Physics of a Ruck (bump) in a Rug
LF12532: The motion of a ruck in a rug is used as an analogy to explain the role of dislocations in crystalline
solids. We take literally one side of this analogy and study the shape and motion of a bump, wrinkle or
ruck in a thin sheet in partial contact with a rough substrate in a gravitational field. Using a combination of
experiments, scaling analysis and numerical solutions of the governing equations, we quantify the static
shape of a ruck on a horizontal plane. When the plane is inclined, the ruck becomes asymmetric and
moves by rolling only when the inclination of the plane reaches a critical angle, at a speed determined by a
simple power balance. We find that the angle at which rolling starts is larger than the angle at which the
ruck stops; i.e., static rolling friction is larger than dynamic rolling friction. We conclude with a
generalization of our results to wrinkles in soft adherent extensible films.
LE12368:We consider the familiar problem of a bump, or ruck, in a rug. Under lateral compression, a rug bends to
form a ruck—a localized region in which it is no longer in contact with the floor. We show that when the
external force that created the ruck is removed, the ruck flattens out unless the initial compression is
greater than a critical value, which we determine. We also study the inertial motion of a ruck that is
generated when one end of the rug is moved rapidly. We show that the equations of motion admit a
traveling ruck solution for which a linear combination of the tension and kinetic energy is determined by
the ruck size. We confirm these findings experimentally. We end by discussing the potential implications
of our work for the analogous propagation of localized slip pulses in the sliding of two bodies in contact.
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LW11697
Quick Clay and Landslides of Clayey Soils
We study the rheology of quick clay, an unstable soil responsible for many landslides. We show that
above a critical stress the material starts flowing abruptly with a very large viscosity decrease caused by
the flow. This leads to avalanche behavior that accounts for the instability of quick clay soils. Reproducing
landslides on a small scale in the laboratory shows that an additional factor that determines the violence of
the slides is the inhomogeneity of the flow.We propose a simple yield stress model capable of reproducing
the laboratory landslide data, allowing us to relate landslides to the measured rheology.
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LH12203
Molecules in a glass are arranged differently from those in a liquid
Conventional wisdom states that a glass is a frozen liquid, in the
sense that the arrangement of molecules relative to each other is the
same in both cases. A team of scientists from Northwestern
University, Argonne National Laboratory and Brookhaven National
Laboratory has found that in fact there are distinct differences in
the surface structure that can be observed using X-ray scattering.
Both liquids and glasses are thought to be isotropic and disordered,
but when isotropic liquids are cooled to sufficiently low
temperatures, they develop molecular-scale layers (anisotropic,
liquid-crystalline order) near the surface. When such a liquid is
cooled to below the glass transition, this layering becomes sharper
and penetrates much further into the bulk of the glass. This change
happens suddenly at the transition temperature, i.e. it is an
apparently discontinuous signal of the glass transition.
Glassy materials play an important role in everyday life, from
containers to lenses to insulating fibers, and the glass transition
is important in materials processing and also a common phenomenon in
everyday life (for example, wrinkles in clothes are removed by
heating them with an iron to above the glass transition). The new
observations may provide a test of the various competing theoretical
models of glasses, and thus lead to a clearer view of this
poorly-understood transition.
