Alessandro Gomez

Charge and fission of droplets in electrostatic sprays

Measurements of the charge and size of heptane droplets generated by electrostatic sprays showed that the droplet charge‐to‐volume ratio is a monotonically decreasing function of size. In the useful range of electrospray operation, characterized by droplets smaller than the size of the orifice from which the liquid is issued, it was found that the larger were the droplets the closer they were to the Rayleigh limit. In particular, when droplets had charging levels between 70% and 80% of such limit, they were observed to rupture because the repulsive force due to surface charge evidently overcame surface tension. The rupture phenomenon, here termed Coulomb fission, was also captured in microphotographs that typically showed a droplet with one or two, diametrically opposed, conical protrusions terminating in a fine jet ejecting a stream of much smaller, apparently equisized offsprings. The process appeared swift and, yet, well ordered, quite different from the common view of a violent, convulsive explosion. ...

On the structure of an electrostatic spray of monodisperse droplets

An experimental study has been performed on the structure of an electrostatic spray of monodisperse droplets. Such a spray is established when a liquid with sufficient electric conductivity and moderate surface tension, in the present case heptane doped with an antistatic additive, is fed through a small metal tube maintained at several kilovolts relative to a ground electrode a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up into charged droplets that disperse into a fine spray. Flash shadowgraph of the breakup region showed that the jet initially breaks into droplets of bimodal size distribution by varicose wave instabilities. The spray monodispersity is established farther downstream by a segregation process of electrostatic and inertial nature that confines the bulk of the mass flow rate (97%) and 85% of the total current in a core of nearly monodisperse...

Controlling the morphology of electrospray-generated PLGA microparticles for drug delivery

We developed a well-controlled method to generate PLGA microparticles of different morphologies using the electrospray drying route. By judiciously selecting polymer molecular weight, concentration, and solution flow rate, we can control the order in which polymer entanglements and Coulomb fission occur in the droplets and their relative importance, and subsequently govern the morphology of the resulting polymer particles. We show that spherical, monodisperse particles are generated when sufficiently strong polymer entanglements set in the evaporating droplets before they undergo any Coulomb fission. On the other hand, tailed and elongated particles are obtained if the Coulomb fission occurs first and if the droplets/particles are sufficiently evaporated to freeze in their irregular shape. Strictly spherical particles are unachievable for polymer solutions below a critical concentration, because the onset of Coulomb fission always sets in prior to the development of a sufficiently entangled polymer network. An extension of a simple model, originally used to determine the onset of electrospinning of polymer solutions, adequately predicts when non-spherical particles are produced. We conclude by demonstrating the scale-up of this approach to the synthesis of polymer particles using a compact, microfabricated, multiplexed electrospray system, which would make it suitable for practical applications.

Generation by electrospray of monodisperse water droplets for targeted drug delivery by inhalation

Abstract An experimental investigation was performed on the feasibility of using an electrospray to produce monodisperse droplets of water in the diameter range 2–10 μm, with the ultimate objective of employing the droplets for targeted delivery of inhaled drugs. Because of the high surface tension of water, the establishment of stable sprays required the use of a sheath flow of CO2 to prevent breakdown in the gas surrounding the spray and its destabilizing consequences on the electrospray performance. With this expedience, stable sprays of water could be operated at flow rates ranging from 5.8 to 42.4 μl min−1, which resulted in the generation of droplets in the diameter range 5–12 μm. Droplet size was controlled primarily by varying the liquid flow rate. An increase in solution conductivity by the addition of a small amount of NaCl showed that this physical variable can also be used to control droplet size and to produce droplets with diameter smaller than 5 μm. Solutions of too large conductivity ( > 1.2 × 10 −4 Ω −1 cm −1 ) may, however, produce droplets too small for targeted drug delivery. Since the typical ratio of droplet standard deviation over mean diameter was less than 0.10, the droplets can be considered monodisperse. A prototype delivery system was also developed in which provisions were made for complete or partial neutralization of the droplet charge by corona discharge. A co-flow of air was provided to carry the discharged droplets through a delivery line and to evaluate its effects on the droplet monodispersity. It was found that the droplet size distribution broadens because of size-dependent evaporation effects. However, at typical conditions of inhalation, the effect should not compromise the system efficiency for targeted deposition.

