Accounting for effective interactions among charged microgels.

We introduce a theoretical approach to describe structural correlations among charged permeable spheres at finite particle concentrations. This theory explicitly accounts for correlations among microions and between microions and macroions and allows for the proposal of an effective interaction among macroions that successfully captures structural correlations observed in poly-N-isopropyl acrylamide microgel systems. In our description the bare charge is fixed and independent of the microgel size, the microgel concentration, and the ionic strength, which contrasts with results obtained using linear response approximations, where the bare charge needs to be adapted to properly account for microgel correlations obtained at different conditions.

Impact of volume transition on the net charge of poly-N-isopropyl acrylamide microgels.

We explore the electrostatic properties of poly-N-isopropyl acrylamide microgels in dilute, quasi-de-ionized dispersions and show that the apparent net charge of these thermosensitive microgels is an increasing function of their size, the size being conveniently varied by temperature. Our experimental results obtained in a combination of light scattering, conductivity, and mobility experiments are consistent with those obtained in Poisson-Boltzmann cell model calculations, effectively indicating that upon shrinking the number of counterions entrapped within the microgels increases. Remarkably, this behavior shows that the electrostatic energy per particle remains constant upon swelling or deswelling the microgel, resulting in a square root dependence of the net charge on the particle radius.

Co-nonsolvency of PNiPAM at the transition between solvation mechanisms.

We investigate the co-nonsolvency of poly-N-isopropyl acrylamide (PNiPAM) in different water-alcohol mixtures and show that this phenomenon is due to two distinct solvation contributions governing the phase behavior of PNiPAM in the water-rich and alcohol-rich regime respectively. While hydrophobic hydration is the predominant contribution governing the phase behavior of PNiPAM in the water-rich regime, the mixing contributions governing the phase behavior of classical polymer solutions determine the phase behavior of PNiPAM in the alcohol-rich regime. This is evidenced by distinct scaling relations denoting the energetic state of the aqueous medium as a key parameter for the phase behavior of PNiPAM in the water-rich regime, while the volume fractions of respectively water, alcohol and PNiPAM become relevant parameters in the alcohol-rich regime. Adding alcohol to water decreases the energetics of the aqueous medium, which gradually suppresses hydrophobic hydration, while adding water to alcohol decreases the solvent quality. Consequently, PNiPAM is insoluble in the intermediate range of solvent composition, where neither hydrophobic hydration nor the mixing contributions prevail. This accounts for the co-nonsolvency phenomenon observed for PNiPAM in water-alcohol mixtures.

Hydrophobic hydration of poly-N-isopropyl acrylamide: a matter of the mean energetic state of water.

The enthalpically favoured hydration of hydrophobic entities, termed hydrophobic hydration, impacts the phase behaviour of numerous amphiphiles in water. Here, we show experimental evidence that hydrophobic hydration is strongly determined by the mean energetics of the aqueous medium. We investigate the aggregation and collapse of an amphiphilic polymer, poly-N-isopropyl acrylamide (PNiPAM), in aqueous solutions containing small amounts of alcohol and find that the thermodynamic characteristics defining the phase transitions of PNiPAM evolve relative to the solvent composition at which the excess mixing enthalpy of the water/alcohol mixtures becomes minimal. Such correlation between solvent energetics and solution thermodynamics extends to other mixtures containing neutral organic solutes that are considered as kosmotropes to induce a strengthening of the hydrogen bonded water network. This denotes the energetics of water as a key parameter controlling the phase behaviour of PNiPAM and identifies the excess mixing enthalpy of water/kosmotrope mixtures as a gauge of the kosmotropic effect on hydrophobic assemblies.

Rheology and structural arrest of casein suspensions.

The rheology of milk powder suspensions is investigated up to very high concentrations, where structural arrest occurs. The main component of the milk powder investigated is casein, so that the suspensions can be regarded as casein suspensions. Four concentration regimes are identified. For effective casein volume fractions less than 0.54 the concentration dependence of the zero-shear viscosity is similar to that of hard-sphere suspensions. However, due to the elastic deformation of the caseins, the viscosity does not diverge at the hard sphere glass transition. In the volume-fraction range of 0.55-0.61 the viscosity exhibits a surprisingly weak dependence on concentration. The shape of the curve of the shear viscosity versus concentration deviates from hard sphere behavior in an unusual way, due to the observation of a region of almost constant viscosity. This concentration regime is followed by a regime where the viscosity steeply increases, eventually diverging at an effective volume fraction of 0.69. Frequency dependent rheology and diffusing wave spectroscopy measurements indicate that the suspensions are jammed for volume fractions above 0.69. Finally we found the concentration dependence of the relative zero-shear viscosity of casein suspensions to be very similar with the one of the micro-gels at volume fractions below 0.50 and above 0.55, which are know to shrink above a certain volume fraction, due to osmotic stress.

Dynamic arrest in charged colloidal systems exhibiting large-scale structural heterogeneities.

Suspensions of charged liposomes are found to exhibit typical features of strongly repulsive fluid systems at short length scales, while exhibiting structural heterogeneities at larger length scales that are characteristic of attractive systems. We model the static structure factor of these systems using effective pair interaction potentials composed of a long-range attraction and a shorter range repulsion. Our modeling of the static structure yields conditions for dynamically arrested states at larger volume fractions, which we find to agree with the experimentally observed dynamics.

Resolving long-range spatial correlations in jammed colloidal systems using photon correlation imaging.

