Studies on diffusion dynamics of single molecules (SMs) have been useful in revealing inhomogeneity of polymer thin films near and above the glass-transition temperature (T(g)). However, despite several applications of polymer thin films where exposure to solvent (or vapor) is common, the effect of absorbed solvent molecules on local morphology and rigidity of polymer matrices is yet to be explored in detail. High-T(g) hydrophilic polymers such as poly(vinylpyrrolidone) (PVP) are used as pharmaceutical coatings for drug release in aqueous medium, as they readily absorb moisture, which results in effective lowering of the T(g) and thereby leads to plasticization. The effect of moisture absorption on swelling and softening of PVP thin films was investigated by visualizing the diffusion dynamics of rhodamine 6G (Rh6G) tracer molecules at various ambient relative humidities (RH). Wide-field epifluorescence microscopy, in conjunction with high-resolution SM tracking, was used to monitor the spatiotemporal evolution of individual tracers under varied moisture contents of the matrix. In the absence of atmospheric moisture, Rh6G molecules in dry PVP films are translationally inactive, suggestive of rigid local environments. Under low moisture contents (RH 30-50%), translational mobility remains arrested but rotational motion is augmented, indicating slight swelling of the polymer network which marks the onset of plasticization. The translational mobility of Rh6G was found to be triggered only at a threshold ambient RH, beyond which a large proportion of tracers exhibit extensive diffusion dynamics. Interestingly, SM tracking data at higher moisture contents of the film (RH ≥ 60%) reveal that the distributions of dynamic parameters (such as diffusivity) are remarkably broad, spanning several orders of magnitude. Furthermore, Rh6G molecules display a wide variety of translational motion even at a fixed ambient RH, clearly pointing out the extremely inhomogeneous environment of plasticized PVP network. Intriguingly, it is observed that a majority of tracers undergo anomalous subdiffusion even under high moisture contents of the matrix. Analyses of SM trajectories using velocity autocorrelation function reveal that subdiffusive behaviors of Rh6G are likely to originate from fractional Brownian motion, a signature of tracer dynamics in viscoelastic medium.

From a direct numerical simulation of the MHD equations we show, for the first time, that velocity and magnetic-field structure functions exhibit multiscaling, extended self similarity (ESS), and generalized extended self similarity (GESS). We also propose a new shell model for homogeneous and isotropic MHD turbulence, which preserves all the invariants of ideal MHD, reduces to a well-known shell model for fluid turbulence for zero magnetic field, has no adjustable parameters apart from Reynolds numbers, and exhibits the same multiscaling, ESS, and GESS as the MHD equations. We also study dissipation-range asymptotics and the inertial- to dissipation-range crossover.Comment: 5 pages, REVTEX, 4 figures (eps

We present an extensive pseudospectral study of the randomly forced Navier-Stokes equation (RFNSE) stirred by a stochastic force with zero mean and a variance ∼ k 4−d−y , where k is the wavevector and the dimension d = 3. We present the first evidence for multiscaling of velocity structure functions in this model for y ≥ 4. We extract the multiscaling exponent ratios ζp/ζ2 by using extended self similarity (ESS), examine their dependence on y, and show that, if y = 4, they are in agreement with those obtained for the deterministically forced Navier-Stokes equation (3dNSE). We also show that well-defined vortex filaments, which appear clearly in studies of the 3dNSE, are absent in the RFNSE. PACS : 47.27.Gs, 47.27.Eq, 05.45.+b, 05.70.Jk Kolmogorov's classic work (K41) on homogeneous, isotropic fluid turbulence focussed on the scaling behavior of velocity v structure functions S p (r) = |v i (x + r) − v i (x)| p , where the angular brackets denote an average over the statistical steady state [1]. He suggested that, for separations r ≡ |r| in the inertial range, which is substantial at large Reynolds numbers Re and lies between the forcing scale L and the dissipation scale η d , these structure functions scale as S p ∼ r ζp , with ζ p = p/3. Subsequent experiments [2] have suggested instead that multiscaling obtains with p/3 > ζ p , which turns out to be a nonlinear, monotonically increasing function of p; this has also been borne out by numerical studies of the three-dimensional Navier-Stokes equation forced deterministically (3dNSE) at large spatial scales [2,3]. The determination of the exponents ζ p has been one of the central, but elusive, goals of the theory of turbulence. One of the promising starting points for such a theory is the randomly forced Navier-Stokes equation (RFNSE) [4][5][6], driven by a Gaussian random force whose spatial Fourier transform f (k, t) has zero mean and a covariancehere k, k ′ are wave numbers, t, t ′ times, i, j Cartesian components in d dimensions, and P ij (k) the transverse projector which enforces the incompressiblity condition. One-loop renormalization-group (RG) studies of this RFNSE yield [4,5] a K41 energy spectrum, namely, E(k) ∼ k 2 S 2 (k) ≡ k 2 |v(k)| 2 ∼ k −5/3 , if we set d = 3 and y = 4; this has also been verified numerically [6]. Nevertheless, these RG studies have been criticised for a variety of reasons [7,8] such as using a large value for y in a small-y expansion and neglecting an infinity of marginal operators (if y = 4). These criticisms of the approxima-tions used in these studies might well be justified; but they clearly cannot be used to argue that the RFNSE is in itself inappropriate for a theory of turbulence. It is our purpose here to check if, indeed, the RFNSE is a good starting point for such a theory. Specifically we want to test whether structure functions in the RFNSE display the same multiscaling as in the 3dNSE for some value of y; if they do, then we can argue that both equations are in the same universality class and the RFNSE can, defensi...

When a bacterium divides, its cell wall at the division site grows radially inward like the shutter of a camera and guillotines the cell into two halves. The wall is pulled upon from inside by a polymeric ring, which itself shrinks in radius. The ring is made of an intracellular protein FtsZ (filamenting temperature sensitive Z) and thus is called the Z ring. It is not understood how the Z ring generates the required contractile force. We propose a theoretical model and simulate it to show how the natural curvature of the FtsZ filaments and lateral attraction among them may facilitate force generation.

Narrow membrane tubes are commonly pulled out from the surface of phospholipid vesicles using forces applied either through laser or magnetic tweezers or through the action of processive motor proteins. Recent examples have emerged where array of such tubes spontaneously grow from vesicles coated with bioactive cytoskeletal filaments (e.g. FtsZ, microtubule) in the presence GTP/ATP. We show how a soft vesicle deforms as a result of the interplay between its topology, local curvature and the forces due to filament bundles. We present results from Dynamically Triangulated Monte Carlo simulations of a spherical continuum membrane coated with a nematic field (the filaments) and show how the intrinsic curvature of the filaments and their bundling interactions drive membrane tubulation. We predict interesting patterns consisting of large number of nematic defects which accompany tubulation. A common theme emerges that defect locations on vesicle surfaces are hot spots of membrane deformation activity, which could be useful for vesicle origami. Although our equilibrium model is not applicable to the nonequlibrium shape dynamics exhibited by active microtubule coated vesicles, we show that some the features like size dependent vesicle shape can still be understood from our equilibrium model.