Physics Commons^™

Mechanical Response Of Active Gels, Tanniemola B. Liverpool, M. Cristina Marchetti, J-F. Joanny, J. Prost

Physics - All Scholarship

We study a model of an active gel of cross-linked semiflexible filaments with additional active linkers such as myosin II clusters. We show that the coupling of the elasticity of the semiflexible filaments to the mechanical properties of the motors leads to contractile behavior of the gel, in qualitative agreement with experimental observations. The motors, however, soften the zero frequency elastic constant of the gel. When the collective motor dynamics is incorporated in the model, a stiffening of the network at high frequencies is obtained. The frequency controlling the crossover between low and high frequency network elasticity is estimated in …

Enhanced Diffusion And Ordering Of Self-Propelled Rods, Aparna Baskaran, M. Cristina Marchetti

Physics - All Scholarship

Starting from a minimal physical model of self propelled hard rods on a substrate in two dimensions, we derive a modified Smoluchowski equation for the system. Self -propulsion enhances longitudinal diffusion and modifies the mean field excluded volume interaction. From the Smoluchowski equation we obtain hydrodynamic equations for rod concentration, polarization and nematic order parameter. New results at large scales are a lowering of the density of the isotropic-nematic transition and a strong enhancement of boundary effects in confined self-propelled systems.

Complex Spontaneous Flows And Concentration Banding In Active Polar Films, Luca Giomi, M. Cristina Marchetti, Tanniemola B. Liverpool

Physics - All Scholarship

We study the dynamical properties of active polar liquid crystalline films. Like active nematic films, active polar films undergo a dynamical transitions to spontaneously flowing steady-states. Spontaneous flow in polar fluids is, however, always accompanied by strong concentration inhomogeneities or "banding" not seen in nematics. In addition, a spectacular property unique to polar active films is their ability to generate spontaneously oscillating and banded flows even at low activity. The oscillatory flows become increasingly complicated for strong polarity.

Plasticity In Current-Driven Vortex Lattices, Panayotis Benetatos, M. Cristina Marchetti

Physics - All Scholarship

We present a theoretical analysis of recent experiments on current-driven vortex dynamics in the Corbino disk geometry. This geometry introduces controlled spatial gradients in the driving force and allows the study of the onset of plasticity and tearing in clean vortex lattices. We describe plastic slip in terms of the stress-driven unbinding of dislocation pairs, which in turn contribute to the relaxation of the shear, yielding a nonlinear response. The steady state density of free dislocations induced by the applied stress is calculated as a function of the applied current and temperature. A criterion for the onset of plasticity at …

Poisson-Bracket Approach To The Dynamics Of Bent-Core Molecules, William Kung, M. Cristina Marchetti

Physics - All Scholarship

We generalize our previous work on the phase stability and hydrodynamic of polar liquid crystals possessing local uniaxial

C1v-symmetry to biaxial systems exhibiting local C2v -symmetry. Our work is motivated by the recently discovered examples of thermotropic biaxial nematic liquid crystals comprising bent-core mesogens, whose molecular structure is characterized by a non-polar body axis (n) as well as a polar axis (p) along the bisector of the bent mesogenic core which is coincident with a large, transverse dipole moment. The free energy for this system differs from that of biaxial nematic liquid crystals in that it contains terms violating the …

Depinning And Plasticity Of Driven Disordered Lattices, M. Cristina Marchetti

Physics - All Scholarship

We review in these notes the dynamics of extended condensed matter systesm, such as vortex lattices in type-II superconductors and charge density waves in anisotropic metals, driven over quenched disorder. We focus in particular on the case of strong disorder, where topological defects are generated in the driven lattice. In this case the repsonse is plastic and the depinning transition may become discontinuous and hysteretic.

Nematic And Polar Order In Active Filament Solutions, A. Ahmadi, Tanniemola B. Liverpool, M. Cristina Marchetti

Physics - All Scholarship

Using a microscopic model of interacting polar biofilaments and motor proteins, we characterize the phase diagram of both homogeneous and inhomogeneous states in terms of experimental parameters. The polarity of motor clusters is key in determining the organization of the filaments in homogeneous isotropic, polarized and nematic states, while motor-induced bundling yields spatially inhomogeneous structures.

