Topological Dynamics of Micelles Formed by Geometrically Varied Surfactants.

The molecular architecture of sugar-based surfactants strongly affects their self-assembled structure, i.e., the type of micelles they form, which in turn controls both the dynamics and rheological properties of the system. Here, we report the segmental and mesoscopic structure and dynamics of a series of C16 maltosides with differences in the anomeric configuration and degree of tail unsaturation. Neutron spin-echo measurements showed that the segmental dynamics can be modeled as a one-dimensional array of segments where the dynamics increase with inefficient monomer packing. The network dynamics as characterized by dynamic light scattering show different relaxation modes that can be associated with the micelle structure. Hindered dynamics are observed for arrested networks of worm-like micelles, connected to their shear-thinning rheology, while nonentangled diffusing rods relate to Newtonian rheological behavior. While the design of novel surfactants with controlled properties poses a challenge for synthetic chemistry, we demonstrate how simple variations in the monomer structure can significantly influence the behavior of surfactants.

Extending depolarized DLS measurements to turbid samples.

The application of dynamic light scattering to soft matter systems has strongly profited from advanced approaches such as the so-called modulated 3D cross correlation technique (Mod3D-DLS) that suppress contributions from multiple scattering, and can therefore be used for the characterization of turbid samples. Here we now extend the possibilities of this technique to allow for depolarized light scattering (Mod3D-DDLS) and thus obtain information on both translational and rotational diffusion, which is important for the characterization of anisotropic particles. We describe the required optical design and test the performance of the approach for increasingly turbid samples using well defined anisotropic colloidal models systems. Our measurements demonstrate that 3D-DDLS experiments can be performed successfully for samples with a reduced transmission due to multiple scattering as low as 1%. We compare the results from this approach with those obtained by standard DDLS experiments, and point out the importance of using an appropriate optical design when performing depolarized dynamic light scattering experiments with turbid systems.

Adsorption of Anionic Dyes Using a Poly(styrene-block-4-vinylpyridine) Block Copolymer Organogel.

An organogel was prepared by chemically cross-linking a poly(styrene-block-4-vinylpyridine) diblock copolymer using dibromododecane in dimethylformamide. Analysis of the prominent structure peak in small-angle X-ray scattering along with the results of light scattering and rheological profile suggests the bridging of the spherical micelles to one another to form an interconnected network after gelation. The use of this organogel as a selective adsorbent for removing anionic dyes from individual aqueous dye solutions and in a mixture of cationic and anionic dye solutions has shown more than 90% removal of the anionic dyes within 2 h. The regeneration and reusability studies showed that even after 20 cycles, the adsorption property of the organogel holds extremely well still beyond 90%. These results are indicative of the potential use of poly(styrene-block-4-vinylpyridine) organogel for the anionic ions removal in wastewater treatment.

Deswelling behaviour of ionic microgel particles from low to ultra-high densities.

The swelling of ionic microgel particles is investigated at a wide range of concentrations using a combination of light, X-ray and neutron scattering techniques. We employ a zero-average contrast approach for small-angle neutron scattering experiments, which enables a direct determination of the form factor at high concentrations. The observed particle size initially decreases strongly with the particle concentration in the dilute regime but approaches a constant value at intermediate concentrations. This is followed by a further deswelling at high concentrations above particle overlap. Theory and experiments point at a pivotal contribution of dangling polymer ends to the strong variation in size of ionic microgels, which presents itself mainly through the hydrodynamics properties of the system.

Short- and long-time diffusion and dynamic scaling in suspensions of charged colloidal particles.

We report on a comprehensive theory-simulation-experimental study of collective and self-diffusion in concentrated suspensions of charge-stabilized colloidal spheres. In theory and simulation, the spheres are assumed to interact directly by a hard-core plus screened Coulomb effective pair potential. The intermediate scattering function, fc(q, t), is calculated by elaborate accelerated Stokesian dynamics (ASD) simulations for Brownian systems where many-particle hydrodynamic interactions (HIs) are fully accounted for, using a novel extrapolation scheme to a macroscopically large system size valid for all correlation times. The study spans the correlation time range from the colloidal short-time to the long-time regime. Additionally, Brownian Dynamics (BD) simulation and mode-coupling theory (MCT) results of fc(q, t) are generated where HIs are neglected. Using these results, the influence of HIs on collective and self-diffusion and the accuracy of the MCT method are quantified. It is shown that HIs enhance collective and self-diffusion at intermediate and long times. At short times self-diffusion, and for wavenumbers outside the structure factor peak region also collective diffusion, are slowed down by HIs. MCT significantly overestimates the slowing influence of dynamic particle caging. The dynamic scattering functions obtained in the ASD simulations are in overall good agreement with our dynamic light scattering (DLS) results for a concentration series of charged silica spheres in an organic solvent mixture, in the experimental time window and wavenumber range. From the simulation data for the time derivative of the width function associated with fc(q, t), there is indication of long-time exponential decay of fc(q, t), for wavenumbers around the location of the static structure factor principal peak. The experimental scattering functions in the probed time range are consistent with a time-wavenumber factorization scaling behavior of fc(q, t) that was first reported by Segrè and Pusey [Phys. Rev. Lett. 77, 771 (1996)] for suspensions of hard spheres. Our BD simulation and MCT results predict a significant violation of exact factorization scaling which, however, is approximately restored according to the ASD results when HIs are accounted for, consistent with the experimental findings for fc(q, t). Our study of collective diffusion is amended by simulation and theoretical results for the self-intermediate scattering function, fs(q, t), and its non-Gaussian parameter α2(t) and for the particle mean squared displacement W(t) and its time derivative. Since self-diffusion properties are not assessed in standard DLS measurements, a method to deduce W(t) approximately from fc(q, t) is theoretically validated.

