Production of homogeneous particles by controlled neutralization of electrosprays

Resumen

This thesis is focused on the efficient reduction of charge of electrospray droplets to produce homogeneously sized polymer particles by electrospraying a polymer solution. In the electrospraying process, a steady micro-jet is emitted, from the tip of the so-called Taylor cone, breaking up into highly charged tiny droplets. When using a polymer-containing solution, it results in narrowly dispersed particle sizes, from micrometer to nanometer scale, depending on liquid properties and operating conditions. Therefore, electrospray is gaining research interest in different fields such as pharmaceutics. Among its advantages over other liquid atomization methods includes its ability to produce much smaller particles (in the few-microns and nano-metric size ranges) with size homogeneity and high energy efficiency. But one problem is that the high charge induced on the droplets by this technique may distort the size uniformity. This occurs if the droplet undergoes a Coulombic instability. Just before that, while the volatile solvent is being evaporated, thus reducing its diameter, the droplet is maintaining its charge. Then, the so-called Rayleigh limit is reached when the repulsive Coulomb forces overcome the surface tension and the droplet explodes. After that, the created polymer particle loses its shape and size. An electrospray-neutralization setup has been developed in which the droplet charge was reduced significantly, preventing Coulombic instabilities, and being able to generate homogeneous polymer particles in controlled atmosphere conditions, obtaining the desired particle morphology, and preventing filamentous particles. To achieve an appropriate particle charge reduction, it was necessary to produce an opposite polarity ion current which must be comparable to the electrospray current, usually from tens to hundreds of nano-amperes. For this reason, we have developed and optimized a unipolar ion source based on corona discharge. This home-made tool was strategically attached to the system, allowing the electrospray droplets to be effectively discharged before they explode. Another interesting feature of this system is that we can easily collect those particles by properly incorporating an extraction system. Due to the fact that, after the particle charge has been reduced sufficiently, they do not follow (at least with the same strength) the electric field anymore, they could be extracted by the gas velocity field, obtaining a high collection efficiency. To check the robustness of our system, different polymer solutions with different electrical conductivities have been used, producing PS, PVP, chitosan particles. Another possible advantage of reducing the droplet charge is that, since the throughput of a single emitter is too low from an industrial point of view, a barrier needs to be overcome: scaling-up the process. But in a multiple-emitter electrospray, the space charge might a problem in terms of developing a miniaturized system. Then, managing the space charge by controlling the charge reduction might be a key point in order to scale-up the process.