Rob Van Hooghten

Rough nanoparticles at the oil–water interfaces: their structure, rheology and applications

A two dimensional suspension is used as a model system to study structural and dynamical responses of weakly attractive particles. An attractive glassy monolayer is produced by spreading carbon black particles at the interface, with attractive interactions between the particles stemming from lateral capillary forces caused by the surface roughness and the subsequent contact line undulations. By controlling the surface concentration, strong and elastic interfacial layers are formed which wrinkle reversibly upon compression. The rheological properties of the resulting monolayers can be described by the framework of the ‘soft glassy rheology’ model. For example, the effects of concentration for a given type of particle on the frequency dependence of the moduli can be scaled. Data for two types of particles with different surface roughness reveal the effect of the interaction strength on the elasticity. The monolayers are also observed to ‘age’ as a function of time, with the elasticity increasing over time. Based on the soft glassy rheology model, a master-curve can be obtained using a time–concentration superposition procedure, where the effect of interaction strength also can be scaled and evaluated indirectly.

Interfacial rheology of sterically stabilized colloids at liquid interfaces and its effect on the stability of Pickering emulsions

Particle-laden interfaces can be used to stabilize a variety of high-interface systems, from foams over emulsions to polymer blends. The relation between the particle interactions, the structure and rheology of the interface, and the stability of the system remains unclear. In the present work, we experimentally investigate how micron-sized, near-hard-sphere-like particles affect the mechanical properties of liquid interfaces. In particular, by comparing dried and undried samples, we investigate the effect of aggregation state on the properties of the particle-laden liquid interface and its relation to the stability of the corresponding Pickering emulsions. Partially aggregated suspensions give rise to a soft-solid-like response under shear, whereas for stable PMMA particulate layers a liquid-like behavior is observed. For interfacial creep-recovery measurements, we present an empirical method to correct for the combined effect of the subphase drag and the compliance of the double-wall ring geometry, which makes a significant contribution to the apparent elasticity of weak interfaces. We further demonstrate that both undried and dried PMMA particles can stabilize emulsions for months, dispelling the notion that particle aggregation, in bulk or at the interface, is required to create stable Pickering emulsions. Our results indicate that shear rheology is a sensitive probe of colloidal interactions but is not necessarily a predictor of the stability of interfaces, e.g., in quiescent Pickering emulsions, as in the latter the response to dilatational deformations can be of prime importance.

A study of the aggregation of cyclodextrins: Determination of the critical aggregation concentration, size of aggregates and thermodynamics using isodesmic and K2–K models

The aggregation of three different cyclodextrins (CDs): 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), 2-hydroxypropyl-γ-cyclodextrin (HP-γ-CD) and sulfobutylether-β-cyclodextrin (SBE-β-CD) was studied. The critical aggregation concentration (cac) of these three CDs is quite similar and is situated at ca. 2% (m/v). There was only a small difference in the cac values determined by DLS and 1H NMR. DLS measurements revealed that CDs in solution have three size populations wherein one of them is that of a single CD molecule. The size of aggregates determined by TEM appears to be similar to the size of the aggregates in the second size distribution determined by DLS. Isodesmic and K2-K self-assembly models were used for studying the aggregation process of HP-β-CD, HP-γ-CD and SBE-β-CD. The results showed that the aggregation process of these CDs is a cooperative one, where the first step of aggregation is less favorable than the next steps. The determined thermodynamic parameters showed that the aggregation process of all three CDs is spontaneous and exothermic and it is driven by an increase of the entropy of the environment.

Limiting coalescence by interfacial rheology: over-compressed polyglycerol ester layers

The stabilisation of foam bubbles is of both scientific interest and technological importance. In this respect, the study of polyglycerol ester-based nonionic surfactants is highly relevant as they lead to remarkable foam stability. The present work investigates the thermodynamic and rheological properties leading to this stability with a particular focus on out-of-equilibrium conditions of the surface. These may occur after bubble coalescence events, which lead to a compression of the interface. To separate the effects of such a compression on surface tension and the extra stresses which arise due to deformation of the interface, a double wall ring geometry mounted in a ribbon trough was used. Surface tension and the linear viscoelastic properties are shown to be affected differently by compression of the surface. The surface tension relaxes to an equilibrium value while the surface moduli will continue to rise which each compression step. The presence of multilayered structures, which have been reported for polyglycerol esters, explains how these differences arise. The polyglycerol ester-based nonionic surfactants are an example of materials where a true surface rheological response, related to the extra and deviatoric stresses, can be used to limit coalescence.

Extensional rheology, cellular structure, mechanical behavior relationships in HMS PP/montmorillonite foams with similar densities

The main goal of this work is to analyze the relationships between the extensional rheological behavior of solid nanocomposites based on high melt strength polypropylene (HMS PP) and montmorillonites (MMT) and the cellular structure and mechanical properties of foams produced from these materials. For this purpose two systems have been analyzed. The first one incorporates organomodified MMT and a compatibilizer and the second system contains natural clays and is produced without the compatibilizer. Results indicate that the extensional rheological behavior of both materials is completely different. The strain hardening of the polymer containing organomodified clays decreases as the clay content increases. As a consequence, the open cell content of this material increases with the clay content and hence, the mechanical properties get worse. However, in the materials produced with natural clays this relationship is not so clear. While no changes are detected in the extensional rheological behavior by adding these particles, the nano-filled materials show an open cell structure, opposite to the closed cell structure of the pure polymer, which is caused by the fact of having particle agglomerates with a size larger than the thickness of the cell walls and a poor compatibility between the clays and the polymer.