Micro-Mechanics of Wet Solids

Britta Crüger, M.Sc.



Although gas-solid processes with liquid injection are widely used in industry e.g. in chemical, pharmaceutical and food technology, a fundamental description of the micro-processes inside the apparatus is still missing. The particles collide with each other and with the apparatus walls, while they are coated by the injected liquid, thus transferring liquid between single particles. Depending on the collision properties particles may stick together after the collision due to liquid bonds or solidified liquids or separate again. The energy dissipation during such collisions is essential for describing and modelling the dynamic behaviour of single particles as well as of the macroscopic particle bed of industrial processes. It can be described by means of the effective coefficient of restitution which is defined as the ratio between the collision velocity after the collision and directly before impact.

The coefficient of restitution characterizes the overall energy dissipation during the collision including the effects of liquid injection. It depends on several parameters including particle properties (size, surface roughness, deformation behaviour), collision parameters (relative velocity, collision angle) and properties of the liquid (viscosity, layer thickness).
Therefore the goal of this project is to find a closure equation to represent the dependence of the effective coefficient of restitution on essential parameters and give a fundamental description of the mechanisms leading to the complex behaviour of wet particles.


To investigate micro-mechanics during wet collisions dry particles impacting on a wet target plate are recorded by two synchronized high-speed cameras allowing a three-dimensional analysis.

The collision is initiated by an inhouse-designed particle accelerator, which is driven by pressurized air and shoots a particle held on its tip onto the target plate. The impact angle can be varied between 0 and 60°. For the investigation of oblique collisions the coefficient of restitution is often represented in one normal component, one tangential part and a rotational component. All three components of the coefficient of restitution are calculated from a series of images.

The target is a glass plate and features a polymer border to keep the liquid from running off. The thickness of the liquid layer applied on the target is measured by a confocal sensor at the exact area of impact. Various particle materials, sizes and forms can be investigated as well as the influence of various liquid layers. In addition to the properties of particles and liquids different parameters are to be investigated, such as the influence of collision velocity and angle, of surface roughness and initial rotation of the particle before impact.

Besides experimental investigations the mechanisms leading to energy dissipation during impact and rebound of colliding particles are analysed by force and energy balances for all phases of the collision.

Selected publications

Tang, Y., Buck, B., Heinrich, S., Deen, N.G., Kuipers, J.A.M: Interface-Resolved Simulations of Normal Collisions of Spheres on a Wet Surface, AIChE Journal, (2017), accepted.

Buck, B., Tang, Y., Heinrich, S., Deen, N.G., Kuipers, J.A.M.: Collision dynamics of wet solids: Rebound and rotation. Powder Technology, 316 (2017), 218-224.

Crüger, B., Heinrich, S., Antonyuk, S., Deen, N.G., Kuipers, J.A.M.: Experimental study of oblique impact of particles on wet surfaces, Chemical Engineering Research and Design, 110 (2016), 209-219.

Crüger, B., Salikov, V., Heinrich, S., Antonyuk, S., Sutkar, V., Deen, N.G., Kuipers, J.A.M.: Coefficient of restitution for particles impacting on wet surfaces: An improved experimental approach, Particuology 25 (2016), 1-9.

Sutkar, V.S., Deen, N.G., Padding, J.T., Salikov, V., Crüger, B., Antonyuk, S., Heinrich, S., Kuipers, J.A.M.: A novel approach to determine wet restitution coefficients through a unified correlation and energy analysis, AIChE Journal, 61 (2015) 3, 769-779.

Cooperation partners

•    Multiphase Reactors Group, Department of Chemical Engineering and Chemistry Eindhoven University of Technology (J.A.M. Kuipers)
•    Multiphase and Reactive Flows Group, Department of Mechanical Engineering, Eindhoven University of Technology (N.G. Deen)

Project funding and Start Date

December 2013