(98n) Dynamics and CFD Modeling of Viscoelastic Polymeric Jets Under Rayleigh Instability As a Result of a Controlled Vibration to Produce Size- and Span-Controlled Microdroplets
We explore the dynamics of laminar Rayleigh-type instabilities of viscoelastic jets in inviscid mediums (namely a liquid jet injection into quiescent air at atmospheric pressure). It has been long known that this type of surface-tension-driven instabilities evolve as free-surface waves whose amplitudes grow with time and leads to an eventual break of the jet, forming droplets.
These instabilities are of primary importance for certain processes such as ink-jet printing, drug processing or biomedical applications, where a proper control of the interfacial instability leads to a controlled breakup into mono-sized droplets.
Besides the understanding of the physical process, the primary goal of this study lies in optimizing a modified commercial method to produce solid microparticles (diameters ranging from 300 to 600 um) with applications in areas such as biomedicine.
The system that we are examining applies a mechanical vibration to a capillary laminar flow of high viscous polymer in order to destabilize the jet in a controlled way. The natural polymer used – sodium alginate – shows high biocompatibility and biodegradability. In order to produce microparticles with high mechanical stability, a medium- to high-viscosity sodium alginate solution must be used. In a previous work we obtained semiempirical expressions from experimental conditions in which the effect of surface tension, flow rate and zero-shear viscosity of the samples enabled us to roughly predict microparticle size but where the effect of the vibration conditions or the viscoelastic nature of the samples were not considered.
We found that the equations from a linear approximation considering Oldroyd-B as the viscoelastic model describe the process accurately, taking into account a larger number of parameters than those analyzed before. The dispersion equation derived from the system of equations relates perturbation growth rate and superficial wave characteristics and the obtained dispersion curves enable us to establish the best conditions for the perturbation source (theoretical dominant wavelength and the optimal frequency) and to control and minimize the droplet size and distribution span.
On the other hand, we have conducted CFD studies to validate the system behaviour with an Euler-Euler approach. We found good qualitative results as for the bulk liquid characteristics (breakup length and droplet size) by applying generalized-Newtonian models. We are carrying out further simulations to obtain a more accurate ligament-bead description.