[Show abstract][Hide abstract] ABSTRACT: In this article we report on a study of the near-wall dynamics of suspended
colloidal hard spheres over a broad range of volume fractions. We present a
thorough comparison of experimental data with predictions based on a virial
approximation and simulation results. We find that the virial approach
describes the experimental data reasonably well up to a volume fraction of
$\phi=0.25$ which provides us with a fast and non-costly tool for the analysis
and prediction of Evanescent Wave DLS data. Based on this we propose a new
method to assess the near-wall self-diffusion at elevated density. Here, we
qualitatively confirm earlier results [Michailidou et al., Phys. Rev. Lett.,
2009, 102, 068302], which indicate that many-particle hydrodynamic interactions
are diminished by the presence of the wall at increasing volume fractions as
compared to bulk dynamics. Beyond this finding we show that this diminishment
is different for the particle motion normal and parallel to the wall.
[Show abstract][Hide abstract] ABSTRACT: Soft colloids-such as polymer-coated particles, star polymers, block-copolymer micelles, microgels-constitute a broad class of materials where microscopic properties such as deformability and penetrability of the particle play a key role in tailoring their macroscopic properties which is of interest in many technological areas. The ability to access these microscopic properties is not yet demonstrated despite its great importance. Here we introduce novel DNA-coated colloids with star-shaped architecture that allows accessing the above local structural information by directly visualizing their intramolecular monomer density profile and arm's free-end locations with confocal fluorescent microscopy. Compression experiments on a two-dimensional hexagonal lattice formed by these macromolecular assemblies reveal an exceptional resistance to mutual interpenetration of their charged corona at pressures approaching the MPa range. Furthermore, we find that this lattice, in a close packing configuration, is surprisingly tolerant to particle size variation. We anticipate that these stimuli-responsive materials could aid to get deeper insight in a wide range of problems in soft matter, including the study and design of biomimetic lubricated surfaces.
[Show abstract][Hide abstract] ABSTRACT: Flow-induced instabilities that lead to non-uniform stationary flow profiles have been observed in many different soft-matter systems. Two types of instabilities that lead to banded stationary states have been identified, which are commonly referred to as gradient- and vorticity-banding. The molecular origin of these instabilities is reasonably well understood. A third type of instability that has been proposed phenomenologically [Europhys. Lett., 1986, 2, 129 and Phys. Rev. E, 1995, 52, 4009] is largely unexplored. Essential to this “Shear-gradient Concentration Coupling” (SCC-) instability is a mass flux that is induced by spatial gradients of the shear rate. A possible reason that this instability has essentially been ignored is that the molecular origin of the postulated mass flux is not clear, and no explicit expressions for the shear-rate and concentration dependence of the corresponding transport coefficient exist. It is therefore not yet known what types of flow velocity- and concentration-profiles this instability gives rise to. In this paper, an expression for the transport coefficient corresponding to the shear-gradient induced mass flux is derived in terms of the shear-rate dependent pair-correlation function, and Brownian dynamics simulations for hard-spheres are presented that specify the shear-rate and concentration dependence of the pair-correlation function. This allows to explicitly formulate the coupled advection–diffusion equation and an equation of motion for the suspension flow velocity. The inclusion of a non-local contribution to the stress turns out to be essential to describe the SCC-banding transition. The coupled equations of motion are solved numerically, and flow- and concentration-profiles are discussed. It is shown that the SCC-instability occurs within the glass state at sufficiently small shear rates, leading to a banded flow-profile where one of the bands is non-flowing.
[Show abstract][Hide abstract] ABSTRACT: We discuss the interfacial migration due to Marangoni forces of clay particles within an oil droplet that is immersed in water, which is relevant for the formation kinetics of Pickering emulsions. Hydrophilic (MMT) and hydrophobic (OMT) clays are studied, where the hydrophilic clay particles adsorb in the oil–water interface, contrary to the hydrophobic clays. The feasibility of an “image–time correlation technique” is discussed, in order to probe the local interfacial migration velocities of clay particles in the oil droplet. Here, correlation functions are constructed from time-resolved images, in order to quantify the local migration of clay particles. Correlation functions are measured at different waiting times, that is, the time after formation of the droplet. The initial decay rate and the baseline of these correlation functions depend on the waiting time in a qualitatively different way for the two clays, which is attributed to the different interfacial migration behavior for the hydrophilic, adsorbing clays and the nonadsorbing, hydrophobic clays as a result of the Marangoni effect.
