Removal of particles from fouling layers due to ail incident particle impact is affected by the fluid fluctuations in industrial applications if the contact time is larger than the fluctuations time scales. The contact time is an important parameter when analysing the influence of the fluid structure interaction on a fouling process. The contact time for a particle hitting a bed of particles is defined as the time it takes for the incident particle to bounce off the bed. The contact time for a particle hitting a bed of particles arranged in a rectangular and a hexagonal array is measured experimentally and calculated numerically based oil the discrete element method. The incident particle and the bed particles are of the same size and material. It is found that the contact time is proportional to the number of bed layers in case of a rectangular bed array and independent of the number of bed layers in case of a hexagonal bed of particles. The contact time is inversely proportional to the impact speed. The rebound speed of the incident particle is independent of the number of bed layers in case of a hexagonal arrangement of particles and is exponentially dependent on the number of bed layers in case of a rectangular arrangement. A hexagonal bed of particles acts as a massive particle due to its large coordination number compared to a rectangular bed of particles. The force propagation speed in granular matter could be calculated by plotting the path of the force as a function of the contact time and finding the gradient of this graph. (C) 2008 Elsevier B.V. All rights reserved.