Repulsive Colloidal Glasses Research Articles

Interparticle interaction-dependent jamming in colloids: insights into glass transition and morphology modulation during rapid evaporation-induced assembly.

Understanding the role of interparticle interactions in jamming phenomena is essential for gaining insights into the intriguing glass transition behavior observed in atomic and molecular systems. In this study, we investigate the jamming behavior of colloids with tunable interparticle interactions during evaporation-induced assembly (EIA). By manipulating the interaction among charged colloids using cationic polyethyleneimine (PEI) through electro-sorption and subsequent free polymer induced repulsion, we observe distinct jamming behavior in silica colloids during EIA, depending on the interparticle interactions. Silica colloids with strong repulsive interactions exhibit a repulsive colloidal glass state with a volume fraction of silica colloids in supraparticle ϕ ∼ 0.70. On the other hand, PEI-mediated attractive interactions among silica colloids lead to an attractive colloidal glass phase with a significantly lower ϕ ∼ 0.43. Free polymer induced repulsion of colloids at higher PEI concentration once again results in a repulsive glassy state with ϕ ∼ 0.61. Furthermore, we revealed that interparticle interactions not only influence the jamming behavior but also play a significant role in shaping the morphology of self-assembled structures during EIA, and the assembled structure undergoes a morphological reentrant transition from a doughnut-like shape to a spherical form and again back to a doughnut-like configuration. Jamming-dependent evolution of micropores and dynamics of the confined PEI have been probed using positron annihilation lifetime spectroscopy (PALS) and broadband dielectric spectroscopy (BDS). PALS reveals distinct variations in the micropores of the supraparticles with different PEI loadings, confirming the impact of jamming on the evolution of the micropores within the supraparticles. BDS measurements uncover non-monotonic dynamics of PEI molecules confined in the evolved pore network. It is revealed that the reentrant jamming behavior of colloids, modulated by PEI, holds profound significance for the long-term stability of supraparticles.

Microscopic pathways for stress relaxation in repulsive colloidal glasses.

Residual stresses are well-known companions of all glassy materials. They affect and, in many cases, even strongly modify important material properties like the mechanical response and the optical transparency. The mechanisms through which stresses affect such properties are, in many cases, still under study, and their full understanding can pave the way to a full exploitation of stress as a primary control parameter. It is, for example, known that stresses promote particle mobility at small length scales, e.g., in colloidal glasses, gels, and metallic glasses, but this connection still remains essentially qualitative. Exploiting a preparation protocol that leads to colloidal glasses with an exceptionally directional built-in stress field, we characterize the stress-induced dynamics and show that it can be visualized as a collection of "flickering," mobile regions with linear sizes of the order of ≈20 particle diameters (≈2 μm here) that move cooperatively, displaying an overall stationary but locally ballistic dynamics.

Sub-diffusive dynamics and two-step yielding in dense thermo-responsive microgel glasses

We have prepared dense colloidal suspensions of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgel particles with different volume fractions by subjecting the particles to different amounts of osmotic compression. Structural ordering and dynamics have been investigated using light scattering and yielding behaviour by oscillatory rheology. At room temperature, suspensions have found to exhibit a glassy state with the microgel particle size to be smaller than that determined under dilute conditions (i.e. deswelling of particles under osmotic compression). Quite interestingly, we observed (a) sub-diffusive mean square displacement (MSD) at short times and (b) two-step yielding (i.e. loss modulus, G″(ω), exhibiting two peaks as a function of shear strain amplitude, γ o ) in the glassy state. These findings are interpreted in terms of the overlap of dangling polymer chains between shells of the neighbouring PNIPAM microgel particles which are known to have a core-shell structure. At elevated temperatures, the two-step yielding behaviour of the glasses has been observed to change into a single-step yielding. With further increase in temperature, it has been found that PNIPAM microgel glasses melt into a liquid-like order and the behaviour of MSD changes from sub-diffusive to diffusive type. The results are discussed in light of those reported for attractive and repulsive colloidal glasses.

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

In purely repulsive colloidal systems a glass transition can be reached by increasing the particle volume fraction beyond a certain threshold. The resulting glassy state is governed by configurational cages which confine particles and restrict their motion. A colloidal glass may also be formed by inducing attractive interactions between the particles. When attraction is turned on in a repulsive colloidal glass a re-entrant solidification ensues. Initially, the repulsive glass melts as free volume in the system increases. As the attraction strength is increased further, this weakened configurational glass gives way to an attractive glass in which motion is hindered by the formation of physical bonds between neighboring particles. In this paper, we study the transition from repulsive-to-attractive glasses using three-dimensional imaging at the single-particle level. We show how the onset of cage weakening and bond formation is signalled by subtle changes in local structure. We then demonstrate the discontinuous nature of the solid-solid transition, which is marked by a critical onset at a threshold bonding energy. Finally, we highlight how the interplay between bonding and caging leads to complex and heterogeneous dynamics at the microscale.

