1,603 research outputs found
Force-velocity correlations in a dense, collisional, granular flow
We report measurements in a 2-dimensional, gravity-driven, collisional,
granular flow of the normal force delivered to the wall and of particle
velocity at several points in the flow. The wall force and the flow velocity
are negatively correlated. This correlation falls off only slowly with distance
transverse to the flow, but dies away on the scale of a few particle diameters
upstream or downstream. The data support a picture of short-lived chains of
frequently colliding particles that extend transverse to the flow direction,
making transient load-bearing bridges that cause bulk fluctuations in the flow
velocity. The time-dependence of these spatial correlation functions indicate
that while the force-bearing structures are local in space, their influence
extends far upstream in the flow, albeit with a time-lag. This leads to
correlated velocity fluctuations, whose spatial range increases as the jamming
threshold is approached.Comment: to be submitted for publicatio
Requirements for contractility in disordered cytoskeletal bundles
Actomyosin contractility is essential for biological force generation, and is
well understood in highly organized structures such as striated muscle.
Additionally, actomyosin bundles devoid of this organization are known to
contract both in vivo and in vitro, which cannot be described by standard
muscle models. To narrow down the search for possible contraction mechanisms in
these systems, we investigate their microscopic symmetries. We show that
contractile behavior requires non-identical motors that generate large enough
forces to probe the nonlinear elastic behavior of F-actin. This suggests a role
for filament buckling in the contraction of these bundles, consistent with
recent experimental results on reconstituted actomyosin bundles.Comment: 10 pages, 6 figures; text shortene
Contractile units in disordered actomyosin bundles arise from F-actin buckling
Bundles of filaments and motors are central to contractility in cells. The
classic example is striated muscle, where actomyosin contractility is mediated
by highly organized sarcomeres which act as fundamental contractile units.
However, many contractile bundles in vivo and in vitro lack sarcomeric
organization. Here we propose a model for how contractility can arise in
actomyosin bundles without sarcomeric organization and validate its predictions
with experiments on a reconstituted system. In the model, internal stresses in
frustrated arrangements of motors with diverse velocities cause filaments to
buckle, leading to overall shortening. We describe the onset of buckling in the
presence of stochastic actin-myosin detachment and predict that
buckling-induced contraction occurs in an intermediate range of motor
densities. We then calculate the size of the "contractile units" associated
with this process. Consistent with these results, our reconstituted actomyosin
bundles contract at relatively high motor density, and we observe buckling at
the predicted length scale.Comment: 5 pages, 4 figures, Supporting text and movies attache
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