Soft Colloidal Systems Research Articles

Softness mapping of the concentration dependence of the dynamics in model soft colloidal systems

The dynamics of a series of soft colloids comprised of polystyrene cores with poly(N-isopropylacrylamide) (PNIPAM) coronas was investigated by diffusing wave spectroscopy (DWS). The modulus of the coronas was varied by changing the cross-link density and we were able to interpret the results within a hard-soft mapping framework. The soft, swellable particle properties were modeled using an extended Flory-Rehner theory and a Hertzian pair potential. Following volume fraction jumps, softness effects on the concentration dependence of dynamics were determined, with a ‘soft colloids make strong glass-forming liquid’-type of behavior observed close to the nominal glass transition volume fraction, φg. Such behavior from the current systems cannot be fully explained by the osmotic deswelling model alone. However, inspired by the soft-hard mapping from Schmiedeberg et al, [Europhys. Lett. 2011, 96(3), 36010] we estimated effective hard-sphere diameters and achieved a successful mapping of the α-relaxation times to a master curve below φg. Above φg, the curves no longer collapse but show strong deviations from a Vogel-Fulcher type of divergence onto soft jamming plateaux. Our results provide evidence that osmotic deswelling itself cannot fully explain the observed dynamics. Softness also plays an important role in the dynamics of soft, concentrated colloids.

A machine learning approach to analyze the structural formation of soft matter via image recognition

ABSTRACT A novel method is developed to analyze the structural formation of colloidal particles based on image recognition via a convolutional neural network (CNN). This makes it possible to analyze various complex structures that are difficult to study using a traditional bond-order parameter analysis. Molecular dynamics simulations on soft colloidal systems are performed in quasi two-dimensional and three-dimensional systems, and the efficiency of the proposed method is demonstrated.

Resolving the internal morphology of core-shell microgels with super-resolution fluorescence microscopy.

We investigate the internal morphology of smart core–shell microgels by super-resolution fluorescence microscopy exploiting a combination of 3D single molecule localization and structured illumination microscopy utilizing freely diffusing fluorescent dyes. This approach does not require any direct chemical labeling and does not perturb the network structure of these colloidal gels. Hence, it allows us to study the morphology of the particles with very high precision. We found that the structure of the core-forming seed particles is drastically changed by the second synthesis step necessary for making the shell, resulting in a core region with highly increased dye localization density. The present work shows that super-resolution microscopy has great potential with respect to the study of soft colloidal systems.

Fouling minimization at membranes having a 3D surface topology with microgels as soft model colloids

Cross-flow ultrafiltration of colloidal suspensions is mainly limited by the accumulation of retained particles adjacent to the membrane surface. The hydrodynamic conditions in the feed channel strongly affect the mass transfer rate of colloids from the membrane surface to the bulk and is therefore an important control measure. A common approach for enhancing mass transfer through hydrodynamics is to modify the geometry of the feed channel. In flat-sheet membrane configurations, feed spacers are typically installed, which function as turbulence promoters. In tubular and hollow-fiber membranes implementation of feed spacers is difficult, thus different strategies are sought for. Here we study the effect of a spiralling surface pattern (Helix) on the hydrodynamics and flux performance of tubular ultrafiltration membrane operated in cross-flow mode at inside-out configuration. We apply flow Magnetic Resonance Imaging (MRI) to visualize the flow-field inside the lumen and asses the length dependency of the permeate flux. We combine this information with classical filtration experiments at a wider range of operational parameters. Microgel suspensions were used as a model for soft colloidal systems. The transmembrane flux for the Helix as compared to the reference tubular membrane only increases when non-laminar flow developed inside the lumen channel. Yet the onset of transient non-laminar flow conditions starts at much lower Reynolds numbers of above 1700 for the Helix, while the regular tube shows a transition at high velocities and Re>3150. In consecutive filtration experiments, the Helix shows less flux decline as well. This improved fouling resistance is based on less flux decline at the end of the Helix membrane. Flow-MRI proves that the helical corrugations impose swirling flow patterns with contributions normal to the membrane surface opposing the deposition of microgels onto the membrane surface.

A simple analysis of Brillouin spectra from opaque liquids and its application to aqueous suspensions of poly-N-isopropylacrylamide microgel particles

Brillouin spectroscopy is a powerful technique to probe the viscoelastic properties of materials. However, the phenomenon of multiple scattering makes getting information from opaque liquids quite difficult, thus limiting the use of this spectroscopy. In this paper we present a new method that greatly simplifies the problem of analyzing Brillouin spectra affected by multiple scattering from samples of moderate opacity. Our approach is based on the observation that multiple-scattered contributions broaden the spectrum acquired in external backscattering geometry, while preserving in the external side the information related to internally backscattered light. The new strategy avoids unnecessary approximations and requires minimum numerical effort to extract physical information. Here, we show the results of two Brillouin light scattering experiments performed on prototypical hard and soft colloidal systems. First, measurements on latex suspensions as a function of depth are used to validate the method and to derive new relations between the back-scattered and multiple-scattered components of the Brillouin spectrum. Second, measurements on poly-N-isopropylacrylamide (PNIPAM) microgels in water as a function of temperature are used as a testing ground to demonstrate the method's capabilities. Our analysis confirms that sound waves are extremely sensitive to the volume-phase transition of thermoresponsive particles. The presented approach, however, shows that a marked increase of attenuation is accompanied by only a moderate decrease of sound velocity. The study revises the viscoelastic properties of PNIPAM suspensions; more generally, it provides a new guideline in the characterization of moderately opaque media and fosters new theoretical investigations.

