ABSTRACT
Polymeric composite microspheres consisting of a poly(D,L-lactic-co-glycolic acid) (PLGA) core surrounded by a poly(D,L-lactic acid) (PDLLA) shell layer were successfully fabricated by coaxial electrohydrodynamic atomization (CEHDA) process. Process conditions, including nozzle voltage and polymer solution flow rates, as well as solution parameters, such as polymer concentrations, were investigated to ensure the formation of composite microspheres with a doxorubicin-loaded PLGA core surrounded by a relatively drug-free PDLLA shell layer. Various microsphere formulations were fabricated and characterized in terms of their drug distribution, encapsulation efficiency and in vitro release. Numerical simulation of CEHDA process was performed based on a computational fluid dynamics (CFD) model in Fluent by employing the process conditions and fluid properties used in the experiments. The simulation results were compared with the experimental work to illustrate the capability of the CFD model to predict the production of consistent compound droplets, and hence, the expected core-shell structured microspheres.
ABSTRACT
The initial motion of two-dimensional capsule in microchannel flow just after release is investigated in this paper by a numerical simulation method, which combines the finite volume method with the front tracking technique. The capsule is modeled as liquid medium enclosed by a thin membrane, for which linear elastic properties are taken into consideration. Three kinds of initial capsule shapes (circle, ellipse, and biconcave) and three initial positions (center-line, near-center, and near-wall positions) are considered in the simulations. Off-center capsules (the near-center and near-wall capsules) experience tilting and membrane tank-treading, and migrate laterally while they move with the fluid flow. After initial rapid tilting, the circular and elliptic near-wall capsules reach quasistationary tilt orientation, while the biconcave near-wall capsules experience steady change in tilt orientation with time. Lateral movements of the capsules indicate the existence of lift effect causing the capsule to move away from the wall. Lift velocities, the velocity components along the transverse direction, of the circular near-wall capsules decrease as they approach the centerline, while those of the elliptic and biconcave near-wall capsules do not show this trend, which might result from the short range of the simulation time. In general, the capsule with higher membrane dilation modulus has lower lift velocity, showing the effect of capsule deformability on the capsule behavior. Both tank-treading and lift velocities are 1-2 orders lower than the capsule translational velocity. For the circular and biconcave capsules, no matter the center-line or off-center capsules, hematocrit ratio increases with the membrane dilation modulus, namely, the capsule moving velocity decreases with the increasing dilation modulus, while the elliptic capsules with nondimensional membrane dilation moduli of 2500 and 500 show inverse trend in some time range. A preliminary study is carried out for long-term simulation of a circular capsule.
ABSTRACT
The pinch-off of a gas bubble from a tiny nozzle immersed vertically in another quiescent viscous fluid due to buoyancy is numerically investigated. The dynamics of bubble growth and pinch-off are described by the full Navier-Stokes equations for both gas and liquid phases. The equations are solved with a finite-volume method based on the SIMPLE scheme, coupled with a front tracking method to locate the interface between the two phases. The effects of liquid viscosity, surface tension, and gas density on the bubble pinch-off dynamics, which are always coupled in experiments, are investigated separately through simulations. The numerical results are compared with experimental observations on the bubble pinch-off for validation purposes. The simulation results show that the radius of the necking region decreases in a power law mode with time as r approximately tau;{alpha} , where tau is the time to pinch-off and the exponent alpha varies in the range 0.5-1.0 depending strongly upon the liquid properties such as viscosity and surface tension. In addition, the surface tension can significantly affect the bubble pinch-off exponent alpha when the surface tension coefficient is smaller than 0.030 N/m with a Bond number higher than 0.72. It is also found that both higher viscosity of the liquid phase and higher surface tension may result in a delayed pinch-off process and a larger bubble. The effect of gas phase density on the pinch-off is also investigated. As reported in the literature, the gas density variation has minimal effect on the necking process because the density ratio of the gas phase to the liquid phase is small.
ABSTRACT
Electrohydrodynamic atomization (EHDA) has many applications such as electrospray ionization in mass spectroscopy, electrospray deposition of thin films, pharmaceutical productions, and polymeric particle fabrications for drug encapsulation. In the present study, EHDA was employed to produce biodegradable polymeric micro- and nanoparticles. The effects of processing parameters such as polymer concentration, flow rate, surfactants, organic salt, and setup configurations on the size and morphology of polymeric particles were investigated systematically. By changing the various processing parameters, controllable particle shape and size can be achieved. PLGA nanoparticles with size of around 250 nm can be obtained by using organic salts to increase the conductivity of the spraying solution even at a relatively high flow rate. A higher flow rate has the advantage of producing a stable cone spray and can be easily reproduced. Solid and porous particles can be fabricated using different experimental setups to control the organic solvent evaporation rate. Also, paclitaxel, a model antineoplastic drug, was encapsulated in polymeric particles which can be employed for controlled release applications. In short, EHDA is a promising technique to fabricate polymeric micro- or nanoparticles which can be used in drug delivery systems.
