Agnes Duri

Length scale dependence of dynamical heterogeneity in a colloidal fractal gel

We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final decay of the average intensity autocorrelation function is well described by g2(q,τ) − 1 ~ exp [ − (τ/τf)p], with τf ~ q−1 and p decreasing from 1.5 to 1 with increasing q. We show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements. We quantify the dynamical fluctuations resulting from intermittency by means of the variance χ(τ,q) of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility χ4 studied in glass formers. The amplitude of χ is found to grow linearly with q. We propose a simple—yet general—model of intermittent dynamics that accounts for the q-dependence of both the average correlation functions and χ.

Time Resolved Correlation measurements of temporally heterogeneous dynamics

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.

Unexpected drop of dynamical heterogeneities in colloidal suspensions approaching the jamming transition

In dense colloidal suspensions, the spatial and temporal fluctuations in the dynamics of the constituent particles are closely related. But very close to the jamming transition—where the suspension becomes rigid—they are found to follow different trends.

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.

Ultra-long range correlations of the dynamics of jammed soft matter

We use photon correlation imaging, a recently introduced space-resolved dynamic light scattering method, to investigate the spatial correlation of the dynamics of a variety of jammed and glassy soft materials. Strikingly, we find that in deeply jammed soft materials spatial correlations of the dynamics are quite generally ultra-long ranged, extending up to the system size, orders of magnitude larger than any relevant structural length scale, such as the particle size, or the mesh size for colloidal gel systems. This has to be contrasted with the case of molecular, colloidal and granular “supercooled” fluids, where spatial correlations of the dynamics extend over a few particles at most. Our findings suggest that ultra long range spatial correlations in the dynamics of a system are directly related to the origin of elasticity. While solid-like systems with entropic elasticity exhibit very moderate correlations, systems with enthalpic elasticity exhibit ultra-long range correlations due to the effective transmission of strains throughout the contact network.

FLUCTUATIONS AND NOISE IN TIME-RESOLVED LIGHT SCATTERING EXPERIMENTS: MEASURING TEMPORALLY HETEROGENEOUS DYNAMICS

We use Time Resolved Correlation (TRC), a recently introduced light scattering method, to study the dynamics of a variety of jammed, or glassy, soft materials. The output of a TRC experiment is cI(t,τ), the time series of the degree of correlation between the speckle patterns generated by the light scattered at time t and t+τ. We characterize the fluctuations of cI by calculating their Probability Density Function, their variance as a function of the lag τ, and their time autocorrelation function. The comparison between these quantities for a Brownian sample and for jammed materials indicate unambiguously that the slow dynamics measured in soft glasses is temporally heterogeneous. The analogies with recent experimental, numerical and theoretical work on temporal heterogeneity in the glassy dynamics are briefly discussed.

Steam-jet agglomeration of skim milk powders: Influence of process parameters

ABSTRACT Steam-jet agglomeration consists in steam–wetting of the surface of fine primary particles, colliding the sticky particles and consolidating the agglomerates by drying in order to obtain the desired properties. The aim of this work was to evaluate the influence of the steam/powder ratio and drying time on growth mechanisms and skim milk agglomerate properties. We demonstrated a predominant influence of the steam/powder ratio on the growth mechanisms and agglomerate properties due to the contribution of the steam flow rate. Moreover, the drying time was identified as a key process parameter to control the final water content, rehydration and mechanical properties of the agglomerates. Highlights A steam-jet agglomeration pilot plant was developed to study the agglomeration of skim-milk powders. The growth mechanisms and agglomerates properties depended on the steam/powder ratio, through the contribution of the steam flow rate. The drying time was identified as a key process parameter to control the final water content, rehydration and mechanical properties of the agglomerates.

Fluctuations and noise in time-resolved light scattering experiments: measuring temporally heterogeneous dynamics

We use Time Resolved Correlation (TRC), a recently introduced light scattering method, to study the dynamics of a variety of jammed, or glassy, soft materials. The output of a TRC experiment is cI(t,r), the time series of the degree of correlation between the speckle patterns generated by the light scattered at time t and t + r. We characterize the fluctuations of cI by calculating their Probability Density Function, their variance as a function of the lag r, and their time autocorrelation function. The comparison between these quantities for a Brownian sample and for jammed materials indicate unambiguously that the slow dynamics measured in soft glasses is temporally heterogeneous. The analogies with recent experimental, numerical and theoretical work on temporal heterogeneity in glassy dynamics are briefly discussed.