Fragility and Strength in Nanoparticle Glasses.

Glasses formed from nano- and microparticles form a fascinating testing ground to explore and understand the origins of vitrification. For atomic and molecular glasses, a wide range of fragilities have been observed; in colloidal systems, these effects can be emulated by adjusting the particle softness. The colloidal glass transition can range from a superexponential, fragile increase in viscosity with increasing density for hard spheres to a strong, Arrhenius-like transition for compressible particles. However, the microscopic origin of fragility and strength remains elusive, both in the colloidal and in the atomic domains. Here, we propose a simple model that explains fragility changes in colloidal glasses by describing the volume regulation of compressible colloids in order to maintain osmotic equilibrium. Our simple model provides a microscopic explanation for fragility, and we show that it can describe experimental data for a variety of soft colloidal systems, ranging from microgels to star polymers and proteins. Our results highlight that the elastic energy per particle acts as an effective fragility order parameter, leading to a universal description of the colloidal glass transition.

Effects of boundaries on structure formation in low-dimensional colloid model systems near the liquid-solid-transition in equilibrium and in external fields and under shear

A brief review focusing on low-dimensional colloidal model systems is given describing both simulation studies and complementary experiments, elucidating the interplay between phase behavior, geometric structures, and transport phenomena. These studies address the response of these very soft colloidal systems to perturbations such as uniform or uniaxial compression, laser fields, randomly quenched disorder, and shear deformation caused by moving boundaries. Binary hard-disk mixtures are studied by Monte Carlo simulation, to investigate ordering on surfaces or in monolayers, modeling the effect of a substrate by an external potential. By weak external laser fields the miscibility of the mixture can be controlled, and the underlying mechanism (laser-induced demixing) is clarified. The stability of various space-filling structures is discussed only for the case where no laser fields are present.Hard spheres interacting with repulsive screened Coulomb or dipolar interaction confined in 2D and 3D narrow constrictions are investigated by Brownian Dynamics simulation. With respect to the structural behavior, it is found that layers or planes throughout the microchannel are formed. The arrangement of the particles is disturbed by diffusion, and can also be modified by an external driving force causing a density gradient along the channel. Then the number of layers or planes gets reduced, adjusting to the density gradient, and this self-organized change of order also shows up in the particle velocities. The experimental work that is reviewed here addresses dipolar colloidal particles confined by gravity on a solid substrate on which a set of pinning sites has been randomly distributed. The dynamics of the system is studied by tracking the trajectories of individual particles, and it is found that the mean square displacements of particles that are nearest neighbors of pinned particles are strongly affected by these defects. The influence of the pinning sites on the order and microscopic mechanism of phase transitions in two dimensions is investigated.

Effects of confinement and external fields on structure and transport in colloidal dispersions in reduced dimensionality

In this work, we focus on low-dimensional colloidal model systems, via simulation studies and also some complementary experiments, in order to elucidate the interplay between phase behavior, geometric structures and transport properties. In particular, we try to investigate the (nonlinear!) response of these very soft colloidal systems to various perturbations: uniform and uniaxial pressure, laser fields, shear due to moving boundaries and randomly quenched disorder. We study ordering phenomena on surfaces or in monolayers by Monte Carlo computer simulations of binary hard-disk mixtures, the influence of a substrate being modeled by an external potential. Weak external fields allow a controlled tuning of the miscibility of the mixture. We discuss the laser induced de-mixing for the three different possible couplings to the external potential. The structural behavior of hard spheres interacting with repulsive screened Coulomb or dipolar interaction in 2D and 3D narrow constrictions is investigated using Brownian dynamics simulations. Due to misfits between multiples of the lattice parameter and the channel widths, a variety of ordered and disordered lattice structures have been observed. The resulting local lattice structures and defect probabilities are studied for various cross sections. The influence of a self-organized order within the system is reflected in the velocity of the particles and their diffusive behavior. Additionally, in an experimental system of dipolar colloidal particles confined by gravity on a solid substrate we investigate the effect of pinning on the dynamics of a two-dimensional colloidal liquid. This work contains sections reviewing previous work by the authors as well as new, unpublished results. Among the latter are detailed studies of the phase boundaries of the de-mixing regime in binary systems in external light fields, configurations for shear induced effects at structured walls, studies on the effect of confinement on the structures and defect densities in three-dimensional systems, the effect of confinement and barriers on two-dimensional flow and diffusion, and the effect of pinning sites on the diffusion.