Field induced anisotropic cooperativity in a magnetic colloidal glass.

The translational dynamics of a repulsive colloidal glass-former is probed by time-resolved X-ray Photon Correlation Spectroscopy. In this dense dispersion of charge-stabilized and magnetic nanoparticles, the interaction potential can be tuned, from quasi-isotropic to anisotropic by applying an external magnetic field. This powerful control parameter finely tunes the anisotropy of the intricate energy landscape in the colloidal glass-former, which is seen here as a new tunable model-system to probe the dynamical heterogeneities at the approach of the glass transition. Both structural and dynamical anisotropies are reported on interparticle lengthscales associated with highly anisotropic cooperativity, almost two orders of magnitude larger in the field direction than in the perpendicular direction and in zero field.

Thermoresponsive polymer-based magneto-rheological (MR) composites as a bridge between MR fluids and MR elastomers

A new class of magnetorheological (MR) material is reported that bridges the gap between conventional MR fluids and MR elastomers. The key point is the use of a thermoresponsive polymer-based suspending medium in the formulation of the MR material whose rheological properties can be externally controlled through changes in the temperature. Under appropriate conditions, it is even possible to induce a thermally driven “liquid-to-solid” transition in the carrier medium. As a result, when the suspending medium is in the “liquid” phase, the MR composite behaves as a conventional MR fluid exhibiting a large MR effect (approx. 1000%). In contrast, when the suspending medium is in the “solid” phase and exhibits an apparent yield stress, the whole material behaves as a MR elastomer where the iron microparticles are entrapped in a polymer network. Two different examples are reported in this communication; on the one hand a triblock copolymer solution is used as a suspending medium that evolves towards a lyotropic liquid crystalline cubic phase upon heating. On the other hand, a concentrated microgel dispersion is used that reaches maximum packing as the temperature decreases forming a repulsive colloidal glass. The rheological properties of the materials are evaluated under dynamic oscillatory and steady simple shear to ascertain the MR effect.

A re-entrant glass transition in colloidal disks with adsorbing polymer

A re-entrant colloidal glass transition has been observed in dispersions of a model disk shaped colloid, laponite®, with the addition of an adsorbing low molecular weight polymer, poly(ethylene oxide) (PEO). In basic solutions laponite® forms a repulsive colloidal glass. With the addition of low molecular weight PEO, we observe a retardation in formation of the glass, and the system behaves as a viscous fluid. Upon aging, a glassy phase re-forms, with the time scale depending on the concentration of polymer. For samples with polymer concentration high enough to saturate the laponite® surface, we believe this arrested state to be an attractive glass where excess PEO chains in the suspension lead to a weak interparticle attraction. For the polymer molecular weight we considered, all the samples eventually age to form a disordered state. We combine our results with previous work on laponite®–PEO dispersions to propose qualitative state diagrams with attractive glass, repulsive glass, liquid and gel regimes as a function of polymer molecular weight, polymer concentration, and aging time. To our knowledge, the progression of repulsive glass–liquid–attractive glass states has not been previously observed in colloids with adsorbing polymers.

Time-resolved viscoelastic properties during structural arrest and aging of a colloidal glass

Evolution of the energy landscape during physical aging of glassy materials can be understood from the frequency and strain dependence of the shear modulus but the nonstationary nature of these systems frustrates investigation of their instantaneous underlying properties. Using a series of time-dependent measurements we systematically reconstruct the frequency and strain dependence as a function of age for a repulsive colloidal glass undergoing structural arrest. In this manner, we are able to unambiguously observe the structural relaxation time, which increases exponentially with sample age at short times. The yield stress varies logarithmically with time in the arrested state, consistent with recent simulation results, whereas the yield strain is nearly constant in this regime. Strikingly, the frequency dependence at fixed times can be rescaled onto a master curve, implying a simple connection between the aging of the system and the change in the frequency dependent modulus.

