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.

We demonstrate the use of an ordinary white-light microscope for the study of the q-dependent dynamics of colloidal dispersions. Time series of digital video images are acquired in bright field with a fast camera and image differences are Fourier-analyzed as a function of the time delay between them. This allows for the characterization of the particle dynamics independent on whether they can be resolved individually or not. The characteristic times are measured in a wide range of wavevectors and the results are found to be in good agreement with the theoretically expected values for Brownian motion in a viscous medium. 1Microscopy and light scattering are widely used in physics, chemistry, biology and medical laboratories to access information on the structure and dynamics of mesoscopic systems.While microscopy gives direct access to real space images, scattering techniques work in reciprocal space, where information on the structure and dynamics of the system is obtained respectively from the angular and time dependence of the scattered light intensity [1]. These two complementary techniques have in general very different experimental requirements.White light sources are usual choices in microscopy, while a certain degree of coherence of the illuminating beam is required in scattering experiments; this is usually achieved by using a laser. In the past many attempts have been made to build a scattering apparatus based on a microscope; all of these attempts involved the use of a laser as an illumination source (see for example Refs. [2-6] and references therein). In some cases special care was taken to ensure the capability to perform microscopy and scattering experiments simultaneously, thereby allowing for a powerful combination of the complementary information obtained by both techniques [2,3]. More recently, microscope-based laser Dynamic Light Scattering (DLS) experiments have been developed to study the dynamic properties of samples of biological interest, such as living macrophage and red blood cells [5,6]. In practice, due to the intrinsic difficulties in building such instruments, the use of such techniques has been restricted to those laboratories, where a sufficient expertise in the realization of optical instrumentation was at hand.In this letter, we present a conceptual scheme to interpret and analyze microscopy images that are obtained from samples containing moving entities. This technique, which we term Differential Dynamic Microscopy (DDM), does not entail any special experimental requirements, being based on the use of a standard light microscope with a normal illumination source and a digital video camera. By using the tools of Fourier Optics [7] we provide the means to access information about the sample dynamics that are equivalent to the one obtained in multi-angle dynamic light scattering (DLS) experiments [8]. We test DDM by analyzing time sequences of microscopy images obtained with an aqueous dispersion of colloidal particles with diameter 73 nm, well below the resolution limit ...

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.

We describe the use of a bright-field microscope for dynamic light scattering experiments on weakly scattering samples. The method is based on collecting a time sequence of microscope images and analyzing them in the Fourier space to extract the characteristic time constants as a function of the scattering wave vector. We derive a theoretical model for microscope imaging that accounts for ͑a͒ the three-dimensional nature of the sample, ͑b͒ the arbitrary coherence properties of the light source, and ͑c͒ the effect of the finite numerical aperture of the microscope objective. The model is tested successfully against experiments performed on a colloidal dispersion of small spheres in water, by means of the recently introduced differential dynamic microscopy technique ͓R. Cerbino and V. Trappe, Phys. Rev. Lett. 100, 188102 ͑2008͔͒. Finally, we extend our model to the class of microscopy techniques that can be described by a linear space-invariant imaging of the density of the scattering centers, which includes, for example, dynamic fluorescence microscopy.

We introduce a new scheme for investigating temporally heterogeneous dynamics, which is termed time-resolved correlation (TRC). TRC is applied to data obtained by diffusing wave spectroscopy probing the slow dynamics of a strongly aggregated colloidal gel. Other examples of TRC data, collected for different jammed materials in single and multiple scattering, are provided to demonstrate the wide range of applicability of this method. In all cases we find evidence that the slow dynamics results from a series of discrete steps rather than from a continuous motion, suggesting temporal heterogeneities to be a general feature of slow dynamics in jammed systems.

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.

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. DOI: 10.1103/PhysRevLett.95.238302 PACS numbers: 82.70.Dd, 64.70.Pf, 82.70.Gg Colloid-polymer mixtures exhibit a rich phase behavior. Depletion of polymer between two colloidal particles induces an attractive interaction whose magnitude U is set by the polymer concentration in the free volume, c p . The range of this interparticle attraction is determined by the radius of the polymer R p and is expressed by R p =a, with a the particle radius. The phase behavior is controlled by both c p and , as well as the colloid volume fraction, . The particles can form equilibrium phases, including crystals, fluids, and gases [1,2]. Additionally, there are a wide variety of nonequilibrium states, characterized by the dynamic arrest of the colloidal particles [3][4][5][6]. In the absence of polymer, crowding drives the system into a repulsive glassy state at 0:58, where particles are caged by their nearest neighbors. This dynamic arrest is captured by mode coupling theory (MCT), a mean-field-like theory which uses the static structure factor, Sq, to predict the dynamics [7,8]. Upon addition of a small amount of polymer, arrest is no longer driven by crowding, but instead by weak attraction between particles; surprisingly, this attractive glass can also be described by MCT [4,9]. In both cases, the dominant length scale in the structure corresponds to the nearest-neighbor particle separation. By contrast, at lower and higher U, the system can gel, forming a space-spanning network characterized by an additional, larger length scale. For systems at very low and sufficiently high U, gelation is a consequence of kinetics; fractal clusters grow and ultimately form a gel by clustercluster aggregation [10,11]. The theory for kinetic aggregation naturally includes the larger length scale, which is the cluster size. By contrast, at higher and lower U, phase separation, such as spinodal decomposition, can drive large-scale structuring, influencing the pathways for gel formation [12 -14]. In this case, the mechanisms for arrest during gelation are still unclear. Gelation requires the formation of a solidified, space-spanning network. In spinodal decomposition this is driven by dynamic arrest within the connected colloid-rich region; this may result from percolation [15][16][17][18][19], pinning [20], or a glasslike transition [6,12,13,17,21,22]. An experimental investigation that distinguishes these possible mechanisms, and that determines the correct underly...

Time resolved correlation (TRC) is a recently introduced light scattering technique that allows one to detect and quantify dynamic heterogeneities. The technique is based on the analysis of the temporal evolution of the speckle pattern generated by the light scattered by a sample, which is quantified by cI(t, tau), the degree of correlation between speckle images recorded at time t and t + tau. Heterogeneous dynamics results in significant fluctuations of cI(t,tau) with time t. We describe how to optimize TRC measurements and how to detect and avoid possible artifacts. The statistical properties of the fluctuations of cI are analyzed by studying their variance, probability distribution function, and time autocorrelation function. We show that these quantities are affected by a noise contribution due to the finite number N of detected speckles. We propose and demonstrate a method to correct for the noise contribution, based on a N--> infinity extrapolation scheme. Examples from both homogeneous and heterogeneous dynamics are provided. Connections with recent numerical and analytical works on heterogeneous glassy dynamics are briefly discussed.