Cone-jet Structure Research Articles

Parametric study on stability and morphology of liquid cone in flow focusing

A parametric study on the stability and morphology of the liquid cone in a flow focusing process is carried out through experimental and numerical methods. For the numerical simulations, we solve the Navier-Stokes equations coupled with a diffuse interface method. A scaling analysis which considers the competition between the viscous shear stress and the interfacial tension force is proposed, showing the effect of flow rates, geometrical parameters and liquid physical properties on the cone instability. The experimental and numerical results validate the scaling law and further show the effects of parameters on the cone morphology. We study the flow fields inside the liquid cone, with emphatic focus on the recirculation flow pattern which occurs at relatively high viscosity ratio between the focusing and the focused liquids. The occurrence of recirculation flow is close related to the tangential velocity distribution at both sides of the cone interface, and the change of flow rates and geometrical parameters is found to significantly affect the size of the recirculation flow. This study is expected to give a guidance to the optimization of process parameters in flow focusing system, which would contribute to the formation of a stable cone-jet structure and the production of microdroplets with high throughput.

Rotating-disk electrospinning: needleless electrospinning of poly(caprolactone), poly(lactic acid) and poly(vinyl alcohol) nanofiber mats with controlled morphology

As a contribution to the feasibility of high-throughput electrospinning, here we report on the investigation into the production of polymer nanofiber mats through needleless electrospinning. For this study, we used poly(caprolactone) (PCL), poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVA), widely used polymers in the field of electrospinning. Optimization studies were carried out to devise the characteristics of the rotating-disk electrospinning system in order to assess the processing window for each polymer. Other factors were also considered, where the production rate, fiber size distribution, and morphology of the specimens acquired by rotating-disk electrospinning and conventional electrospinning techniques were compared. Our studies also illustrate the similarities of this novel “bottom-up” process compared to the conventional one, where a high resolution image confirmed the formation of a cone-jet structure and a Taylor Cone.

A physical insight into electrospray process in cone-jet mode: Role of operating parameters

Abstract This article investigates the formation of cone-jet structure in an electrospray process based on a two-phase numerical simulation. The numerical approach takes account of the coupled governing equations of fluid flow and electrostatics in conjunction with the charge conservation equation and a VOF interface tracking method on the basis of a CSF model. The temporal and spatial evolutions of the cone-jet mode are examined in connection with the operating parameters, i.e. liquid flow rate and electric potential. Under the influence of these parameters, this study elucidates the physical aspects of the geometrical growth and extension along with the electric charge dispersion within the cone-jet structure. Furthermore, the flow patterns developed in the two-phase flow are studied revealing how orderly the operating parameters can alter the flow configuration. The results are compared with experimental data indicating good agreements, which, in turn, confirm the effectiveness of the simulation methodology concerning the electrospray phenomenon.

Photopolymerization of complex emulsions with irregular shapes fabricated by multiplex coaxial flow focusing

We fabricate complex emulsions with irregular shapes in the microscale by a simple but effective multiplex coaxial flow focusing process. A multiphase cone-jet structure is steadily formed, and the compound liquid jet eventually breaks up into Janus microdroplets due to the perturbations propagating along the jet interfaces. The microdroplet shapes can be exclusively controlled by interfacial tensions of adjacent phases. Crescent-moon-shaped microparticles and microcapsules with designated structural characteristics are further produced under ultraviolet light of photopolymerization after removing one hemisphere of the Janus microdroplets. These complex emulsions have potential applications in bioscience, food, functional materials, and controlled drug delivery.

Global stability of stretched jets: conditions for the generation of monodisperse micro-emulsions using coflows

AbstractIn this paper we reveal the physics underlying the conditions needed for the generation of emulsions composed of uniformly sized drops of micrometric or submicrometric diameters when two immiscible streams flow in parallel under the so-called tip streaming regime after Suryo and Basaran (Phys. Fluids, vol. 18, 2006, 082102). Indeed, when inertial effects in both liquid streams are negligible, the inner to outer flow-rate and viscosity ratios are small enough and the capillary number is above an experimentally determined threshold which is predicted by our theoretical results with small relative errors, a steady micrometre-sized jet is issued from the apex of a conical drop. Under these conditions, the jet disintegrates into drops with a very well-defined mean diameter, giving rise to a monodisperse microemulsion. Here, we demonstrate that the regime in which uniformly sized drops are produced corresponds to values of the capillary number for which the cone-jet system is globally stable. Interestingly enough, our general stability theory reveals that liquid jets with a cone-jet structure are much more stable than their cylindrical counterparts thanks, mostly, to a capillary stabilization mechanism described here for the first time. Our findings also limit the validity of the type of stability analysis based on the common parallel flow assumption to only those situations in which the liquid jet diameter is almost constant.

Two-cell circulation in a liquid meniscus driven by a swirling gas jet

A liquid issuing from a capillary needle adopts a cone-jet structure if the liquid is further driven by a coflowing gas jet. In the present work, flow patterns appearing in this cone-jet structure are studied by the volume-of-fluid numerical method. Axisymmetric motions of the liquid and gas, both treated as viscous incompressible fluids, are simulated. As the gas/liquid mass ratio increases, the meridional circulation develops in the meniscus region of the liquid flow. As the ratio exceeds a threshold, the flow becomes time periodic and droplet generating. Swirl, added in the gas jet, affects the liquid flow in two ways. First, the threshold value increases with swirl. Second, the circulation region transforms from the bubble-like into ring-like pattern and then becomes two-cellular. As swirl further increases, the cells separate, one cell disappears, and a new cell emerges being attached to the needle wall. The predicted metamorphoses of the flow topology might be important for atomization of a liquid fuel.

Pulsation Modes and the Effect of Applied Voltage on Current and Flow Rate in Nanoelectrospray

New modes of nanoelectrospray (nES) have been identified that exhibit stable high-frequency current oscillations in the kilohertz range. In-line flow rate measurements during nES have for the first time allowed the accurate determination of the spray current and volumetric flow rate dependence on applied voltage. High-speed video imaging has revealed that each current pulse corresponds to the periodic formation of a short-lived stable cone-jet structure at the capillary tip. These findings are of particular importance and suggest greater process monitoring and control in nES-MS systems is necessary if efficiency and ion yield are to be optimized.