Dynamics and complex formation in charged and uncharged Ficoll70 solutions

We apply pulsed-field-gradient NMR (PFG NMR) technique to measure the translational diffusion for both uncharged and charged polysaccharide (Ficoll70) in water. Analysis of the data indicate that NMR signal attenuation above a certain packing fraction can be adequately fitted with a bi-exponential function. The self-diffusion measurements show also that the Ficoll70, an often-used compact, spherical polysucrose molecule, is itself non-ideal, exhibiting signs of both softness and attractive interactions in the form of a stable suspension consisting of monomers and clusters. Further, we can quantify the fraction of monomer and cluster. This work strengthens the picture of the existence of a bound water layer within and around a porous Ficoll70 particle.Comment: 6 page

Combining Diffusion NMR and Small-Angle Neutron Scattering Enables Precise Measurements of Polymer Chain Compression in a Crowded Environment

The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of “crowding” proteins in cells, colloid-polymer mixtures, and nanoparticle “fillers” in polymer solutions and melts. In this Letter, we study the effect of spherical inert nanoparticles (which we refer to as “crowders”) on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gradient NMR and small-angle neutron scattering (SANS), respectively. The diffusion coefficients exhibit a plateau below a characteristic polymer concentration, which we identify as the overlap threshold concentration c⋆. Above c⋆, in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. The radius of gyration obtained from SANS in the crossover regime changes linearly with an increase in polymer concentration, and must be extrapolated to c⋆ in order to obtain the radius of gyration of an individual polymer chain. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. There is significant chain compression, however, when the crowder size is much smaller than the polymer radius gyration

Contrasting the dynamics of elastic and non-elastic deformations across an experimental colloidal Martensitic transition

We present a framework to segregate the roles of elastic and non-elastic deformations in the examination of real-space experiments of solid-solid Martensitic transitions. The Martensitic transformation of a body-centred-tetragonal(BCT) to a body-centred-orthorhombic(BCO) crystal structure has been studied in a model system of micron-scale ionic microgel colloids. Non-affine fluctuations, i.e., displacement fluctuations that do not arise from purely elastic(affine) deformations, are detected in particle configurations acquired from the experiment. Tracking these fluctuations serves as a highly sensitive tool in signaling the onset of the Martensitic transition and precisely locating particle rearrangements occurring at length scales of a few particle diameters. Particle rearrangements associated with non-affine displacement modes become increasingly favorable during the transformation process. The nature of the displacement fluctuation modes that govern the transformation are shown to be different from those predominant in an equilibrium crystal. We show that BCO crystallites formed through shear may, remarkably, co-exist with those resulting from local rearrangements within the same sample

Characterization of dynamics and internal structure of a mixed-surfactant wormlike micellar system using NMR and rheometry

We use complementary experiments—proton NMR diffusometry and relaxometry, deuterium NMR lineshapes, and rheometry—to construct a comprehensive picture of the microscopic structure of a mixed-surfactant wormlike micellar system composed of a zwitterionic surfactant and an anionic surfactant in brine. In this system, at some surfactant concentrations, the time for micellar breaking and recombination sb is not small compared with the micellar reptation time sR, weakening the condition to obtain a stress relaxation function with just one relaxation time at long times. FromNMRrelaxometry, we determine the overlap concentration. Deuterium NMR spectral lineshapes indicate the presence of a wide angular distribution in the orientational order. NMR diffusometry and rheology probe different timescales and yield complementary information indicating polymer-like behaviour at the corresponding lengthscales. Via NMR, surfactant diffusion coefficients are seen to decrease with increasing diffusion time, consistent with restricted diffusion within a reptating micelle. At the same time, comparison of measurements with protonated and deuterated surfactants strongly suggests that the measured short and long time diffusion coefficients correspond to intra-micellar and micellar diffusion, respectively. Fitting the diffusion results to a simple model, the average end-to-end micellar distance is estimated to be in the 1 mm range and only weakly dependent on concentration. The water diffusion measurements, on the other hand, imply a high degree of water structuring at the micellar surface. We also find that the wormlike micelles obeyed simple polymer-like scaling behaviors, with a crossover from Zimm-like (diffusion) to Rouse-like (rheology) exponents

