We perform local or microrheological measurements on microtubule solutions, as well as composite networks. The viscoelastic properties of microtubules as reported from two-point microrheology agree with the macroscopic measurement at high frequencies, but appear to show a discrepancy at low frequencies, at time scales on the order of a second. A composite of filamentous actin (F-actin) and microtubules has viscoelastic behavior between that of F-actin and pure microtubules. We further show that the Poisson ratio of the composite, measured by the length-scale dependent two-point microrheology, is robustly smaller than that of the F-actin network at time scales tau>1 s, suggesting that a local compressibility is conferred by the addition of microtubules to the F-actin network.

We have produced metal wire grids with 33nm periodicity, using a thin film of a self-assembling diblock copolymer as a template. These grids, supported on fused quartz wafers, function as transmission polarizers for visible and near-ultraviolet lights. Their polarization efficiency is measured to be near 50% in the visible. Quantitative comparison with a new theoretical analysis of such wire grids indicates that they should perform well into the far UV. This analysis also explains the reversal in polarization direction at shorter wavelengths which we observe in our specimens.

The spectrum of charged particles hopping on a kagomé lattice in a uniform transverse magnetic field shows an unusual set of Landau levels at low field. They are unusual in two respects: the lowest Landau levels are paramagnetic so their energies decrease linearly with increasing field magnitude, and the spacings between the levels are not equal. These features are shown to follow from the degeneracy of the energy bands in zero magnetic field. We give a general discussion of Landau levels in the case of two degenerate bands, and show how the kagomé lattice tight-binding model includes one special case of this more general problem. We also discuss the consequences of this for the behavior of the critical temperature of a kagomé grid superconducting wire network, which is the experimental system that originally motivated this work.Typeset using REVT E X 1

The authors demonstrate how a nonsolvent fluid, flowing over the surface of a block copolymer thin film in a complex path defined by a channel, can locally tailor the alignment of cylindrical microdomains. The cylinders have a periodicity of ∼30nm, while the mesoscale pattern has a millimeter length scale. A diverging-converging channel is employed to measure alignment quality over a range of applied stresses in a single experiment; the results are compared with analogous measurements where the stress is applied through a controlled-stress rheometer, and can be satisfactorily described by a melting-recrystallization model.

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We present feature finding and tracking algorithms in 3D in living cells, and demonstrate their utility to measure metrics important in cell biological processes. We developed a computational imaging hybrid approach that combines automated three-dimensional tracking of point-like features, with surface reconstruction from which cell (or nuclear) volume, shape, and planes of interest can be extracted. After validation, we applied the technique to real space context-rich dynamics of the mitotic spindle, and cell volume and its relationship to spindle length, in dividing living cells.These methods are additionally useful for automated segregation of pre-anaphase and anaphase spindle populations. We found that genetic deletion of the yeast kinesin-5 mitotic motor cin8 leads to abnormally large mother and daughter cells that were indistinguishable based on size, and that in those cells the spindle length becomes uncorrelated with cell size. The technique can be used to visualize and quantify tracked feature coordinates relative to cell bounding surface landmarks, such as the plane of cell division. By virtue of enriching the information content of subcellular dynamics, these tools will help advance studies of non-equilibrium processes in living cells.