Modeling the formation of ordered nano-assemblies comprised by dendrimers and linear polyelectrolytes: The role of Coulombic interactions

Models of mixtures of peripherally charged dendrimers with oppositely charged linear polyelectrolytes in the presence of explicit solvent are studied by means of molecular dynamics simulations. Under the influence of varying strength of electrostatic interactions, these systems appear to form dynamically arrested film-like interconnected structures in the polymer-rich phase. Acting like a pseudo-thermodynamic inverse temperature, the increase of the strength of the Coulombic interactions drive the polymeric constituents of the mixture to a gradual dynamic freezing-in. The timescale of the average density fluctuations of the formed complexes initially increases in the weak electrostatic regime reaching a finite limit as the strength of electrostatic interactions grow. Although the models are overall electrically neutral, during this process the dendrimer/linear complexes develop a polar character with an excess charge mainly close to the periphery of the dendrimers. The morphological characteristics of the resulted pattern are found to depend on the size of the polymer chains on account of the distinct conformational features assumed by the complexed linear polyelectrolytes of different length. In addition, the length of the polymer chain appears to affect the dynamics of the counterions, thus affecting the ionic transport properties of the system. It appears, therefore, that the strength of electrostatic interactions together with the length of the linear polyelectrolytes are parameters to which these systems are particularly responsive, offering thus the possibility for a better control of the resulted structure and the electric properties of these soft-colloidal systems.

Single crystal growth and anisotropic crystal-fluid interfacial free energy in soft colloidal systems

We measure the anisotropy of the crystal-fluid interfacial free energy in soft colloidal systems. A temperature gradient is used to direct crystal nucleation and control the growth of large single crystals in order to achieve well-equilibrated crystal-fluid interfaces. Confocal microscopy is used to follow both the growth process and the equilibrium crystal-fluid interface at the particle scale: heterogeneous crystal nucleation, the advancing interface, and the stationary equilibrium interface. We use the measured growth velocity to determine the chemical potential difference between crystal and fluid phases. Well-equilibrated, large crystal-fluid interfaces are then used to determine the interfacial free energy and its anisotropy directly from thermally excited interface fluctuations. We find that while the measured average interfacial free energy is in good agreement with values found in simulations, the anisotropy is significantly larger than simulation values. Finally, we investigate the effect of impurities on the advancing interface. We determine the critical force needed to overcome impurity particles from the local interface curvature.

Swelling and rheology of thermoresponsive gradient copolymer micelles

Poly(laurylmethacrylate-block-styrene-block-laurylmethacrylate) gradient copolymers (PLMA-PS-PLMA) in a selective solvent for PLMA, self-assemble into spherical colloidal micelles. The structure and the dynamics of the solutions are investigated as a function of temperature and concentration using small angle X-ray scattering (SAXS) and rheology. When the temperature is increased, the micellar cores shrink and the coronas increase due to the gradual solubilization of the domains where S and LMA segments are mixed. The swelling of the micelles is perfectly correlated to the increase of the viscosity. The variations of the low-shear viscosity over a wide range of concentrations and temperatures are rescaled onto a universal curve and compared to those for other soft colloidal systems like star polymers, hairy particles and dendrimer solutions.

Counterion distribution and many-body interaction in charged dendrimer solutions

Coarse-grained molecular dynamics simulations are performed to study the structural formation, counterion distribution and effective interaction on charged dendrimers in an aqueous solution. In particular, the many-body effects in triplet systems are clarified. The conformation of a dendrimer molecule depends on the size of counterions, and the counterion distribution can be scaled by the gyration radius of a dendrimer molecule. Two-body and three-body interactions have been numerically investigated, and the results indicate both similarities and differences between dendrimer solutions and other soft colloidal systems, such as star polymer solutions.

Glassy colloidal systems

This review focuses on recent developments in the theoretical, numerical and experimental study of slow dynamics in colloidal systems, with a particular emphasis on the glass transition phenomenon. Colloidal systems appear to be particularly suited for tackling the general problem of dynamic arrest, since they show a larger flexibility compared to atomic and molecular glasses because of their size and the possibility of manipulating the physical and chemical properties of the samples. Indeed, a wealth of new effects, not easily observable in molecular liquids, have been predicted and measured in colloidal systems. The slow dynamic behavior of three classes of colloidal suspension is reviewed – hard colloids, short-range attractive colloids and soft colloidal systems – selecting the model systems among the most prominent candidates for grasping the essential features of dynamic arrest. Emphasis is on the possibility of performing a detailed comparison between experimental data and theoretical predictions based on the mode coupling theory of the glass transition. Finally, the importance of understanding the system's kinetic arrest phase diagram, i.e. the regions in phase space where disordered arrested states can be expected, is stressed. When and how these states are kinetically stabilized with respect to the ordered lowest free energy phases is then examined in order to provide a framework for interpreting and developing new ideas in the study of new materials. Glassy colloidal systemsAll authorsF. Sciortino & P. Tartagliahttps://doi.org/10.1080/00018730500414570Published online:19 February 2007Table Download CSVDisplay Table