The Journal of Physical Chemistry C 10/2014; 118(42):24803-24810. DOI:10.1021/jp508333u · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this article we present a study of the structure and rheology of mixed suspensions of Montmorillonite clay platelets and Ludox TMA silica spheres at pH 5, 7 and 9. Using cryogenic transmission electron microscopy (cryo-TEM) we probe the changes in the structure of the montmorillonite suspensions induced by changing the pH and by adding silica particles. Using oscillatory and transient rheological measurements we examine the changes in storage modulus and yield stress of the montmorillonite suspensions upon changing the pH and adding silica particles. Cryo-TEM images reveal that changes in pH have a significant effect on the structure of the suspensions, which can be related to the change in charge of the edges from positive at pH 5 to negative at higher pH. Furthermore, at pH 7, the cryo-TEM images show indications of a micro phase separation between clay and silica particles. The addition of silica leads to lowering of the storage modulus and yield stress, which we connect to the structural changes of the suspension.
The Journal of Physical Chemistry B 09/2014; 118(40). DOI:10.1021/jp504217m · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this article we extend recent experimental developments [Rogers et al., Phys. Rev. Lett., 2012, 109, 098305] by providing a suitable theoretical framework for the derivation of exact expressions for the first cumulant (initial decay rate) of the correlation function measured in Evanescent Wave Dynamic Light Scattering (EWDLS) experiments. We focus on a dilute suspension of optically anisotropic spherical Brownian particles diffusing near a planar hard wall. In such a system, translational and rotational diffusion are hindered by hydrodynamic interactions with the boundary which reflects the flow incident upon it, affecting the motion of colloids. The validity of the approximation by the first cumulant for moderate times is assessed by juxtaposition to Brownian dynamics simulations, and compared with experimental results. The presented method for the analysis of experimental data allows the determination of penetration-depth-averaged rotational diffusion coefficients of spherical colloids at low density.
[Show abstract][Hide abstract] ABSTRACT: The response of concentrated dispersions of charged colloids to low-frequency electric fields is governed by field-induced inter-colloidal interactions resulting from the polarization of electric double layers and the layer of condensed ions, association and dissociation of condensed ions, as well as hydrodynamic interactions through field-induced electro-osmotic flow. The phases and states that can be formed by such field-induced interactions are an essentially unexplored field of research. Experiments on concentrated suspensions of rod-like colloids (fd-virus particles), within the isotropic-nematic phase coexistence region, showed that a number of phases/states are induced, depending on the field amplitude and frequency [Soft Matter, 2010, 6, 273]. In particular, a dynamical state is found where nematic domains form and melt on a time scale of the order of seconds. We discuss the microscopic origin of this dynamical state, which is attributed to the cyclic, electric-field induced dissociation and association of condensed ions. A semi-quantitative theory is presented for the dynamics of melting and formation of nematic domains, including a model for the field-induced dissociation/association of condensed ions. The resulting equation of motion for the orientational order parameter is solved numerically for parameters complying with the fd-virus system. A limit-cycle is found, with a cycling-time that diverges at the transition line in the field-amplitude versus frequency plane where the dynamical state first appears, in accord with experimental findings.
[Show abstract][Hide abstract] ABSTRACT: We review the effect of shear flow on the phase behavior and structure
of colloidal dispersions with increasing degree of complexity. We
discuss dispersions of colloidal rods, stiff living polymers like
wormlike micelles, and colloidal platelets. In addition, a review is
presented on sheared binary dispersions. For all cases we discuss the
interplay between thermodynamic instabilities and hydrodynamic
The European Physical Journal Special Topics 11/2013; 222(11):2739-2755. DOI:10.1140/epjst/e2013-02055-2 · 1.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The purpose of this study was to validate a new method to investigate the polymerization shrinkage vectors of composite during light curing and to evaluate the overall utility and significance of the technique.