Physical aging and relaxation of residual stresses in a colloidal glass following flow cessation

Dilute Laponite suspensions in water at low salt concentration form repulsive colloidal glasses which display physical aging. This phenomenon is still not completely understood and, in particular, little is known about the connection between the flow history, as a determinant of the initial state of the system, and the subsequent aging dynamics. Using a stress controlled rheometer, we perform stress jump experiments to observe the elastic component of the flow stress that remains on cessation of flow or flow quenching. We investigate the connection between the dynamics of these residual stresses and the rate of physical aging upon quenching from different points on the steady-state flow curve. Quenching from high rates produces a fluid state, G″>G′, with small, fast relaxing residual stresses and rapid, sigmoidal aging of the complex modulus. Conversely, quenching from lower shear rates produces increasingly jammed states featuring slowly relaxing stresses and a slow increase of the complex modulus with system age. Flow cessation from a fixed shear rate with varying quench durations shows that slower quenches produce smaller residual stresses at short times which relax at long times by smaller extents, by comparison with faster quenches. These smaller stresses are correlated with a higher modulus but slower physical aging of the system. The characteristic time for the residual stress relaxation scales inversely with the quench rate. This implies a frustrated approach to any ideal stress-free state that succinctly reflects the frustrated nature of these glassy systems.

Probing heterogeneous dynamics of a repulsive colloidal glass by time resolved x-raycorrelation spectroscopy

We present here experimental results obtained by time resolved x-ray correlationspectroscopy, revealing the heterogeneous nature of the dynamics down to thenanoscale in a dense dispersion of magnetic nanoparticles. The dynamicalsusceptibility, which quantifies the amplitude of the dynamical fluctuations, isinvestigated in a repulsive colloidal glass. Its value is an order of magnitudesmaller than the ones found in gels and is length scale independent in theQ-range of the x-ray scattering experiment.

Low temperature behavior and glass line in the symmetrical colloidal electrolyte

We report on the low temperature behavior of the colloidal electrolyte by means of molecular dynamics simulations, where the electrostatic interactions were modeled using effective screened interactions. As in previous works, we have found a region of gas-liquid coexistence located in the low T-low rho region. At temperatures much lower than the critical one, the system cannot reach equilibrium, that is, the gas-liquid transition is arrested. Two different mechanisms have been identified to cause arrest: crowding at intermediate T values, associated with the crossing point between the binodal and the glass line, and a gelationlike arrest at very low T. To test the latter, the dynamics of the colloidal electrolyte near this crossing point has been computed and compared to the universal predictions of the ideal mode-coupling theory. As in other glass-forming liquids, we found good agreement between this mean field theory and the dynamics of this complex system, although it fails just at the transition. Interestingly, in this region we found that the dynamics of this system is driven mainly by the steric interactions, showing all the typical properties of a repulsive colloidal glass. Finally, the isodiffusivity lines show that in this system with short-range attractions, there is no reentrant glass phenomenon as opposed to monocomponent attractive systems.

Rotational arrest in a repulsive colloidal glass

The rotational dynamics of aqueous colloidal dispersions of magnetic nanoparticles isexamined whilst increasing the volume fraction up to the formation of a macroscopic solid.The freezing of the rotation is studied by magneto-optical relaxation experiments andrelated to small-angle neutron-scattering results showing that the structure, albeit imposedby the Coulombic repulsion, remains amorphous because of the size polydispersity. Thevolume fraction at which the freezing occurs is noticeably dependent on the salinity of thesuspension. The latter point is discussed together with the microscopic mechanism of thearrest.

Mode-coupling theory of colloids with short-range attractions

Within the framework of the mode-coupling theory of super-cooledliquids, we investigate new phenomena in colloidal systems on approachto their glass transitions. When the inter-particle potentialcontains an attractive part, besides the usual repulsive hard core,two intersecting liquid-glass transition lines appear, one of whichextends to low densities, while the other one, at high densities,shows a re-entrant behaviour. In the glassy region a newtype of transition appears between two different types ofglasses. The complex phenomenology can be described in terms of higherorder glass transition singularities. The various glass phases arecharacterized by means of their viscoelastic properties.The glass driven by attractions has been associatedwith particle gels, andthe other glass is the well known repulsive colloidal glass. Thesecorrespondences, in association with the new predictions of glassybehaviour, mean that such phenomena may be expected in colloidal systemswith, for example, strong depletion or other short-ranged attractivepotentials.

The vibrational motions of particle gels

A simple discussion of the nature of repulsive and attractive glasses is given, and results presented in the framework of idealized mode coupling theory (MCT). The attractive glass is considered to be equivalent to the particle gel state observed in colloidal science, and the repulsive glass is equivalent to the dense colloidal glass that has recently been studied in colloidal science. Given the equivalence between a particle gel and a glass, it is possible to use MCT to calculate the dynamical structure factor, and thereby the spectrum of all vibrational motions in the glass. We present such spectra for a more and less dense particle gel state, and interpret the motions predicted by the theory. It is pointed out that such motions should resolve between what we call the particle gel, and the traditional repulsive colloidal glass, and as such are perhaps a most promising manner in which to study particle gels in the future.