2,199 research outputs found
Mechanical fluctuations suppress the threshold of soft-glassy solids : the secular drift scenario
We propose a dynamical mechanism leading to the fluidization of soft-glassy
amorphous mate-rial driven below the yield-stress by external mechanical
fluctuations. The model is based on the combination of memory effect and
non-linearity, leading to an accumulation of tiny effects over a long-term. We
test this scenario on a granular packing driven mechanically below the Coulomb
threshold. We bring evidences for an effective viscous response directly
related to small stress modulations in agreement with the theoretical
prediction of a generic secular drift
Turning bacteria suspensions into a "superfluid"
The rheological response under simple shear of an active suspension of
Escherichia coli is determined in a large range of shear rates and
concentrations. The effective viscosity and the time scales characterizing the
bacterial organization under shear are obtained. In the dilute regime, we bring
evidences for a low shear Newtonian plateau characterized by a shear viscosity
decreasing with concentration. In the semi-dilute regime, for particularly
active bacteria, the suspension display a "super-fluid" like transition where
the viscous resistance to shear vanishes, thus showing that macroscopically,
the activity of pusher swimmers organized by shear, is able to fully overcome
the dissipative effects due to viscous loss
Learning circuits with few negations
Monotone Boolean functions, and the monotone Boolean circuits that compute
them, have been intensively studied in complexity theory. In this paper we
study the structure of Boolean functions in terms of the minimum number of
negations in any circuit computing them, a complexity measure that interpolates
between monotone functions and the class of all functions. We study this
generalization of monotonicity from the vantage point of learning theory,
giving near-matching upper and lower bounds on the uniform-distribution
learnability of circuits in terms of the number of negations they contain. Our
upper bounds are based on a new structural characterization of negation-limited
circuits that extends a classical result of A. A. Markov. Our lower bounds,
which employ Fourier-analytic tools from hardness amplification, give new
results even for circuits with no negations (i.e. monotone functions)
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