Discontinuous nature of the repulsive-to-attractive colloidal glass transition

Rheological study of nanoemulsions with repulsive and attractive interdroplet interactions

Nanoemulsions have adjustable transparency, tunable rheology, high stability, and low sensitivity to changes in pH and temperature, which make them interesting for applications such as low-fat and low-calorie foods. In this research, we study model concentrated nanoemulsions which are stabilized by sodium dodecyl sulfate (SDS). To prepare samples in different structural states, semi-dilute nanoemulsions are prepared at 25% droplet volume fraction (φ), after which evaporating the continuous phase at room temperature leads to concentrated nanoemulsions up to 60% volume fraction. Surfactant concentration is also tuned to induce different interdroplet interactions so that concentrated nanoemulsions in repulsive glass, attractive glass, and gel states are achieved. Rheological properties of nanoemulsions with different structural states are comprehensively studied over a volume fraction range. Utilizing the existing predictive models for (nano)emulsion rheology reveals a more satisfactory prediction for repulsive systems than systems with attractive interactions. In addition, a master curve is constructed for storage and loss moduli of nanoemulsions with different interdroplet interactions. The present work offers control over physicochemical properties of nanoemulsions for design of new food products with enhanced quality and functionality.

Dynamics and microrheology of colloidal clay-polymer glasses and gels: Size-dependent phenomena and re-entrant behavior at early aging times

Colloidal clay Laponite forms a variety of arrested states that display interesting aging behavior. Microrheology has been applied to Laponite-based glasses and gels, but few studies evaluate the influence of probe particle size. In this work, we report the dynamics and microrheology of Laponite-polymer dispersions during aging using passive microrheology with three different probe particle sizes. At early aging times, the neat Laponite dispersion forms an arrested state; the nature of this state (e.g., a repulsive glass or gel) has remained the subject of debate. The addition of polymer retards gelation and melts the arrested state. While this melting has been observed at the macroscale and has been attributed to a re-entrant transition of a repulsive glass to a liquid state, to our knowledge, it has not been observed at the microscale. The delay of the gelation time needed to form an arrested state was found to depend on the polymer concentration and could vary from ∼24 h for neat Laponite to seven days for some Laponite-polymer samples. Significant effects of probe particle sizes are observed from the mean-squared displacement (MSD) curves as small and intermediate probe particles show diffusive motion, while the motion of large particles is restricted. By examining the factor of ⟨Δr² (τ)⟩a, structural heterogeneity can be confirmed through the strong size-dependence displayed. Different MSD trends of probe particles are obtained at longer aging times, but no significant changes occur after 30 days of aging. Our microrheology results also reveal significant effects of probe particle size.

Predicting Solute Diffusivity in Polymers Using Time-Temperature Superposition

We demonstrate a novel application of the time-temperature superposition (TTS) principle to predict solute diffusivity D in glassy polymers using atomistic molecular dynamics simulations. Our TTS approach incorporates the Debye-Waller factor ⟨u²⟩, a measure of solute caging, along with concepts from thermodynamic scaling methods, allowing us to balance contributions to the dynamics from temperature and ⟨u²⟩ using adjustable parameters. Our approach rescales the solute mean-squared displacement curves at several temperatures into a master curve that approximates the diffusive dynamics at a reference temperature, effectively extending the simulation time scale from nanoseconds to seconds and beyond. With a set of "universal" parameters, this TTS approach predicts D with reasonable accuracy in a broad range of polymer/solute systems. Using TTS greatly reduces the computational cost compared to standard MD simulations. Thus, our method offers a means to rapidly and routinely provide order-of-magnitude estimates of D using simulations.

On the dynamically arrested states of equilibrium and non-equilibrium gels: a comprehensive Brownian dynamics study

In this work a systematic study over a wide number of final thermodynamic states for two gel-forming liquids was performed. Such two kind of gel formers are distinguished by their specific interparticle interaction potential. We explored several thermodynamic states determining the thermodynamic, structural and dynamic properties of both liquids after a sudden temperature change. The thermodynamic analysis allows to identify that the liquid with short range attraction and long range repulsion lacks of a stable gas-liquid phase separation liquid, in contrast with the liquid with short range attractions. Thus, although for some thermodynamic states the structural behavior, measured by the static structure factor, is similar to and characteristic of the gel phase, for the short range attractive fluid the gel phase is a consequence of a spinodal decomposition process. In contrast, gelation in the short range attraction and long range repulsion liquid is not due to a phase separation. We also analyze the similarities and differences of the dynamic behavior of both systems through the analysis of the mean square displacement, the self part of the intermediate scattering function, the diffusion coefficient and theαrelaxation time. Finally, using one of the main results of the non-equilibrium self-consistent generalized Langevin equation theory (NE-SCGLE), we determine the dynamic arrest phase diagram in the volume fraction and temperature (φvsT) plane.

