Ulrich et al. explained this phenomenon by measuring the friction on the sides of the container. They found that aging the system increased the friction, while cleaning the system decreased the system to its original value. They suggest a two-mechanism system. First the dense particles sink to the bottom because their buoyancy is smaller than their weight, then, the sidewall-driven convection begins to play a part. The way this convection works is that as the container vibrates up and down, a convection roll going downwards at the walls of the container and upwards in the center of the container begins. When the larger particles move upwards in the center, they have a lower probability to enter the downstream layer at the walls of the container. Since an increase in the friction of the walls would strengthen the convective motion relative to the buoyancy, it would strengthen that segregation method.