Viscous sintering phenomena in liquid-liquid dispersions

Bitumen Emulsion Destabilization Kinetics: Importance of the Crystallized Wax Content

We study the kinetics of bitumen emulsion destabilization after the addition of sodium hydroxide (NaOH) using macroscopic observations and rheology. Destabilization occurs in a two-step process: first, emulsion flocculates, forming a percolated network of contacting drops, and then coalescence provokes the irreversible connection of bitumen drops, leading to a bitumen continuous network that further relaxes the shape. We show that the destabilization kinetics exhibits a rheological easily identifiable signature allowing reproducible and accurate measurement of the connection/coalescence time t_rc (which corresponds to the time, determined by rheology, required to form the network made of drops connected by nonrelaxed coalescence). Using this powerful tool, we show that, even if viscosity is thought to govern the shape relaxation of the connected network it does not determine the connection kinetics. Indeed, emulsions with similar rheological behaviors exhibit very different destabilization times. Instead, we evidence a good correlation between the bitumen crystallized wax content and t_rc. From these experimental results, we discuss the stabilizing effect against coalescence of crystals in bitumen emulsions.

Force percolation of contractile active gels

Living systems provide a paradigmatic example of active soft matter. Cells and tissues comprise viscoelastic materials that exert forces and can actively change shape. This strikingly autonomous behavior is powered by the cytoskeleton, an active gel of semiflexible filaments, crosslinks, and molecular motors inside cells. Although individual motors are only a few nm in size and exert minute forces of a few pN, cells spatially integrate the activity of an ensemble of motors to produce larger contractile forces (∼nN and greater) on cellular, tissue, and organismal length scales. Here we review experimental and theoretical studies on contractile active gels composed of actin filaments and myosin motors. Unlike other active soft matter systems, which tend to form ordered patterns, actin-myosin systems exhibit a generic tendency to contract. Experimental studies of reconstituted actin-myosin model systems have long suggested that a mechanical interplay between motor activity and the network's connectivity governs this contractile behavior. Recent theoretical models indicate that this interplay can be understood in terms of percolation models, extended to include effects of motor activity on the network connectivity. Based on concepts from percolation theory, we propose a state diagram that unites a large body of experimental observations. This framework provides valuable insights into the mechanisms that drive cellular shape changes and also provides design principles for synthetic active materials.

Arrested coalescence of viscoelastic droplets: polydisperse doublets

Arrested droplet coalescence produces stable anisotropic shapes and is a key mechanism for microstructure development in foods, petroleum and pharmaceutical formulations. Past work has examined the dynamic elastic arrest of coalescing monodisperse droplet doublets and developed a simple model of doublet strain as a function of physical variables. Although the work describes experimental data well, it is limited to describing same-size droplets. A new model incorporating a generalized description of doublet shape is developed to describe polydisperse doublet formation in more realistic emulsion systems. Polydisperse doublets are shown to arrest at lower strains than monodisperse doublets as a result of the smaller contribution of surface area in a given pair. Larger droplet size ratios have lower relative degrees of strain because coalescence is arrested at an earlier stage than in more monodisperse cases. Experimental observations of polydisperse doublet formation indicate that the model under-predicts arrest strains at low solid levels and small droplet sizes. The discrepancy is hypothesized to be the result of nonlinear elastic deformation at high strains.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Hindered coarsening of a phase-separating microemulsion due to dispersed colloidal particles

The addition of sterically stabilized colloidal particles to a phase-separating microemulsion leads to dramatic changes in its demixing behavior, especially during the later stages. Our microemulsion is composed of reverse micelles of sodium dodecyl sulfate, pentanol, and water in a dodecane continuous phase which separates into micelle-rich and micelle-poor phases above a lower critical solution temperature. The poly(methyl methacrylate) particles preferentially partition into the less structured, micelle-poor phase. Nucleation of the minority phase or spinodal decomposition close to criticality continue to occur in the presence of particles, albeit with pronounced pretransitional clustering of particles when the micelle-poor phase is in the minority. The coalescence of micelle-poor droplets and the coarsening of micelle-rich domains are both strongly modified due to the presence of colloidal particles. We use our observations of the early stages of phase separation to understand these late stage changes.

