(70dz) Finite Element Simulations of the Impact of Elastic Particles with Liquid Films on Solid Surfaces

The deposition of fine particles onto ?wet' surfaces plays a vital role in processes such as coating, filtration and agglomeration. The main system design criterion is the rebound velocity, which should be zero to achieve a stick condition. Some of the factors controlling the energy dissipated in the liquid bridges formed during particle impacts have been studied in considerable detail and closed-form approximations have been developed. For example, this is the case for elastohydrodynamic collisions involving Hertzian particles and Newtonian fluids [1]. However, the lubrication solution used in this work is significantly more restrictive for power law fluids, even for nominally rigid particles [2]. Moreover, at finite gaps, extensional terms become important and bridge rupture may be stabilised by filamentation. Simple inviscid capillary bridges conserved energy during collisions [3] but wetting hysteresis [4] and also the inertial and hydrodynamic distortion of the meridian curvature [5] will result in energy losses. Other factors that may be important during such impacts include hydrodynamic inertial losses and cavitation and also the shape of the particles. Thus, in order to account for the complexities of the behaviour of real liquid bridges, it is necessary to employ numerical simulation. The current paper will describe the results of finite element analyses involving a parametric study of spherical and non-spherical elastic particles that impact a smooth elastically deforming solid surface, which is coated by a liquid film with different thicknesses and a range of rheological and wetting properties. The analyses are based on an explicit integration scheme with adaptive meshing such that the singularity in the force at the minimum gap (represented by a single element) is eliminated by homogenising the properties of the element with those of the underlying solid surface. The particular aim of this study is to improve the understanding of the impact behaviour of particles onto oil coated surfaces for applications in food coating processes.

References 1. Lian, G., Adams, M. J. and Thornton, C., ?Elastohydrodynamic collisions of solid spheres', J. Fluid Mech., 311 (1996) 141-152. 2. Lian, G., Xu Y., Huang W. and Adams M. J., ?On the squeeze flow of a power-law fluid between rigid spheres', J. Non-Newtonian Fluid Mech., 100 (2001) 151-164. 3. Lian, G., Thornton, C. and Adams, M. J., ?A theoretical study of liquid bridge forces between two rigid spherical bodies', J. Coll. Int. Sci., 161, (1993) 138 ? 147. 4. Willett, C.D., Adams, M. J., Johnson S.A. and Seville, J.P.K., ?Effects of wetting hysteresis on pendular liquid bridges between rigid spheres', Powder Technol., 130 (2003) 63-69. 5. Adams, M.J., Johnson, S.A., Seville, J.P.K. and Willett, C.D., ?Mapping the influence of gravity on pendular liquid bridges', Langmuir, 18 (2002) 6180-6184.