Hydrodynamics Of Polar Liquid Crystals, William Kung, M. Cristina Marchetti, Karl Saunders

Physics - All Scholarship

Starting from a microscopic definition of an alignment vector proportional to the polarization, we discuss the hydrodynamics of polar liquid crystals with local

C1v-symmetry. The free energy for polar liquid crystals differs from that of nematic liquid crystals (D1h) in that it contains terms violating the n −n symmetry. First we show that these Z2-odd terms induce a general splay instability of a uniform polarized state in a range of parameters. Next we use the general Poissonbracket formalism to derive the hydrodynamic equations of the system in the polarized state. The structure of the linear hydrodynamic modes confirms the existence …

Hydrodynamics Of Self-Propelled Hard Rods, Aparna Baskaran, M. Cristina Marchetti

Physics - All Scholarship

Motivated by recent simulations and by experiments on aggregation of gliding bacteria, we study a model of the collective dynamics of self-propelled hard rods on a substrate in two dimensions. The rods have finite size, interact via excluded volume and their dynamics is overdamped by the interaction with the substrate. Starting from a microscopic model with non-thermal noise sources, a continuum description of the system is derived. The hydrodynamic equations are then used to characterize the possible steady states of the systems and their stability as a function of the particles packing fraction and the speed of self propulsion.

Patterned Geometries And Hydrodynamics At The Vortex Bose Glass Transition, M. Cristina Marchetti, David R. Nelson

Physics - All Scholarship

Patterned irradiation of cuprate superconductors with columnar defects allows a new generation of experiments which can probe the properties of vortex liquids by confining them to controlled geometries. Here we show that an analysis of such experiments that combines an inhomogeneous Bose glass scaling theory with the hydrodynamic description of viscous flow of vortex liquids can be used to infer the critical behavior near the Bose glass transition. The shear viscosity is predicted to diverge as

|T − TBG|−z at the Bose glass transition, with z ≃

6 the dynamical critical exponent.

Viscoelasticity From A Microscopic Model Of Dislocation Dynamics, M. Cristina Marchetti, Karl Saunders

Physics - All Scholarship

It is shown that the dynamics of a two-dimensional crystal with a finite concentration of dislocations, as well as vacancy and interstitial defects, is governed by the hydrodynamic equations of a viscoelastic medium. At the longest length scales the viscoelasticity is described by the simplest Maxwell model, whose shear and compressional relaxation times are obtained in terms of microscopic quantities, including the density of free dislocations. At short length scales, bond orientational order effects become important and lead to wavevector dependent corrections to the relaxation times.

Interstitials, Vacancies And Dislocations In Flux-Line Lattices: A Theory Of Vortex Crystals, Supersolids And Liquids, M. Cristina Marchetti, Leo Radzihovsky

Physics - All Scholarship

We study a three dimensional Abrikosov vortex lattice in the presence of an equilibrium concentration of vacancy, interstitial and dislocation loops. Vacancies and interstitials renormalize the long-wavelength bulk and tilt elastic moduli. Dislocation loops lead to the vanishing of the long-wavelength shear modulus. The coupling to vacancies and interstitials - which are always present in the liquid state - allows dislocations to relax stresses by climbing out of their glide plane. Surprisingly, this mechanism does not yield any further independent renormalization of the tilt and compressional moduli at long wavelengths. The long wavelength properties of the resulting state are formally …

Organization And Instabilities Of Entangled Active Polar Filaments, Tanniemola B. Liverpool, M. Cristina Marchetti

Physics - All Scholarship

We study the dynamics of an entangled, isotropic solution of polar filaments coupled by molecular motors which generate relative motion of the filaments in two and three dimensions. We investigate the stability of the homogeneous state for constant motor concentration taking into account excluded volume and entanglement. At low filament density the system develops a density instability, while at high filament density entanglement effects drive the instability of orientational fluctuations.

Rheology Of Active Filament Solutions, Tanniemola B. Liverpool, M. Cristina Marchetti

Physics - All Scholarship

We study the viscoelasticity of an active solution of polar biofilaments and motor proteins. Using a molecular model, we derive the constitutive equations for the stress tensor in the isotropic phase and in phases with liquid crystalline order. The stress relaxation in the various phases is discussed. Contractile activity is responsible for a spectacular difference in the viscoelastic properties on opposite sides of the order-disorder transition.