I understand that the torque caused by the friction must cause an angular acceleration, and it makes sense that the angular velocity increases from the bottom to the top so if there was a particle on the rim, as it went upwards it's speed would increase but what happens when it comes down again? My intuition tells me it would slow down and reach zero at the bottom (when it comes into contact with the floor) but I don't see how that can happen given the direction of the torque? There doesn't seem to be any torque which should cause the angular acceleration to decrease again. Does that mean the angular velocity keeps increasing until it reaches the bottom where it suddenly decreases? But what torque is causing that?