Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: `Sinking' in a bed of grains activated by shearing   We show how a weak force, $f$, enables intruder motion through dense granular
materials subject to external mechanical excitations, in the present case
stepwise shearing. A force acts on a Teflon disc in a two dimensional system of
photoelastic discs. This force is much smaller than the smallest force needed
to move the disc without any external excitation. In a cycle, material +
intruder are sheared quasi-statically from $\gamma = 0$ to $\gamma_{max}$, and
then backwards to $\gamma = 0$. During various cycle phases, fragile and jammed
states form. Net intruder motion, $\delta$, occurs during fragile periods
generated by shear reversals. $\delta$ per cycle, e.g. the quasistatic rate
$c$, is constant, linearly dependent on $\gamma_{max}$ and $f$. It vanishes as,
$c \propto (\phi_c - \phi)^a$, with $a \simeq 3$ and $\phi_c \simeq \phi_J$,
reflecting the stiffening of granular systems under shear as $\phi \rightarrow
\phi_J$. The intruder motion induces large scale grain circulation. In the
intruder frame, this motion is a granular analogue to fluid flow past a
cylinder, where $f$ is the drag force exerted by the flow.

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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat
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The categories of this paper are: bayes-an
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The categories of this paper are: econ
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-th
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The categories of this paper are: supr-con
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The categories of this paper are: adap-org
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The categories of this paper are: alg-geom
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The categories of this paper are: math-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: q-bio
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The categories of this paper are: chao-dyn
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The categories of this paper are: physics
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The categories of this paper are: comp-gas
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The categories of this paper are: atom-ph