Granular Conveyor -- Granular Material acts as a liquid?

[ADB_Glasgow]

TL;DR Summary

My final year experiment involves vibrating a granular conveyor with the same amplitude at at different frequencies.

My final year experiment involves vibrating a granular conveyor with the same amplitude at at different frequencies. (See picture for experiment set-up).
Between 5-115 Hz, the glass beads bounce erratically. As soon as the frequency hits 120Hz or higher, the beads remain on the surface, and can be seen behaving collectively as a liquid. I am however struggling to find a reason why this is the case.
Any ideas of why the beads behave as a liquid when the frequency is higher?
Cheers!

 

[hutchphd]

Do you have images of the two behaviors?

 

[ADB_Glasgow]

Yes no probem.
'Granular Conveyor_1' is at 5Hz, and all the glass beads are gather together at the bottom of the conveyor.
'GC_70Hz' is the granular conveyor at 70 Hz. Notice the glass beads are spread out across the surface and jumping around erratically.
'GC_135Hz' is the granular conveyor at 135 Hz. he glass beads have now spread out fairly evenly across the surface and are not erratically jumping around.
I am however struggling to find a reason why this is the case. Any ideas of why the beads behave as a liquid when the frequency is higher?
Cheers!

 

[anorlunda]

That's an interesting question. My guess is that the conveyer's surface is behaving like the surface of the speaker in the video below. As you can see, at certain frequencies the surface resembles an egg crate. As other frequencies, it hops from one vibrating mode to another.

An ultra slow motion video of the conveyor and beads may make it clear.

Edit: A second video

 

[hutchphd]

Thanks for pictures. I see the amplitude is characterized by a driving voltage. Do you know for certain what the actual displacement amplitude of the conveyer is (i.e. is it really the same at different frequencies for a given voltage). I guess one way to know would be to examine the frequency dependence at different fixed pp voltages. So what do you know?
(My first thought is that the transition would occur when the shake amplitude is similar to ball diameter...which is in fact similar to the step size ... I am not expert in this field...but I have thought about atoms scattering on rough surfaces previously)

 

[256bits]

Summary:: My final year experiment involves vibrating a granular conveyor with the same amplitude at at different frequencies.

On the pictures there is something that says "Amplitude = 50mVpp".
I assume that is the amplitude of the voltage applied to electronics of the vibrating table, which I would guess is that round white thing.

Coupling between electrical / mechanical systems will not mean that the table itself has a steady amplitude in meters peak-to-peak , as one moves through the electrical frequency spectrum.

 

[Baluncore]

For a fixed mechanical amplitude, the velocity of the surface is proportional to frequency.
What is the maximum mechanical surface velocity at 120 Hz ?
Is it a transducer resonance affected by the load? Change the load to test.
Is it surface acceleration that is important ?