## Kamien, Randall

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Publication Direct Determination of DNA Twist-Stretch Coupling(1996-11-01) Kamien, Randall; Lubensky, Tom; Nelson, Philip C; O'Hern, Corey S.The symmetries of the DNA double helix require a new term in its linear response to stress: the coupling between twist and stretch. Recent experiments with torsionally constrained single molecules give the first direct measurement of this important material parameter. We extract its value from a recent experiment of Strick et al. [Science 271 (1996) 1835] and find rough agreement with an independent experimental estimate recently given by Marko. We also present a very simple microscopic theory predicting a value comparable to the one observed.Publication Bubble Kinetics in a Steady-State Column of Aqueous Foam(2006-10-13) Feitosa, K.; Kamien, Randall D; Halt, O. L; Durian, Douglas JWe measure the liquid content, the bubble speeds, and the distribution of bubble sizes, in a vertical column of aqueous foam maintained in steady state by continuous bubbling of gas into a surfactant solution. Nearly round bubbles accumulate at the solution/foam interface, and subsequently rise with constant speed. Upon moving up the column, they become larger due to gas diffusion and more polyhedral due to drainage. The size distribution is monodisperse near the bottom and polydisperse near the top, but there is an unexpected range of intermediate heights where it is bidisperse with small bubbles decorating the junctions between larger bubbles. We explain the evolution in both bidisperse and polydisperse regimes, using Laplace pressure differences and taking the liquid fraction profile as a given.Publication Dynamics of Shallow Impact Cratering(2005-10-01) Kamien, Randall D; Ambroso, M. A; Durian, Douglas JWe present data for the time dependence of wooden spheres penetrating into a loose noncohesive packing of glass beads. The stopping time is a factor of 3 longer than the time d ∕ v0 needed to travel the total penetration distance d at the impact speed v0. The acceleration decreases monotonically throughout the impact. These kinematics are modeled by a position- and velocity-dependent stopping force that is constrained to reproduce prior observations for the scaling of the penetration depth with the total drop distance.Publication Why is Random Close Packing Reproducible?(2007-10-09) Kamien, Randal D.; Liu, Andrea JWe link the thermodynamics of colloidal suspensions to the statistics of regular and random packings. Random close packing has defied a rigorous definition yet, in three dimensions, there is near universal agreement on the volume fraction at which it occurs.We conjecture that the common value of фrcp ≈ 0.64 arises from a divergence in the rate at which accessible states disappear.We relate this rate to the equation of state for a hard-sphere fluid on a metastable, noncrystalline branch.Publication Elongation and Fluctuations of Semi-flexible Polymers in a Nematic Solvent(2004-03-26) Dogic, Z.; Zhang, J.; Discher, Dennis E; Lau, A. W.C.; Janmey, Paul; Aranda-Espinoza, Helim; Kamien, Randall; Dalhaimer, Paul M; Lubensky, Thomas C.; Yodh, ArjunWe directly visualize single polymers with persistence lengths ranging from lp = 0:05 to 16 µm, dissolved in the nematic phase of rod-like fd virus. Polymers with sufficiently large persistence length undergo a coil-rod transition at the isotropic-nematic transition of the background solvent. We quantitatively analyze the transverse fluctuations of semi-flexible polymers and show that at long wavelengths they are driven by the fluctuating nematic background. We extract both the Odijk deflection length and the elastic constant of the background nematic phase from the data.Publication Conformal Smectics and their Many Metrics(2012-05-11) Alexander, Gareth P; Kamien, Randall; Mosna, Ricardo AWe establish that equally spaced smectic configurations enjoy an infinite-dimensional conformal symmetry and show that there is a natural map between them and null hypersurfaces in maximally symmetric spacetimes. By choosing the appropriate conformal factor it is possible to restore additional symmetries of focal structures only found before for smectics on flat substrates.Publication Geometric Theory of Columnar Phases on Curved Substrates(2007-07-06) Santangelo, Christian D.; Vitelli, Vincenzo; Kamien, Randal D.; Nelson, David R.We study thin self-assembled columns constrained to lie on a curved, rigid substrate. The curvature presents no local obstruction to equally spaced columns in contrast with curved crystals for which the crystalline bonds are frustrated. Instead, the vanishing compressional strain of the columns implies that their normals lie on geodesics which converge (diverge) in regions of positive (negative) Gaussian curvature, in analogy to the focusing of light rays by a lens. We show that the out of plane bending of the cylinders acts as an effective ordering field.Publication Colloquium: Disclination loops, point defects, and all that in nematic liquid crystals(2012-06-01) Alexander, Gareth P.; Chen, Bryan Gin-ge; Kamien, Randall; Matsumoto, Elisabetta AThe homotopy theory of topological defects is a powerful tool for organizing and unifying many ideas across a broad range of physical systems. Recently, experimental progress was made in controlling and measuring colloidal inclusions in liquid crystalline phases. The topological structure of these systems is quite rich but, at the same time, subtle. Motivated by experiment and the power of topological reasoning, the classification of defects in uniaxial nematic liquid crystals was reviewed and expounded upon. Particular attention was paid to the ambiguities that arise in these systems, which have no counterpart in the much-storied XY model or the Heisenberg ferromagnet.Publication Helical Tubes in Crowded Environments(2007-05-18) Snir, Yehuda; Kamien, Randal D.When placed in a crowded environment, a semiflexible tube is forced to fold so as to make a more compact shape. One compact shape that often arises in nature is the tight helix, especially when the tube thickness is of comparable size to the tube length. In this paper we use an excluded volume effect to model the effects of crowding. This gives us a measure of compactness for configurations of the tube, which we use to look at structures of the semiflexible tube that minimize the excluded volume. We focus most of our attention on the helix and which helical geometries are most compact. We found helices of specific pitch to radius ratio 2.512 to be optimally compact. This is the same geometry that minimizes the global curvature of the curve defining the tube. We further investigate the effects of adding a bending energy or multiple tubes to begin to explore the more complete space of possible geometries a tube could form.Publication Hard Disks on the Hyperbolic Plane(2007-12-03) Modes, Carl D.; Kamien, Randall D.We examine a simple hard disk fluid with no long range interactions on the two-dimensional space of constant negative Gaussian curvature, the hyperbolic plane. This geometry provides a natural mechanism by which global crystalline order is frustrated, allowing us to construct a tractable model of disordered monodisperse hard disks. We extend free-area theory and the virial expansion to this regime, deriving the equation of state for the system, and compare its predictions with simulation near an isostatic packing in the curved space.

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