439 research outputs found
Thermalization of plastic flow versus stationarity of thermomechanical equilibrium in SGR theory
We discuss issues related to thermalization of plastic flow in the context of
soft glassy rheology (SGR) theory. An apparent problem with the theory in its
current form is that the stationarity of thermomechanical equilibrium obtained
by requiring that its flow rule satisfy detailed balance in the absence of
applied deformation requires plastic flow to be athermal. This prevents proper
application of SGR to small-molecule and polymer glasses where plastic flow is
often well-thermalized. Clearly, one would like to have a SGR-like theory of
thermalized plastic flow that satisfies stationarity. We discuss reasons why
such a theory could prove very useful and clarify obstacles that must be
overcome in order to develop it.Comment: substantially revised in response to referee comment
Structure and dynamics of model colloidal clusters with short-range attractions
We examine the structure and dynamics of small isolated -particle clusters
interacting via short-ranged Morse potentials. "Ideally preprared ensembles"
obtained via exact enumeration studies of sticky hard sphere packings serve as
reference states allowing us to identify key statistical-geometrical properties
and to quantitatively characterize how nonequilibrium ensembles prepared by
thermal quenches at different rates differ from their equilibrium
counterparts. Studies of equilibrium dynamics show nontrival temperature
dependence: nonexponential relaxation indicates both glassy dynamics and
differing stabilities of degenerate clusters with different structures. Our
results should be useful for extending recent experimental studies of small
colloidal clusters to examine both equilibrium relaxation dynamics at fixed
and a variety of nonequilibrium phenomena.Comment: Noro-Frenkel analysis added. Published in PR
TAX REFORM AND BEEF COW REPLACEMENT STRATEGY
This paper models optimal beef cow replacement strategy in a stochastic environment under U.S. income tax rules effective before and after the Tax Reform Act of 1986. Under each tax regime, the producer's buy versus raise decision and optimal culling age choice are analyzed. Per-cow profit levels are also calculated. Results of the numerical analysis indicate that tax law changes, particularly the loss of the capital gains exclusion and restrictions on preproduction expensing, will have significant effects on both optimal decisions and profitability of beef cow operations. When provisions of the Tax Reform Act of 1986 are fully effective in 1988, the optimum age for culling beef cow will increase, as will the after-tax costs of beef cow operations.Agricultural Finance, Livestock Production/Industries, Public Economics,
Minimal energy packings and collapse of sticky tangent hard-sphere polymers
We enumerate all minimal energy packings (MEPs) for small single linear and
ring polymers composed of spherical monomers with contact attractions and
hard-core repulsions, and compare them to corresponding results for monomer
packings. We define and identify ``dividing surfaces" in polymer packings,
which reduce the number of arrangements that satisfy hard-sphere and covalent
bond constraints. Compared to monomer MEPs, polymer MEPs favor intermediate
structural symmetry over high and low symmetries. We also examine the
packing-preparation dependence for longer single chains using molecular
dynamics simulations. For slow temperature quenches, chains form crystallites
with close-packed cores. As quench rate increases, the core size decreases and
the exterior becomes more disordered. By examining the contact number, we
connect suppression of crystallization to the onset of isostaticity in
disordered packings. These studies represent a significant step forward in our
ability to predict how the structural and mechanical properties of compact
polymers depend on collapse dynamics.Comment: Supplementary material is integrated in this versio
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