Validity of the Stokes-Einstein Relation in Soft Colloids up to the Glass Transition.

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering, and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q,t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

Lyotropic smectic B phase formed in suspensions of charged colloidal platelets.

Here, we present the first observation of a smectic B (Sm(B)) phase in a system of charged colloidal gibbsite platelets suspended in dimethyl sulfoxide (DMSO). The use of DMSO, a polar aprotic solvent, leads to a long range of the electrostatic Coulomb repulsion between platelets. We believe this to be responsible for the formation of the layered liquid crystalline phase consisting of hexagonally ordered particles, that is, the Sm(B) phase. We support our finding by high-resolution X-ray scattering experiments, which additionally indicate a high degree of ordering in the Sm(B) phase.

Thermal diffusion of a stiff rod-like mutant Y21M fd-virus.

We investigated the thermal diffusion phenomena of a rodlike mutant filamentous fd-Y21M virus in the isotropic phase by means of an improved infrared thermal-diffusion-forced Rayleigh scattering (IR-TDFRS) setup optimized for measurements of slowly diffusing systems. Because this is the first thermal diffusion study of a stiff anisotropic solute, we investigate the influence of the shape anisotropy on the thermal diffusion behavior. The influence of temperature, fd-Y21M concentration, and ionic strength in relation with the thermodiffusion properties is discussed. We characterize and eliminate the effect of these parameters on the absolute diffusion of the rods and show that diffusion determines the behavior of the Soret coefficient because the thermal diffusion coefficient is constant in the investigated regime. Our results indicate that for the thermal diffusion behavior structural changes of the surrounding water are more important than structural changes between the charged macroions. In the investigated temperature and concentration range, the fd-Y21M virus is thermophobic for the low salt content, whereas the solutions with the high salt content change from thermophobic to thermophilic behavior with decreasing temperature. A comparison with recent measurements of other charged soft and biological matter systems shows that the shape anisotropy of the fd-virus becomes not visible in the results.

Pair structure of the hard-sphere Yukawa fluid: an improved analytic method versus simulations, Rogers-Young scheme, and experiment.

We present a comprehensive study of the equilibrium pair structure in fluids of nonoverlapping spheres interacting by a repulsive Yukawa-like pair potential, with special focus on suspensions of charged colloidal particles. The accuracy of several integral equation schemes for the static structure factor, S(q), and radial distribution function, g(r), is investigated in comparison to computer simulation results and static light scattering data on charge-stabilized silica spheres. In particular, we show that an improved version of the so-called penetrating-background corrected rescaled mean spherical approximation (PB-RMSA) by Snook and Hayter [Langmuir 8, 2880 (1992)], referred to as the modified PB-RMSA (MPB-RMSA), gives pair structure functions which are in general in very good agreement with Monte Carlo simulations and results from the accurate but nonanalytical and therefore computationally more expensive Rogers-Young integral equation scheme. The MPB-RMSA preserves the analytic simplicity of the standard rescaled mean spherical (RMSA) solution. The combination of high accuracy and fast evaluation makes the MPB-RMSA ideally suited for extensive parameter scans and experimental data evaluation, and for providing the static input to dynamic theories. We discuss the results of extensive parameter scans probing the concentration scaling of the pair structure of strongly correlated Yukawa particles, and we determine the liquid-solid coexistence line using the Hansen-Verlet freezing rule.

Structures and phase behavior in mixtures of charged colloidal spheres and platelets.