Negative resistance for colloids driven over two barriers in a microchannel

When considering the flow of currents through obstacles, one core expectation is that the total resistance of sequential single resistors is additive. While this rule is most commonly applied to electronic circuits, it also applies to other transport phenomena such as the flow of colloids or nanoparticles through channels containing multiple obstacles, as long as these obstacles are sufficiently far apart. Here we explore the breakdown of this additivity for fluids of repulsive colloids driven over two energetic barriers in a microchannel, using real-space microscopy experiments, particle-resolved simulations, and dynamical density functional theory. If the barrier separation is comparable to the particle correlation length, the resistance is highly non-additive, such that the resistance added by the second barrier can be significantly higher or lower than that of the first. Surprisingly, in some cases the second barrier can even add a negative resistance, such that two identical barriers are easier to cross than a single one. We explain this counterintuitive observation in terms of the structuring of particles trapped between the barriers.

Cage dynamics leads to double relaxation of the intermediate scattering function in a binary colloidal system

A system of binary colloids where one fraction of particles is aggregating by forming irreversible bonds and the other fraction of particles only interacts as hard spheres, is simulated using Brownian cluster dynamics. These aggregating species always formed percolating clusters for the case of diffusing hard spheres while for the static case, formation of percolating clusters depended on the fraction of static hard spheres in the system. The dynamics of the hard spheres inside the percolating clusters was studied by restarting the simulation after the kinetics of aggregation was arrested. Two cases were studied, one where the percolated particles moved within the bonds or cage dynamics was allowed and another where the movement within the bonds was not allowed or the cages were static. The hard spheres showed anomalous diffusion in both cases. The mean square displacement showed that for the case of dynamic cages we always had diffusive hard spheres irrespective of the fraction of hard spheres for volume fractions below 0.49. Static cages, depending on the fraction of hard spheres, showed either diffusive or arrested behavior of hard spheres. The intermediate scattering function of only the hard sphere particles showed double relaxation similar to the colloidal glass system for low volume fraction, where the fraction of hard sphere particles was small. For higher fractions we observed only a single stretched exponential. We could differentiate between slow and fast particles for both static and dynamic cages. For the case of static cages the hard spheres were permanently stuck inside the cages while for the case of dynamic cages almost all the hard spheres were moving in and out of the cages.

Apparent strength versus universality in glasses of soft compressible colloids

Microgel colloids, solvent swollen hydrogel particles of microscopic size, are in osmotic equilibrium with their surroundings. This has a profound effect on the behaviour of dense solutions of these polymeric colloids, most notably their ability to swell and deswell depending on the osmotic pressure of the system as a whole. Here we develop a minimal simulation model to treat this intrinsic volume regulation in order to explore the effects this has on the properties of dense solutions close to a liquid-solid transition. We demonstrate how the softness dependent volume regulation of particles gives rise to an apparent change in the fragility of the colloidal glass transition, which can be scaled out through the use of an adjusted volume fraction that accounts for changes in particle size. Moreover, we show how the same model can be used to explain the selective deswelling of soft microgels in a crystalline matrix of harder particles leading to robust crystals free of defects. Our results not only highlight the non-trivial effects of osmotic regulation in governing the apparent physics of microgel suspensions, but also provides a platform to efficiently account for particle deswelling in simulations.

Tracer Transport Probes Relaxation and Structure of Attractive and Repulsive Glassy Liquids

Dynamic coupling of small penetrants to slow, cooperative relaxations within crowded cells, supercooled liquids, and polymer matrices has broad consequences for applications ranging from drug delivery to nanocomposite processing. Interactions between the constituents of these and other disordered media alter the cooperative relaxations, but their effect on penetrant dynamics remains incompletely understood. We use molecular dynamics simulations to show that the motions of hard-sphere tracer particles probe differences in local structure and cooperative relaxation processes in attractive and repulsive glassy liquid matrices with equal bulk packing fractions and long-time diffusivities. Coupling of the tracer dynamics to collective relaxations in each matrix affects the shape of tracer trajectories, which are fractal within the repulsive matrix and more compact in the attractive. These results reveal that the structure of relaxations controls penetrant transport and dispersion in cooperatively relaxing systems and provide insight into dynamical heterogeneity within glassy liquids.