Thermophoretic Effects on Particles in Counterflow Laminar Diffusion Flames

Abstract Thermophoresis, meaning particle drift down a local gas temperature gradient, is now known to be important to many combustion-related technologies. Until now, however, no direct experimental determinations of primary and aggregated particle thermophoretic diffusivities, αT D, in high temperature combustion environments have been reported. To perform such measurements, we selected a seeded laminar counterflow diffusion flame (CDF) operated at low strain-rate as a well-defined combustion system, offering at the same time a low velocity and high temperature gradient environment. We established a CH4/ O2Inert opposed jet diffusion flame in which the gaseous fuel/oxygen ratio, and the diluent flow rates were adjusted to obtain a flat, stable flame, approximately coincident with the gas stagnation plane (GSP). Particles fed to or formed on either or both sides of the GSP move toward this plane until the local axial velocity is exactly counterbalanced by the thermophoretic velocity. As a result of this ...

Mesoscale power generation by a catalytic combustor using electrosprayed liquid hydrocarbons

The development of a mesoscale catalytic combustor to be coupled with direct energy conversion modules for power production is presented. The combustor has a volume on the order of a few cubic centimeters and operates on JP8 jet fuel, which is electrosprayed at a flow rate on the order of 10 g/hr and equivalence ratios varying from 0.35 to 0.70. Temperatures in the range 900–1300 K are achieved with a � 5% uniformity over the top circular surface of the burner. Using gas chromatographic analysis of the exhaust gases, a combustion efficiency on the order of 97% is estimated. Remarkably, no fouling, nor soot, nor NOx were detected in the exhaust gases. The system resulted in clean and efficient combustion of even environmentally problematic liquid hydrocarbons.

A multiplexed electrospray process for single-step synthesis of stabilized polymer particles for drug delivery

While conventional methods for biodegradable particle production rely predominately on batch, emulsion preparation methods, an alternative process based on multiplexed electrospray (ES) can offer distinct advantages. These include enhanced encapsulation efficiency of hydrophilic and hydrophobic agents, scale-up potential, tight control over particle size and excellent particulate reproducibility. Here we developed a well-controlled ES process to synthesize coated biodegradable polymer particles. We demonstrate this process with the Poly(DL-lactic-co-glycolic acid) system encapsulating amphiphilic agents such as doxorubicin (DOX), Rhodamine B (RHO(B)) and Rhodamine B octadecyl ester perchlorate (RHO(BOEP)). We show that in a single-step flow process particles can be made encapsulating the agent with high efficiency and coated either with emulsifiers that stabilize them in solution or that may facilitate further functionalization for targeted drug delivery. The coating process allows for the surface modification of the particles without further changes in particle size or morphology, and with minimal loss of drug (>94% encapsulation efficiency). This synthesis technique is well suited for massive scale-up using microfabricated, multiplexed arrays consisting of multiple electrospray nozzles operating in parallel. A simple analytical model of the diffusion of the encapsulated agent within the polymer reveals two distinct phases in the cumulative release profile: a first phase in which the release is dominated by diffusion and a second phase with a slower release related to the erosion of the polymer matrix. The first, diffusion-driven stage is highly affected by particle agglomeration properties, whereas the second one shows a much less pronounced dependence on particle size. Modeling suggests that the size of the particles will substantially influence the initial burst in both the percentage of drug released and the rate at which it is released. It will also affect to a smaller extent the secondary slow and sustained release. Our study highlights the importance of tight control over particle size and morphology and the avoidance of particle aggregation for control over the release kinetics and formulation repeatability.

