When a small quantity of water (of volume typically one-tenth of the dense bed total volume) is added to a gas-fluidized bed of relatively large non-wettable particles, such as 6 mm polystyrene spheres, the added water can cause a dramatic improvement in the quality of fluidization. By reducing the water requirement, the resulting fluidized bed is advantageous for washing of farm products or ... [Show full abstract] fragile industrial devices. In this article, the characteristics of washing in this unique multi-phase flow system are investigated. Three kinds of experiments were conducted on target cylinders immersed in a water-assisted fluidized bed to measure particle collision forces on the cylinder surface, to determine attrition of a cylinder surface and to test whitening of a cylinder covered with soiled cloth. It is shown that the washing results from a combination of gentle abrasive washing due to liquid movement and direct attrition and fragment removal caused by particle collisions with the surface.