We have sought to test if increasing wall friction will improve or diminish the development of the U-tube instability, i.e., the migration of grains to one side of the tube when submitted to vertical oscillation. Our results show that the effect of friction is to slow down the exponential growth shown by the height difference between the level of grains in each branch of the tube. This is the expected effect that increasing wall friction will have on the flow if we think of a liquid flowing through a pipe, thus our results rein-force the assumptions made by previous work about the possibility of modeling the granular material as a liquid during a portion of
the oscillation cycle. For the roughest tube wall explored, besides
observing a dramatic decrease of the overall exponential growth
rate, the range of vibration intensities over which the instability
takes place was shortened, i.e., no instability was observed for high
amplitudes and large wall friction. We have tested the predictions
of a simple model based on the cyclic fluidization of the granular material inside the tube. The model was able to describe the
experimental results only for low wall roughnesses