66,578 research outputs found
A non-monotonic constitutive model is not necessary to obtain shear banding phenomena in entangled polymer solutions
In 1975 Doi and Edwards predicted that entangled polymer melts and solutions
can have a constitutive instability, signified by a decreasing stress for shear
rates greater than the inverse of the reptation time. Experiments did not
support this, and more sophisticated theories incorporated Marrucci's idea
(1996) of removing constraints by advection; this produced a monotonically
increasing stress and thus stable constitutive behavior. Recent experiments
have suggested that entangled polymer solutions may possess a constitutive
instability after all, and have led some workers to question the validity of
existing constitutive models. In this Letter we use a simple modern
constitutive model for entangled polymers, the non-stretching Rolie-Poly model
with an added solvent viscosity, and show that (1) instability and shear
banding is captured within this simple class of models; (2) shear banding
phenomena is observable for weakly stable fluids in flow geometries that impose
a sufficiently inhomogeneous total shear stress; (3) transient phenomena can
possess inhomogeneities that resemble shear banding, even for weakly stable
fluids. Many of these results are model-independent.Comment: 5 figure
Super-hard Superconductivity
We present a study of the magnetic response of Type-II superconductivity in
the extreme pinning limit, where screening currents within an order of
magnitude of the Ginzburg-Landau depairing critical current density develop
upon the application of a magnetic field. We show that this "super-hard" limit
is well approximated in highly disordered, cold drawn, Nb and V wires whose
magnetization response is characterized by a cascade of Meissner-like phases,
each terminated by a catastrophic collapse of the magnetization. Direct
magneto-optic measurements of the flux penetration depth in the virgin
magnetization branch are in excellent agreement with the exponential model in
which J_c(B)=J_co exp(-B/B_o), where J_co~5x10^6 A/cm^2 for Nb. The
implications for the fundamental limiting hardness of a superconductor are
discussed.Comment: corrected Fig.
Mdm2 Is Required for Survival and Growth of p53-Deficient Cancer Cells.
p53 deletion prevents the embryonic lethality of normal tissues lacking Mdm2, suggesting that cells can survive without Mdm2 if p53 is also absent. Here we report evidence challenging this view, with implications for therapeutically targeting Mdm2. Deletion of Mdm2 in T-cell lymphomas or sarcomas lacking p53 induced apoptosis and G2 cell-cycle arrest, prolonging survival of mice with these tumors. p53-/- fibroblasts showed similar results, indicating that the effects of Mdm2 loss extend to pre-malignant cells. Mdm2 deletion in p53-/- cells upregulated p53 transcriptional target genes that induce apoptosis and cell-cycle arrest. Mdm2 deletion also increased levels of p73, a p53 family member. RNAi-mediated attenuation of p73 rescued the transcriptional and biological effects of Mdm2 loss, indicating that p73 mediates the consequences of Mdm2 deletion. In addition, Mdm2 deletion differed from blocking Mdm2 interaction with p53 family members, as Nutlin-3 induced G1 arrest but did not activate apoptosis in p53-/- sarcoma cells. Our results indicate that, in contrast to current dogma, Mdm2 expression is required for cell survival even in the absence of p53. Moreover, our results suggest that p73 compensates for loss of p53 and that targeting Mdm2 in p53-deficient cancers has therapeutic potential. ©2017 AACR
Fermi-liquid effects in the gapless state of marginally thin superconducting films
We present low temperature tunneling density-of-states measurements in Al
films in high parallel magnetic fields. The thickness range of the films, t=6-9
nm, was chosen so that the orbital and Zeeman contributions to their parallel
critical fields were comparable. In this quasi-spin paramagnetically limited
configuration, the field produces a significant suppression of the gap, and at
high fields the gapless state is reached. By comparing measured and calculated
tunneling spectra we are able to extract the value of the antisymmetric
Fermi-liquid parameter G^0 and thereby deduce the quasiparticle density
dependence of the effective parameter G^0_{eff} across the gapless state.Comment: 6 pages, 4 figure
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