accounts for the ratio of the gas pressure inside the device to the liquid viscous pressure drop from the orifices where the liquid is injected to the exit, where the BLS is obtained. This parameter permits the correct scaling of the BLS gas volume fraction of all the experiments presented.