Interplay between Caging and Bonding in Binary Concentrated Colloidal Suspensions

When a liquid becomes dynamically arrested, a gel, a repulsive glass, or an attractive glass state will form. Bonding and caging mechanisms decide their static structures and dynamic properties. To better understand their interplay, the competition between bonding and caging in a binary mixture of polystyrene core/poly( N-isopropylacrylamide) shell (CS) microgels and sulfonated polystyrene (PSS) particles is studied. CS microgels have short-range attraction above the volume phase transition temperature, whereas PSS species experiences relatively long-range electrostatic repulsion. Adding more PSS into the binary mixture will, of course, increase the total effective volume fraction but lead to different properties in gel or glass states. For instance, in gels, it increases the localization length and weakens the gel, whereas in glass, it decreases the localization length and strengthens the glass. This thus implies that the static and dynamic properties of gels are mainly controlled by bonding and those of both repulsive and attractive glasses are governed by caging. On the other hand, increasing the temperature will decrease the effective volume fraction because of the volume phase transition of the CS microgels. A discontinuous repulsive glass-to-liquid-to-gel transition can be observed when the PSS concentration is low, but a continuous repulsive glass-to-gel transition can also be observed with the increase of the PSS concentration. This may hint that glass transition and physical gelation share a similar mechanism, whereas the former has a longer relaxation time.

Cooling-rate induced softening in a colloidal glass

Contrary to crystalline solids, amorphous solids always become softer when vitrifying the melts under higher cooling rates. Understanding this phenomenon is of utmost importance in providing a basis for the mechanical-performance control of amorphous solids. However, the underlying mechanisms leading to this cooling-rate-induced softening of amorphous solids have remained elusive, especially the dynamic reasons are neglected. Here, we use a colloidal glass as the model system to directly study this issue. Shear modulus is used as the representative parameter to monitor the stress-bearing properties of colloidal glass. The space-spanning immobile particles, whose population is sensitive to the cooling rate, are found to make the dominant contribution to the shear modulus. The rapid solidification induced softening of colloidal glass is observed to originate from fewer immobile particles formed at higher cooling rates.

Study of dynamical heterogeneities in colloidal nanoclay suspensions approaching dynamical arrest

The dynamics of aqueous Laponite clay suspensions slow down with increasing sample waiting time (t w ). This behavior, and the material fragility that results, closely resemble the dynamical slowdown in fragile supercooled liquids with decreasing temperature, and are typically ascribed to the increasing sizes of distinct dynamical heterogeneities in the sample. In this article, we characterize the dynamical heterogeneities in Laponite suspensions by invoking the three-point dynamic susceptibility formalism. The average time-dependent two-point intensity autocorrelation and its sensitivity to t w are probed in dynamic light scattering experiments. Distributions of relaxation time scales, deduced from the Kohlrausch-Williams-Watts equation, are seen to widen with increasing t w . The calculated three-point dynamic susceptibility of Laponite suspensions exhibits a peak, with the peak height increasing with evolving t w at fixed volume fraction or with increasing volume fraction at fixed t w , thereby signifying the slowdown of the sample dynamics. The number of dynamically correlated particles, calculated from the peak-height, is seen to initially increase rapidly with increasing t w , before eventually slowing down close to the non-ergodic transition point. This observation is in agreement with published reports on supercooled liquids and hard sphere colloidal suspensions and offers a unique insight into the colloidal glass transition of Laponite suspensions.

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.

Attraction Controls the Inversion of Order by Disorder in Buckled Colloidal Monolayers

We show how including attraction in interparticle interactions reverses the effect of fluctuations in ordering of a prototypical artificial frustrated system. Buckled colloidal monolayers exhibit the same ground state as the Ising antiferromagnet on a deformable triangular lattice, but it is unclear if ordering in the two systems is driven by the same geometric mechanism. By a real-space expansion we find that, for buckled colloids, bent stripes constitute the stable phase, whereas in the Ising antiferromagnet straight stripes are favored. For generic pair potentials we show that attraction governs this selection mechanism, in a manner that is linked to local packing considerations. This supports the geometric origin of entropy in jammed sphere packings and provides a tool for designing self-assembled colloidal structures.

Linking Particle Dynamics to Local Connectivity in Colloidal Gels

Colloidal gels are a prototypical example of a heterogeneous network solid whose complex properties are governed by thermally activated dynamics. In this Letter we experimentally establish the connection between the intermittent dynamics of individual particles and their local connectivity. We interpret our experiments with a model that describes single-particle dynamics based on highly cooperative thermal debonding. The model, in quantitative agreement with experiments, provides a microscopic picture for the structural origin of dynamical heterogeneity in colloidal gels and sheds new light on the link between structure and the complex mechanics of these heterogeneous solids.