Imaging techniques applied to characterize bitumen and bituminous emulsions

The purpose of this article is to present some important advances in the imaging techniques currently used in the characterization of bitumen and bituminous emulsions. Bitumen exhibits some properties, such as a black colour and a reflecting surface at rest, which permit the use of optical techniques to study the macroscopic behaviour of asphalt mixes in the cold mix technology based on emulsion use. Imaging techniques allow monitoring in situ the bitumen thermal sensitivity as well as the complex phenomenon of emulsion breaking. Evaporation-driven breaking was evaluated from the shape of evaporating emulsion drops deposited onto non-porous and hydrophobic substrates. To describe the breaking kinetics, top-view images of a drying emulsion drop placed on an aggregate sheet were acquired and processed properly. We can conclude that computer-aided image analysis in road pavement engineering can elucidate the mechanism of breaking and curing of bituminous emulsion.

Small-angle neutron scattering study of particle coalescence and SDS desorption during film formation from carboxylated acrylic latices

Four monodisperse core-shell latices were synthesized for small-angle neutron scattering (SANS) studies, differing by the acrylic acid content in the particle shell (1 or 4 wt%) and the T(g) of the acrylic core (around -40 or 10 degrees C). In a first part, the coalescence kinetics of the surfactant-free latices were studied. It was shown that coalescence was hindered by an increase in the acrylic acid content of the shell, pH of the latex, and Tg of the core. These results could be interpreted in terms of chain mobility in the shell and in the core. Upon coalescence, the hydrophilic phase was segregated in spherical, polydisperse domains with an average diameter of 110 nm. In a second part, labeled SDS was used to follow desorption of the surfactant during film formation. It was shown that desorption occurred early in the film formation process when the latex still contained around 20% of water. A small fraction of the surfactant remained irreversibly adsorbed at the particle surface.

Effect of polymer-surfactant association on colloidal force

We investigate the forces between emulsion droplets in the presence of neutral polymer-surfactant complexes. The polymer used in our experiment was statistical copolymer of polyvinyl alcohol. The anionic surfactant used is sodiumdodecyl sulphate, the cationic surfactants are cetyltrimethylammonium bromide and tetradecyltrimethylammonium bromide, and the nonionic surfactant is nonylphenol ethoxylate (NP10). It has been found that the force profiles in the presence of surfactant-polymer complexes follow an exponential scaling with a characteristic decay length, close to the radius of gyration of the polymer alone. A continuous increase in the onset of repulsion is observed in the case of all three ionic surfactants, whereas no such variation was noticed in the case of nonionic surfactant, NP10. The experimental observations suggest that in the presence of charged surfactant molecules or micelles, the neutral polymer chain at the interface is converted into partial polyelectrolytes, where the charges on the chain repel each other and the electrostatic repulsion collectively leads to chain stretching. These results suggest that the associative polymers can be potential candidates for making the emulsions stable for a sufficiently long period.

Effects of structural rearrangements on the rheology of rennet-induced casein particle gels

During ageing of casein or skim milk gels, structural changes take place that affect gel parameters, such as pore size and storage modulus. These changes can be explained in terms of rearrangements of the gel network at various length scales. In this paper, rheological experiments on rennet-induced casein gels and a general model on rearrangements are presented. The results of experiments (e.g. microscopy, permeametry) and computer simulations, the model, and recent literature on casein gels and other types of particle gels are compared to each other. Experiments presented include measurements of storage and loss moduli and maximum linear strain of the casein gels. Parameters varied were pH (5.3 and 6.65) and temperature (25 and 30 degrees C). In addition, the casein volume fraction (5-9 vol.%) was varied, which enables application of fractal scaling models. For rennet-induced casein gels, it is demonstrated that at the lower pH, all types of rearrangements proceed significantly faster. The rearrangements include: an increase in the size of compact building blocks; partial disappearance of fractal structure; and the formation of straightened strands, some of which eventually break. All of these rearrangements seem to be a consequence of particle fusion. There are indications of universality of the relation between particle fusion and gel syneresis for gels composed of viscoelastic particles.