PROPAGATION OF EDGE FLAMES IN COUNTERFLOW MIXING LAYERS: EXPERIMENTS AND THEORY

Edge flames were investigated in a methane/O 2 /N 2 counterflow diffusion flame burner. In a typical experiment, a stable counterflow diffusion flame in an axysymmetric configuration was perturbed by lowering the relevant Damkohler number slightly below the extinction value, Da ext . As a result, the flame extinguished in the vicinity of the burner axis where conditions were uniform. An edge flame extinction front quickly propagated in the radial direction, turned into an ignition edge flame, and eventually stabilized as a standing triple flame at a radial position larger than the burner radius. This sequence of events resulted from an increase of Da as a function of the radial direction, consequent to a decrease in the strain rate in the radial direction. The edge flame propagation velocity in the ignition mode was measured for propagating edge flames at moderate Da and for standing triple flames at large Da , using a combination of laser Doppler velocimetry of seeded particles, formaldehyde planar laser-induced fluorescence, and natural chemiluminescence imaging. The propagation velocity, nondimensionalized with the premixed laminar flame speed of the unburned stoichiometric mixture, was correlated with Da . The latter was calculated using a thermal diffusive model and velocity measurements. The nondimensional velocity reached a value of 2.6 at large Da , in good agreement with the estimated square root of the ratio of the unburned gas density to the burned gas density, as suggested by scaling considerations.

An experimental study of well-defined turbulent nonpremixed spray flames

Abstract An experiment system was designed for the study of well-defined turbulent nonpremixed spray flames. Particular emphasis was placed on minimizing the influence of the injector design and on maximizing turbulence within the spray flames. A comprehensive description of the structure of such flames was obtained by applying a variety of complementary diagnostic techniques, including: broadband chemiluminescence imaging, CH∗ emission imaging, phase Doppler interferometric techniques, and spontaneous Raman spectroscopy. Two methanol spray flames were examined in detail, with Reynolds number ranging from 2.1 × 10 4 to 2.8 × 10 4 . Flame appearance and detailed measurements confirmed the occurrence of group combustion. Near the burner mouth, a dense column of drops enveloped by a common flame was observed. Further up, large corrugated structures were visualized which eventually developed into separate “islands.” A significant fraction of the spray escaped unburned, which implies that droplet evaporation is slow in this configuration. Detailed scanning of the flames provided an extensive database of average and fluctuating components of gas velocity and temperature, as well as spray and droplet size–classified properties. Key conclusions from such measurements include: the evidence of two-way coupling between the two phases along the centerline near the burner mouth; a velocity acceleration in the densest areas of the spray flames, as a result of momentum addition through vaporization, followed by deceleration farther downstream as the jet spreading predominates; and the droplet inertial behavior, especially for the large size classes, as confirmed by estimates of some relevant Stokes numbers. The average flame height was found to correlate with an overall equivalence ratio and with the initial concentration of droplets at the burner mouth.

Counterflow diffusion flames of quasi-monodisperse electrostatic sprays

The structure of counterflow diffusion flames of quasi-monodisperse electrostatic sprays of heptane has been studied by measuring droplet size, velocity and gas-phase temperature in two flames characterized by the same overall equivalence ratio. The first flame was found to behave much like a purely gaseous diffusion flame, since droplets evaporate completely before directly interacting with the flame. In the second spray flame, whose strain rate, initial mean droplet size and relative velocity were respectively 43%, 30% and 200% larger than in the first one, droplets have sufficient momentum to penetrate the flame and continue to burn in an oxygen rich environment. The flame appearance is also dramatically different the two cases. The flow strain rate flame appears as a thin blue sheet, whereas the other flame exhibits an additional thick orange region on the oxidizer side, whose luminosity is presumably due to the presence of small soot particles. A comparison between temperature profiles shows that the flame with the higher strain rate has a substantially broader temperature profile, in contrast with the typical behavior of purely gaseous diffusion flames. Furthermore, even though the two flames have identical overall equivalence ratios and nearly equal adiabatic flame temperatures, the high strain rate flame has a peak temperature 240 K higher than the other. This finding can be explained in terms of N 2 dilution effects: droplets cross the blue flame and continue to burn stoichiometrically in an oxygen rich environment while the inert gas concentration progressively decreases. In the course of droplet evaporation, the size distribution, initially monodisperse, broadens because of size-dependent evaporation rate and residence time.