(260av) Discrete Element Model of Nanoparticle Deposition during Electrospraying

Authors: 
Kroupa, M., Institute of Chemical Technology Prague
Kosek, J., University of Chemistry and Technology Prague
Electrospray is one of the devices used in the fast developing sector of the nanostructured materials production. Its advantages: low costs, high efficiency and easy operation as well as the possibility of the deposition of various materials are consequences of the simple principle of its operation: charged nozzle and grounded substrate cause electric field responsible for the flight of highly charged droplets. Liquid evaporates from the droplets during the flight which together with the increasing charge density causes the disintegration of the droplets. But unfortunately the deposition of the particles and also the morphology of the layer strongly depend on the operating conditions and they are therefore hardly predictable.
Our goal is to clarify the influence of various effects taking place during the final part of the dropletâ??s motion on the morphology of the layer. A convenient way how to do that is the mathematical modelling. We use the Discrete Element Method (DEM) based on the second Newtonâ??s law and on the summation of the forces affecting the particles. The forces between the particles include non-contact van der Waals and contact elastic adhesive forces, while the drag force acts on a particle surrounded by a fluid. An integral part of the model is the computation of the electric force. The resulting electric force affecting each charged particle depends on many effects: differently charged particles, the permittivity of the particles and the surrounding environment, the electric field between the nozzle and the substrate, etc. Therefore, the force has to be computed numerically. For this purpose, we used the Finite Volume Method (FVM) to discretize the Poissonâ??s equation and the electric force was evaluated from the resulting field of electric potential.
The predictions of the model were validated by layers created using a custom-built electrospray device. These deposited layers were characterized using a scanning electron microscope and an atomic force microscope.
Simulations in a multi-particle system were initially performed for particles with large electric conductivity, which are discharged immediately after their deposition and we used two configurations. In the first configuration, the so-called frozen deposit, the particles were not allowed to move after their deposition on the substrate or another deposited particle. The second configuration involved interparticle forces acting after the particle deposition. With the assumption of the frozen deposit, the operational parameters caused only minor changes in the final structures. On the other hand, when contact interparticle forces were considered, the resultant layers were more compact and more similar to the layers obtained from the experiments. The influence of each parameter was clearly observed. This clearly shows the importance of the after-deposition processes on the resulting morphology of the layer.
To extend the usability of the model for materials with lower conductivity, the model was equipped with the description of the discharging process. Due to the absence of the established description of the nanoparticles discharging, we had to come up with our own model. It computes the particle discharging mainly from the charge difference between the particle and the objects in the contact with it. The new results prove the importance of this extension and show that the morphology of the layer is significantly altered when particles are discharging slowly.
The obtained results bring more insight into the complicated process of the nanoparticle deposition and allow us to compare the importance of various effects occurring in the system. However, there is still a long and certainly very interesting way to go.