Self organization of exotic oil-in-oil phases driven by tunable electrohydrodynamics

Self organization of large-scale structures in nature - either coherent structures like crystals, or incoherent dynamic structures like clouds - is governed by long-range interactions. In many problems, hydrodynamics and electrostatics are the source of such long-range interactions. The tuning of electrostatic interactions has helped to elucidate when coherent crystalline structures or incoherent amorphous structures form in colloidal systems. However, there is little understanding of self organization in situations where both electrostatic and hydrodynamic interactions are present. We present a minimal two-component oil-in-oil model system where we can control the strength and lengthscale of the electrohydrodynamic interactions by tuning the amplitude and frequency of the imposed electric field. As a function of the hydrodynamic lengthscale, we observe a rich phenomenology of exotic structure and dynamics, from incoherent cloud-like structures and chaotic droplet dynamics, to polyhedral droplet phases, to coherent droplet arrays

Nature of an Electric-Field-Induced Colloidal Martensitic Transition

We study the properties of a solid-solid close-packed to body-centered tetragonal transition in a colloidal suspension via fluorescence confocal laser scanning microscopy, in three dimensions and in real space. This structural transformation is driven by a subtle competition between gravitational and electric dipolar field energy, the latter being systematically varied via an external electric field. The transition threshold depends on the local depth in the colloidal sediment. Structures with order intermediate between close-packed and body-centered tetragonal were observed, with these intermediate structures also being stable and long lived. This is essentially a colloidal analogue of an ‘‘atomiclevel’’ interfacial structure. We find qualitative agreement with theory (based purely on energetics). Quantitative differences can be attributed to the importance of entropic effects

Polymer conformation and dynamics in crowded environments: A combined diffusion NMR and small-angle neutron scattering study

The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of “crowding” proteins in cells, colloid-polymer mixtures, and nanoparticle “fillers” in polymer solutions and melts. In this talk, I will present a study of the effect of spherical inert nanoparticles (which we refer to as “crowders”) on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gra- dient NMR and small-angle neutron scattering (SANS), respectively. In addition, the role of enthalpic crowder- crowder interactions on the crowding process is unknown: we can control this by varying charge on the crowder particle. Below a characteristic polymer concentration, which we identify as the overlap threshold concentration c⋆, the diffusion coefficients exhibit a plateau. Above c⋆, in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. We find that crowder charge only weakly affects polymer size and dynamics in the crowding limit, but that local macromolecular mobility depends strongly on molecular flexibility

Two Experimental Tests of the Halperin-Lubensky-Ma Effect at the Nematic-Smectic-A Phase Transition

We have conducted two quantitative tests of predictions based on the Halperin-Lubensky-Ma (HLM) theory of fluctuation-induced first-order phase transitions. First, we explore the effect of an external magnetic field on the nematic-smectic-A (NA) transition in a liquid crystal. Second, we examine the dependence of the first-order discontinuity as a function of mixture concentration in pure 8CB and three 8CB-10CB mixtures. We find the first quantitative evidence for deviations from the HLM theory.Comment: 4 pages, 2 figure

Clusters in sedimentation equilibrium for an experimental hard-sphere-plus-dipolar Brownian colloidal system

In this work, we use structure and dynamics in sedimentation equilibrium, in the presence of gravity, to examine, $via$ confocal microscopy, a Brownian colloidal system in the presence of an external electric field. The zero field equation of state (EOS) is hard sphere without any re-scaling of particle size, and the hydrodynamic corrections to the long-time self-diffusion coefficient are quantitatively consistent with the expected value for hard spheres. Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number $Pe_g$, and dipolar strength $\Lambda$ are of order unity. In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength $\Lambda \sim 1$. The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes