Suspension droplet wall impingement and particle deposition

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The project aims to analyse suspension droplet wall impacts experimentally and by numerical simulations. This is a complex and rarely studied topic, but is of fundamental importance for numerous technical processes. These are: spray painting, spray drying, composite surface coating, coating of pharmaceuticals, and single droplet processes, such as ink-jet printing, printing of biomaterials and electronic circuits. Experiments will be conducted for obtaining the regimes of droplet impact, such as deposition, rebound and splashing, summarised by relevant non-dimensional parameters. Moreover, droplet spreading, formation of deposits on the walls and the properties of rebound fragments will be studied.Experiments are planned for millimetres droplet sizes, different impact angles, velocities, and wall materials. Various liquids and different volume fractions of solid particles will be considered to obtain a comprehensive understanding of the complex droplet wall impact. Also, different sizes and material densities of the enclosed particles will be considered to analyse the effect of particle inertia on the impact. For visualising the droplet impingement direct shadowgraph imaging techniques will be applied in combination with LED illumination and high-speed camera recording. These techniques will allow not only to study possible drop impact outcomes but also to monitor particle motion inside spreading drops along with final particle distribution.Complementary numerical simulations will be conducted for predicting droplet behaviour during impact as well as the particle motion inside the droplet and eventually the structure of wall deposition. For dealing with such a complex three-phase flow problem a number of extensions of the available numerical code based on the Euler/Lagrange principle have to be done. The basis of the numerical simulations will be the OpenFOAM library, which was already extended at the institute with a number of features needed for the present task. The behaviour of the droplets itself during wall impact will be solved by a VoF method with geometrical reconstruction of the liquid interface using the isoAdvector. The solid particles dispersed inside the droplet will be treated by a point-particle Lagrangian method, considering all relevant forces. Two-way coupling and inter-particle collisions will be also considered. Moreover, the interaction of the particles with the droplet interface and the solid wall requires additional forces to be considered. Finally, particle wall adhesion will be considered in order to predict the formation of the particle deposits on the wall. Model parameters needed, will be provided by the detailed experimental studies, e.g. for the particle-wall collisions in a liquid. The extended numerical code will be verified with the thorough experiments. Overall, this project will lead to a detailed knowledge and provide design rules for processes applying suspension droplet wall collisions.