Silo Hiccups
What are silo hiccups?  It is a
phenomena which has no analogue
in the flow of liquids, it's uniquely granular.
When particles flow out of a
constricted tube, as shown in the images,
the flow is not continuous,
it stops and starts again, even
when the tube is open at both
ends!  This is unlike the ticking
hourglass where  pressure changes  in the
upper or lower part of the glass must
be established for the grain flow to stop.
Here the flow stops without
external pressure changes. Why ?
The explanation is based on
the phenomenon  known as 'dilatancy'.
When a granular packing is distorted
the grains come in conflict as they
have to move by each other.
The result is that the packing must open up
to allow the motion. This happens i
n the tube as well, and when the
packing suddenly opens up, there
is more room for the air. But when
the  air is forced to expand the
pressure drops and there is all the sudden
a pressure force from the air on the grains.
This force, as it turns out, is sufficient to stop the flow of the grains.
When this happens air is sucked into the packing ,  as in a 'hiccup'.
The understanding of this phenomenon is important because it  may take place,
not  only in our lab, but also in  any other hopper or tube.
 

ead about it in
Silo Hiccups: Dynamic effects of dilatancy
T. le Pennec, K.J. Måløy, E.G. Flekkøy, Phys. of Fluids 10 p. 3072 (1998).
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