Finite Element Method Modeling of Self-Limiting Electrospray Deposition

Electrospray deposition (ESD) is a useful manufacturing tool by which a solution is passed through a charged capillary, resulting in the formation of a Taylor cone at the end of the capillary and the dispersion of the solution into child droplets through Coulomb fission. In doing so, a thin film of material can be deposited onto a grounded spray substrate. Each droplet deposits both material and charge onto the substrate, and this charge build-up can result in the deflection of further spray for materials that fall into the self-limiting electrospray deposition (SLED) regime when sprayed. This results in an evenly coated substrate as the material begins to spread out over the substrate as it is sprayed. SLED coatings are extremely advantageous in manufacturing as the spray will evenly coat 2D and 3D surfaces without moving the target or the spray nozzle. The spray is also attracted to grounded surfaces, resulting in little loss of material, making it more efficient and controllable. Due to these advantages, we are exploring the use of SLED to deposit a host of functional polymer and polymer composite coatings; however, we currently lack a method to predict the thickness of the SLED coatings before spraying. In this study, COMSOL finite element method simulation was used to model the grounded substrate, charged needle, surface charge of the film, and resulting particle trace of the spray. Using this particle trace, a MATLAB interpolation was used to compute the resulting surface charge density, and a wrapper in MATLAB would run this simulation several times to determine the deposition of each particle thickness and charge on each iteration. This was done on a flat substrate as well as on a substrate with a 3D grounded needle on the surface. In addition, we included nonconductive surfaces by altering the MATLAB function to have no charge decay in the regions designated nonconductive. In doing so, the amount of spray lost to nonconductive surfaces was able to be determined. The distribution of film thickness on the conductive and non-conductive surfaces was then evaluated to determine the efficiency and distribution of the spray, which was then compared to experimental results to fit the empirical charge decay parameter. This method may be used by manufacturers to determine the proper amount of spray needed for a desired coating thickness on a multitude of 2D and 3D surfaces, allowing for the eventual industrial application of the SLED process.