An optical instrument was developed to measure the location and direction of the polymerization shrinkage strain vectors of dental composite during light curing using a particle tracking method with computer vision. The measurement system consisted of a CCD color camera, a lens and a filter, and software for multi-particle tracking. A universal hybrid composite (Z250, 3M ESPE, St. Paul MN, USA) was molded into thin disk-shaped specimens (un-bonded and bonded) or filled into a cavity within a tooth slab (bonded). The composite surface was coated with fluorescent particles prior to light curing. The images of the fluorescent particles were stored at 2 frames/s for 10min, and the movements of the particles on the composite surface were tracked with computer vision during curing. The polymerization shrinkage strain vectors as a function of time and location were analyzed. The volume shrinkage of the composite was also measured for comparison.
The linear and volume shrinkage of the composite at 10min were 0.75 (0.12)% and 2.26 (0.18)%, respectively. The polymerization shrinkage vectors were directed toward the center of the specimen and were isotropic in all directions when the composite was allowed to shrinkage freely without bonding. In contrast, the shrinkage vectors were directed toward the bonding surface and were anisotropic when the composite was bonded to a fixed wall. The regional displacement vectors of composite in a tooth cavity were dependent on the location, depth and time.
The new instrument was able to measure the regional linear shrinkage strain vectors over an entire surface of a composite specimen as a function of time and location. Therefore, this instrument can be used to characterize the shrinkage behaviors for a wide range of commercial and experimental visible-light-cure materials in relation to the composition, boundary condition and cavity geometry.
Dental materials: official publication of the Academy of Dental Materials 10/2013; 29(12). DOI:10.1016/j.dental.2013.09.011 · 4.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microemulsion-based synthesis of inorganic nanoparticles is a versatile method, where many parameters can be varied to control the properties of products. In this work, solely by modifying the morphology of nonionic reverse microemulsions from spheres to cylinders, while keeping the water-to-surfactant ratio (W/S), salt concentration, volume fraction of surfactants, and temperature constant, we successfully controlled the variation of the length of PbCO3 nanorods with the same diameter. Short nanorods formed in the spherical microemulsions, while long nanorods are produced in the cylindrical microemulsions. In addition, an amorphous-to-amorphous transformation from spherical nanoparticles to nanorods is observed and nanorods self-assemble into one-dimensional chains in the microemulsion solution.
[Show abstract][Hide abstract] ABSTRACT: There is an abundance of experiments and theories on the glass transition of colloidal systems consisting of spherical particles. Much less is known about possible glass transitions in suspensions of rod-like colloids. In this study we present observations of a glass transition in suspensions of very long and thin rod-like, highly charged colloids. We use as a model system fd-virus particles (a DNA strand covered with coat proteins) at low ionic strength, where thick electric double layers are present. Structural arrest as a result of particle-caging is observed by means of dynamic light scattering. The glass-transition concentration is found to be far above the isotropic–nematic coexistence region. The morphology of the system thus consists of nematic domains with different orientations. Below the glass-transition concentration the initial morphology with large shear-aligned domains breaks up into smaller domains, and equilibrates after typically 50–100 hours. We quantify the dynamics of the transitional and the equilibrated texture by means of image time-correlation. A sharp increase of relaxation times of image time-correlation functions is found at the glass-transition concentration. The texture dynamics thus freezes at the same concentration where structural arrest occurs. We also observe a flow instability, which sets in after very long waiting times (typically 200–300 hours), depending on the rod concentration, which affects the texture morphology.
[Show abstract][Hide abstract] ABSTRACT: Self-assembly of nanoparticles triggered by attractive depletion forces presents a versatile pathway to build nanostructural superlattices directly in solution. In this work, the synthesis of square-like lead chromate (PbCrO4) nanoplatelets is described and their assembly into well-defined stacks by introducing various types of micelles as depletion agents is studied. The kinetics of the reversible assembling process in solution is probed by light scattering and the depletion-induced self-assembly of the nanoplatelets is investigated by transmission electron microscopy. For nonionic surfactants, rod-like micelles are found to be more efficient to produce self-assembly than sphere-like micelles, in accordance with an increased depletion force. Adding ionic surfactants leads additionally to a segregation of PbCrO4 rod reaction-byproducts.
