82 research outputs found
Dynamical heterogeneity in aging colloidal glasses of Laponite
Glasses behave as solids due to their long relaxation time; however the
origin of this slow response remains a puzzle. Growing dynamic length scales
due to cooperative motion of particles are believed to be central to the
understanding of both the slow dynamics and the emergence of rigidity. Here, we
provide experimental evidence of a growing dynamical heterogeneity length scale
that increases with increasing waiting time in an aging colloidal glass of
Laponite. The signature of heterogeneity in the dynamics follows from dynamic
light scattering measurements in which we study both the rotational and
translational diffusion of the disk-shaped particles of Laponite in suspension.
These measurements are accompanied by simultaneous microrheology and
macroscopic rheology experiments. We find that rotational diffusion of
particles slows down at a faster rate than their translational motion. Such
decoupling of translational and orientational degrees of freedom finds its
origin in the dynamic heterogeneity since rotation and translation probe
different length scales in the sample. The macroscopic rheology experiments
show that the low frequency shear viscosity increases at a much faster rate
than both rotational and translational diffusive relaxation times.Comment: 12 pages, 5 figures, Accepted in Soft Matter 201
On phase behavior and dynamical signatures of charged colloidal platelets
We investigate the competition between anisotropic excluded-volume and
repulsive electrostatic interactions in suspensions of thin charged colloidal
discs, by means of Monte-Carlo simulations and dynamical characterization of
the structures found. We show that the original intrinsic anisotropy of the
electrostatic potential between charged platelets, obtained within the
non-linear Poisson-Boltzmann formalism, not only rationalizes the generic
features of the complex phase diagram of charged colloidal platelets such as
Gibbsite and Beidellite clays, but also predicts the existence of novel
structures. In addition, we find evidences of a strong slowing down of the
dynamics upon increasing density.Comment: 6 pages, 6 Figure
Experimental study of out of equilibrium fluctuations in a colloidal suspension of Laponite using optical traps
This work is devoted to the study of displacement fluctuations of
micron-sized particles in an aging colloidal glass. We address the issue of the
validity of the fluctuation dissipation theorem (FDT) and the time evolution of
viscoelastic properties during aging of aqueous suspensions of a clay (Laponite
RG) in a colloidal glass phase. Given the conflicting results reported in the
literature for different experimental techniques, our goal is to check and
reconcile them using \emph{simultaneously} passive and active microrheology
techniques. For this purpose we measure the thermal fluctuations of micro-sized
brownian particles immersed in the colloidal glass and trapped by optical
tweezers. We find that both microrheology techniques lead to compatible results
even at low frequencies and no violation of FDT is observed. Several
interesting features concerning the statistical properties and the long time
correlations of the particles are observed during the transition
Aging of rotational diffusion in colloidal gels and glasses
We study the rotational diffusion of aging Laponite suspensions for a wide
range of concentrations using depolarized dynamic light scattering. The
measured orientational correlation functions undergo an ergodic to non-ergodic
transition that is characterized by a concentration-dependent
ergodicity-breaking time. We find that the relaxation times associated with
rotational degree of freedom as a function of waiting time, when scaled with
their ergodicity-breaking time, collapse on two distinct master curves. These
master curves are similar to those previously found for the translational
dynamics; The two different classes of behavior were attributed to colloidal
gels and glasses. Therefore, the aging dynamics of rotational degree of freedom
provides another signature of the distinct dynamical behavior of colloidal gels
and glasses.Comment: 12 pages, 7 figure
Active motion of tangentially driven polymers in periodic array of obstacles
One key question about transport of active polymers within crowded environments is how spatial order of obstacles influences their conformation and dynamics when compared to disordered media. To this end, we computationally investigate the active transport of tangentially driven polymers with varying degrees of flexibility and activity in two-dimensional square lattices of obstacles. Tight periodic confinement induces notable conformational changes and distinct modes of transport for flexible and stiff active filaments. It leads to caging of low activity flexible polymers inside the inter-obstacle pores while promoting more elongated conformations and enhanced diffusion for stiff polymers at low to moderate activity levels. The migration of flexible active polymers occurs via hopping events, where they unfold to move from one cage to another, similar to their transport in disordered media. However, in ordered media, polymers are more compact and their long-time dynamics is significantly slower. In contrast, stiff chains travel mainly in straight paths within periodic inter-obstacle channels while occasionally changing their direction of motion. This mode of transport is unique to periodic environment and leads to more extended conformation and substantially enhanced long-time dynamics of stiff filaments with low to moderate activity levels compared to disordered media. At high active forces, polymers overcome confinement effects and move through inter-obstacle pores just as swiftly as in open spaces, regardless of the spatial arrangement of obstacles. We explain the center of mass dynamics of semiflexible polymers in terms of active force and obstacle packing fraction by developing an approximate analytical theory.</p
Calculation of Four Point Correlation Function of Logarithmic Conformal Field Theory Using AdS/CFT Correspondence
We use the correspondence between scalar field theory on AdS and induced
conformal field theory on its boundary to calculate correlation functions of
logarithmic conformal field theory in arbitrary dimensions.Our calculations
utilize the newly proposed method of nilpotent weights.We derive expressions
for the four point function assuming a generic interaction termComment: 7 pages, no figure
Dynamical steady-states of active colloids interacting via chemical fields
We study the dynamical steady-states of a monolayer of chemically active self-phoretic colloids as a function of packing fraction and self-propulsion speed by means of Brownian dynamics simulations. We focus on the case that a chemical field induces competing attractive positional and repulsive orientational interactions. Analyzing the distribution of cluster size and local density as well as the hexatic order parameter, we distinguish four distinct dynamical states which include collapsed, active gas, dynamical clustering, and motility-induced phase-separated states. The long-range chemical field-induced interactions shift the onset of motility-induced phase separation (MIPS) to very low packing fractions at intermediate self-propulsion speeds. We also find that the fraction of particles in the largest clusters is a suitable order parameter characterizing the dynamical phase transitions from an active gas or dynamical clustering steady-state to a phase-separated state upon increase of the packing fraction. The order parameter changes discontinuously when going from an active gas to a MIPS-like state at intermediate self-propulsion speeds, whereas it changes continuously at larger activities where the system undergoes a transition from a dynamical clustering state to MIPS-like state
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