We introduce a new dynamic light scattering method, termed photon correlation imaging, which enables us to resolve the dynamics of soft matter in space and time. We demonstrate photon correlation imaging by investigating the slow dynamics of a quasi-two-dimensional coarsening foam made of highly packed, deformable bubbles and a rigid gel network formed by dilute, attractive colloidal particles. We find the dynamics of both systems to be determined by intermittent rearrangement events. For the foam, the rearrangements extend over a few bubbles, but a small dynamical correlation is observed up to macroscopic length scales. For the gel, dynamical correlations extend up to the system size. These results indicate that dynamical correlations can be extremely long-ranged in jammed systems and point to the key role of mechanical properties in determining their nature.

Investigation of q-dependent dynamical heterogeneity in a colloidal gel by x-ray photon correlation spectroscopy.

We use time-resolved x-ray photon correlation spectroscopy to investigate the slow dynamics of colloidal gels made of moderately attractive carbon black particles. We show that the slow dynamics is temporally heterogeneous and quantify its fluctuations by measuring the variance chi of the instantaneous intensity correlation function. The amplitude of dynamical fluctuations has a nonmonotonic dependence on scattering vector q, in stark contrast with recent experiments on strongly attractive colloidal gels [Duri and Cipelletti, Europhys. Lett. 76, 972 (2006)]. We propose a simple scaling argument for the q-dependence of fluctuations in glassy systems that rationalizes these findings.

Glasslike arrest in spinodal decomposition as a route to colloidal gelation.

Colloid-polymer mixtures can undergo spinodal decomposition into colloid-rich and colloid-poor regions. Gelation results when interconnected colloid-rich regions solidify. We show that this occurs when these regions undergo a glass transition, leading to dynamic arrest of the spinodal decomposition. The characteristic length scale of the gel decreases with increasing quench depth, and the nonergodicity parameter exhibits a pronounced dependence on scattering vector. Mode coupling theory gives a good description of the dynamics, provided we use the full static structure as input.

Heterogeneous dynamics of coarsening systems.

We show by means of experiments, theory, and simulations that the slow dynamics of coarsening systems displays dynamic heterogeneity similar to that observed in glass-forming systems. We measure dynamic heterogeneity via novel multipoint functions which quantify the emergence of dynamic, as opposed to static, correlations of fluctuations. Experiments are performed on a coarsening foam using time-resolved correlation, a recently introduced light scattering method. Theoretically we study the Ising model, and present exact results in one dimension, and numerical results in two dimensions. For all systems the same dynamic scaling of fluctuations with domain size is observed.

Limits to gelation in colloidal aggregation.

We show that the dynamics of large fractal colloid aggregates are well described by a combination of translational and rotational diffusion and internal elastic fluctuations, allowing both the aggregate size and internal elasticity to be determined by dynamic light scattering. The comparison of results obtained in microgravity and on Earth demonstrates that cluster growth is limited by gravity-induced restructuring. In the absence of gravity, thermal fluctuations ultimately inhibit fractal growth and set the fundamental limitation to the lowest volume fraction which will gel.

Rideal lecture. Universal features of the fluid to solid transition for attractive colloidal particles.

Attractive colloidal particles can exhibit a fluid to solid phase transition if the magnitude of the attractive interaction is sufficiently large, if the volume fraction is sufficiently high, and if the applied stress is sufficiently small. The nature of this fluid to solid transition is similar for many different colloid systems, and for many different forms of interaction. The jamming phase transition captures the common features of these fluid to solid translations, by unifying the behavior as a function of the particle volume fraction, the energy of interparticle attractions, and the applied stress. This paper describes the applicability of the jamming state diagram, and highlights those regions where the fluid to solid transition is still poorly understood. It also presents new data for gelation of colloidal particles with an attractive depletion interaction, providing more insight into the origin of the fluid to solid transition.

Jamming phase diagram for attractive particles.

A wide variety of systems, including granular media, colloidal suspensions and molecular systems, exhibit non-equilibrium transitions from a fluid-like to a solid-like state, characterized solely by the sudden arrest of their dynamics. Crowding or jamming of the constituent particles traps them kinetically, precluding further exploration of the phase space. The disordered fluid-like structure remains essentially unchanged at the transition. The jammed solid can be refluidized by thermalization, through temperature or vibration, or by an applied stress. The generality of the jamming transition led to the proposal of a unifying description, based on a jamming phase diagram. It was further postulated that attractive interactions might have the same effect in jamming the system as a confining pressure, and thus could be incorporated into the generalized description. Here we study experimentally the fluid-to-solid transition of weakly attractive colloidal particles, which undergo markedly similar gelation behaviour with increasing concentration and decreasing thermalization or stress. Our results support the concept of a jamming phase diagram for attractive colloidal particles, providing a unifying link between the glass transition, gelation and aggregation.

Scaling of the viscoelasticity of weakly attractive particles

The rheological data of weakly attractive colloidal particles are shown to exhibit a surprising scaling behavior as the particle volume fraction, straight phi, or the strength of the attractive interparticle interaction, U, are varied. There is a critical onset of a solid network as either straight phi or U increase above critical values. For all solidlike samples, both the frequency-dependent linear viscoelastic moduli, and the strain-rate dependent stress can be scaled onto universal master curves. A model of a solid network interspersed in a background fluid qualitatively accounts for this behavior.

Light-scattering microscope.

We demonstrate a new design for a light-scattering microscope that is convenient to use and that allows simultaneous imaging and light scattering. The design is motivated by the growing use of thermal fluctuations to probe the viscoelastic properties of complex inhomogeneous environments. We demonstrate measurements of an optically nonergodic sample, one of the most challenging light-scattering applications.