The experimental phase diagram for aqueous mixtures of charged gibbsite platelets and silica spheres is presented. The platelets are 95 nm in diameter, and the diameter ratio between the spheres and the platelets is 0.18. Here the spheres are acting as depletants in the mixtures perturbing the phase behavior of the pure platelet suspensions. An important finding is that a large isotropic/columnar coexistence region has been identified in the phase diagram, which appeared already at low concentrations of the platelets. Microradian X-ray diffraction measurements revealed the structure of the liquid crystalline phases and the orientational order of platelets. An interesting observation is that in the columnar phase the silica spheres are located between the columnar stacks. All samples were in equilibrium because sedimentation did not affect the system because of the relatively small size of the colloidal particles and the charges present at their surfaces.

Long-time dynamics of concentrated charge-stabilized colloids.

Dynamic light scattering was used to study the dynamic structure factor, S(q,t), of suspensions of charged colloidal silica spheres over the full colloidal time range. We show that a dynamic scaling relation for S(q,t) found by Segrè and Pusey [Phys. Rev. Lett. 77, 771 (1996)10.1103/PhysRevLett.77.771] for hard spheres, relating long-time and short-time dynamics, and collective and self-diffusion, also applies to charged colloids up to the freezing concentration. The universality of this scaling is analyzed theoretically. Our experimental data confirm dynamic freezing criteria proposed for the long-time self- and cage-diffusion coefficients, along with a theoretical prediction for the self-diffusion coefficient.

Laparoscopic aided cholecystostomy as a treatment of inspissated bile syndrome.

We report a case of a newborn girl with inspissated bile syndrome (IBS) that did not respond to treatment with oral ursodeoxycholic acid (Ursofalk). A solution was found using laparoscopic aided cholecystostomy with an indwelling catheter for local Ursofalk flushing in the gallbladder and the choledochus. This is the first report of a laparoscopic aided management of IBS without cholecystectomy or exploration of the bile ducts. This minimal invasive approach showed a clear advantage for the patient. There were no complications. The method is recommended in the treatment of IBS.

Unexpected slow near wall dynamics of spherical colloids in a suspension of rods.

In this paper, we will show the influence of an additional rodlike component, that is, fd-virus, on the diffusion of spherical polystyrene colloids close to a wall. The sphere diffusivity normal to the wall, D perpendicular, is strongly affected by the presence of the rods, while the effect on the parallel diffusivity, D||, is less pronounced except in the immediate vicinity of the wall. We show that this observation cannot be explained by describing the effect of the rods as a simple mean field depletion potential alone.

Colloidal dynamics near a wall studied by evanescent wave light scattering: experimental and theoretical improvements and methodological limitations.

The dynamics of colloidal spheres near to a wall is studied with an evanescent wave scattering setup that allows for an independent variation of the components of the scattering wave vector normal and parallel to the wall. The correlation functions obtained with this novel instrumentation are interpreted on the basis of an expression for their short time behavior that includes hydrodynamic interactions between the colloidal spheres and the wall. The combination of the evanescent wave scattering setup and the exact expression for the short time behavior of correlation functions allows for an unambiguous measurement of the particle mobility parallel and normal to the wall by means of light scattering. It is possible to measure the viscous wall drag effect on the dynamics of particles with radii as small as 27 nm, where, however, the method reaches its limits due to the low scattering intensities of such small particles.

Anisotropy of Brownian motion caused only by hydrodynamic interaction with a wall.

The diffusivity of spherical colloidal particles close to a planar hard wall is studied by dynamic light scattering with evanescent illumination. A novel setup allows us to independently vary the scattering vector components parallel Q parallel and normal Q perpendicular to the wall. An expression for the initial decay rate Gamma of the time autocorrelation functions is derived as a function of both Q parallel and Q perpendicular, as well as the penetration depth of the evanescent wave, where hydrodynamic interactions of particles with the wall are included. This makes it possible to study the viscous wall drag effect quantitatively for particles as small as 85 nm in radius.

Many-body hydrodynamic interactions in charge-stabilized suspensions.

In this joint experimental-theoretical work we study hydrodynamic interaction effects in dense suspensions of charged colloidal spheres. Using x-ray photon correlation spectroscopy we have determined the hydrodynamic function H(q), for a varying range of electrosteric repulsion. We show that H(q) can be quantitatively described by means of a novel Stokesian dynamics simulation method for charged Brownian spheres, and by a modification of a many-body theory developed originally by Beenakker and Mazur. Very importantly, we can explain the behavior of H(q) for strongly correlated particles without resorting to the controversial concept of hydrodynamic screening, as was attempted in earlier work by Riese [Phys. Rev. Lett. 85, 5460 (2000)].

Nematic-isotropic spinodal decomposition kinetics of rodlike viruses.

We investigate spinodal decomposition kinetics of an initially nematic dispersion of rodlike viruses. Quench experiments are performed from a flow-stabilized homogeneous nematic state at a high shear rate into the two-phase isotropic-nematic coexistence region at a zero shear rate. We present experimental evidence that spinodal decomposition is driven by orientational diffusion, in accordance with a very recent theory.