[Show abstract][Hide abstract] ABSTRACT: We recently observed glass-like transitions in suspensions of the
charged fibrous viruses (fd), which are very long (880 nm) and thin (7
nm), stiff, rod-like DNA macromolecules. These charged fibrous viruses
are used as a good model system for charged colloidal rods, since they
carry a large surface charge through dissociation of negatively charged
coat proteins. We report here on a glass transition that leads to the
structural arrest as well as freezing of a texture dynamics. For the
determination of glass transition concentration, we have performed,
separately, the dynamicimage time correlation spectroscopy for
texture-dynamics, and dynamic light scattering to probe the structure
arrest in particle-dynamics.
[Show abstract][Hide abstract] ABSTRACT: We report on the observation of a glass transition in suspensions of very long and thin, highly charged colloidal rods (fd-virus particles). Structural particle arrest is found to occur at a low ionic strength due to caging of the charged rods in the potential setup by their neighbors through long-ranged electrostatic interactions. The relaxation time of density fluctuations as probed by dynamic light scattering is found to diverge within a small concentration range. The rod concentration where structural particle arrest occurs is well within the full chiral-nematic state, far beyond the two-phase isotropic-nematic coexistence region. The morphology of the suspensions thus consists of nematic domains with various orientations. We quantify the dynamics of the resulting texture with image-time correlation spectroscopy. Interestingly, the decay times of image correlation functions are found to diverge in a discontinuous fashion at the same concentration of charged rods where structural particle arrest is observed. At the glass-transition concentration, we thus find both structural arrest and freezing of the texture dynamics.
[Show abstract][Hide abstract] ABSTRACT: The thermal diffusion behavior of dilute solutions of very long and thin, charged colloidal rods (fd-virus particles) is studied using a holographic grating technique. The Soret coefficient of the charged colloids is measured as a function of the Debye screening length, as well as the rod-concentration. The Soret coefficient of the fd-viruses increases monotonically with increasing Debye length, while there is a relatively weak dependence on the rod-concentration when the ionic strength is kept constant. An existing theory for thermal diffusion of charged spheres is extended to describe the thermal diffusion of long and thin charged rods, leading to an expression for the Soret coefficient in terms of the Debye length, the rod-core dimensions, and the surface charge density. The thermal diffusion coefficient of a charged colloidal rod is shown to be accurately represented, for arbitrary Debye lengths, by a superposition of spherical beads with the same diameter of the rod and the same surface charge density. The experimental Soret coefficients are compared with this and other theories, and are contrasted against the thermal diffusion behaviour of charged colloidal spheres.
[Show abstract][Hide abstract] ABSTRACT: We determined the phase boundary of an ideal rod-sphere mixture consisting of fd-virus, which is an established model system for mono-disperse colloidal rods, and density matched mono-disperse polystyrene beads employing diffuse wave spectroscopy. The low volume fraction of fd needed to induce a phase separation at relatively low ionic strength exemplifies the fact that slender rods are very effective depletion agents. Confocal microscopy showed that stable clusters are formed during phase separation. Relaxation after shear deformation of these clusters showed that the phase separation is gas-liquid-like and that the interfacial tension involved is very low as in colloid-polymer mixtures.
[Show abstract][Hide abstract] ABSTRACT: There is currently no experimental technique available to probe spatially resolved rotational diffusion of nanoparticles in the vicinity of a wall. We present the first experimental study of rotational diffusion of small spherical colloids, using dynamic evanescent wave scattering. A setup is used where the wave vector components parallel and perpendicular to the wall can be varied independently, and an expression is derived for the first cumulant of the intensity correlation function in VH evanescent wave geometry for optically anisotropic spheres. The experimental results are in agreement with theoretical predictions that take particle-wall hydrodynamic interactions into account.
[Show abstract][Hide abstract] ABSTRACT: A new near-wall velocimetry technique is proposed, based on evanescent wave dynamic light scattering, which allows for the measurement of near-wall velocity profile (characterized by an apparent slip velocity and a shear rate) with a resolution of tens of nanometers. A full theoretical expression of the correlation function is derived for the case of linear flow with negligible Brownian motion. The technique is demonstrated for latex spheres dispersed in water-glycerol mixtures.
The European Physical Journal E 07/2012; 35(7):62. DOI:10.1140/epje/i2012-12062-5 · 2.18 Impact Factor