ILASS–Europe 2019, 29th Conference on Liquid Atomization and Spray Systems, 2-4 September 2019, Paris, France This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0). Drop impact onto a solid sphere: the case of hydrophobic and superhydrophobic surfaces and low Weber numbers. Danial Khojasteh 1 , Reza Kamali 2 , Marco Marengo 3 1 Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Sydney, NSW, Australia - danial.khojasteh@unsw.edu.au 2 School of Mechanical Engineering, Shiraz University, Shiraz 71936-16548, Iran - rkamali@shirazu.ac.ir 3 School of Computing, Engineering and Mathematics, University of Brighton, Brighton BN2 4GJ, UK *Corresponding author: m.marengo@brighton.ac.uk Abstract Encapsulation of solid particles and liquid agents in a liquid shell is of exceptional interest in biotechnological, chemical and pharmaceutical fields such as personalized medicine, fluidized catalytic cracking, wire fabrication, catalytic reactions. Looking at the interaction between liquid drop and solid particles, we can study the special, ideal case of spherical drops impacting solid spheres, in order to understand the basic phenomena and the effect of the physical variables on the spreading behavior. Considering the importance of dynamics of drop-particle collision, which directly affects the quality of film deposition during encapsulation, various aspects of drop impact on dry solid spherical surfaces are still lacking in the existing literature. The cases are studied with a 3D level-set method implemented in a standard finite-element-based solver platform. The impact Weber number, the size ratio (ratio of spherical particle diameter to droplet diameter), and the surface contact angle (CA) are varied throughout the numerical simulations within a suitable range. After providing a strong validation with experimental data, it is found that impact on spherical surfaces generally presents a higher area of liquid to be in contact with the substrate with respect to the case of flat surfaces, when all other impact conditions are the same. The maximum spreading diameter increases with the impact velocity, with an increase of the sphere diameter, for a lower surface wettability and a lower surface tension. Typical outcomes of the impact include 1) complete rebound, 2) splash, and 3) final deposition stage after a series of spreading and recoiling phases. The roles of the centrifugal and the gravitational forces are considered and quantified. Finally, a model is proposed, which can reasonably predict the maximum deformation of low Reynolds number impact of droplets onto hydrophobic (H) or superhydrophobic (SH) spherical solid surfaces. Keywords Encapsulation, wettability, multiphase flows, level-set method. Introduction While a vast number of numerical, analytical and experimental investigations are dedicated to droplet dynamics after hitting a planar surface [1,2], the current literature is limited in understanding this impact on non-planar surfaces (i.e., spherical surfaces) despite the high number of applications of this process in industrial, biotechnological, chemical and pharmaceutical fields [3,4]. To illustrate, droplet-particle collision is significant in process industries during heterogeneous catalytic reactions and wetting of catalyst particles [5]. Brackish water from seas are sprayed on arrays of tubes (carrying hot water) in multi-effect desalination evaporators to produce distilled water. This phenomenon has also attracted a great deal of attention in droplet-based microfluidics, where numerous techniques are being developed to manipulate and functionalise droplet impingement [6]. The encapsulation process provides a powerful tool for various industrial applications including in biotechnological, chemical and pharmaceutical fields. For example, droplets which are encapsulated with H particles are extensively used in microreactors, gas sensing, and water pollution discovery [7]. The so-called spray drying is widely applied to achieve encapsulation because it provides good flexibility and a continuous operation [8]. This procedure is a conventional method of converting feedstock from a fluid state to dried particles through spraying the coating agent on the particles in which the sprayed droplets spread on the surface of the particles [8]. Then, the particles are heated by a flow of hot air to attain a swift liquid evaporation. In ideal case, the target particles need to be dense, and have spherical shape as well as high wettability to keep the momentum exchange of droplet particle and coating material to a minimum. For an effective encapsulation operation, the spreading of encapsulating material should be controlled, and a successful droplet landing should be gained to avoid splashing or bouncing of the droplets [9].