Gelling colloidal suspensions represent an important class of soft materials. Their mechanical response is characterized by a solid-to-liquid transition at a given shear stress level. Moreover, they often exhibit a complex time-dependent rheological behavior known as thixotropy. The viscosity changes find their origin in the microstructure, which depends on flow history. Yet, the structural response of colloidal gels to flow differs fundamentally from most complex fluids, where flow induces orientation. Upon yielding, low to intermediate volume fraction gels break down in a spatially anisotropic way. Bonds in the velocity-velocity gradient plane are broken, whereas microstructural features in other planes are less affected. The subsequent flow-induced microstructural anisotropy is characterized by typical butterfly scattering patterns. However, as yet there was no evidence for the pertinence of this anisotropy for the rheological properties of these systems. In the present work, orthogonal superposition rheometry was first used to evaluate how the flow-induced microstructure affects the viscoelastic properties. It was shown to retain significant elasticity in the velocity-vorticity plane, even when the structure liquefied. Further, the shearinduced mechanical anisotropy was measured using two-dimensional small amplitude oscillatory shear, exploiting the fact that for suitable thixotropic samples the recovery after arresting the flow is relatively slow. It was hence possible to measure the anisotropy of the moduli upon cessation of flow. The mechanical anisotropy was shown to be spectacular, with the storage moduli in perpendicular directions differing by as much as 2 orders of magnitude. V C 2017 The Society of Rheology. [http://dx

High-frequency rheology is a form of mechanical spectroscopy which provides access to fast dynamics in soft materials and hence can give valuable information about the local scale microstructure. It is particularly useful for systems where timetemperature superposition cannot be used, when there is a need to extend the frequency range beyond what is possible with conventional rotational devices. This review gives an overview of different approaches to high-frequency bulk rheometry, i.e. mechanical rheometers that can operate at acoustic (20 Hz-20 kHz) or ultrasound (> 20 kHz) frequencies. As with all rheometers, precise control and know-how of the kinematic conditions are of prime importance. The inherent effects of shear wave propagation that occur in oscillatory measurements will hence be addressed first, identifying the gap and surface loading limits. Different high-frequency techniques are then classified based on their mode of operation. They are reviewed critically, contrasting ease of operation with the dynamic frequency range obtained. A comparative overview of the different types of techniques in terms of their operating window aims to provide a practical guide for selecting the right approach for a given problem. The review ends with a more forward looking discussion of selected material classes for which the use of high-frequency rheometry has proven particularly valuable or holds promise for bringing physical insights.

In the present work a polymeric transient viscoelastic network is used as a model system to investigate several fundamentals of interfacial viscoelasticity and non-linear behavior, in simple shear, compression and for simple mixed deformations. A supramolecular polymer bilayer, characterized by long but finite relaxation times, is created at the water-air interface using a layer-by-layer assembly method. The possibility of studying the individual layers starting from an unstrained reference state enabled the independent quantification of the equilibrium thermodynamic properties, and the viscoelastic response of the bilayer could be studied separately for shear and compressional deformations. Time-and frequency-dependent material functions of the layer were determined in simple shear and uniform compression. Moreover, a quasi linear neo-Hookean model for elastic interfaces was adapted to describe step strain experiments on a viscoelastic system by allowing the material properties to be time-dependent. The use of this model made it possible to calculate the response of the system to step deformations. Within the linear response regime, both stress-strain proportionality and the superposition principle were investigated. Furthermore, the onset of non-linear behavior of the extra stresses was characterized in shear and for the first time in pure compression. We conclude by investigating the multilayer system in a rising bubble setup and show that the neo-Hookean model is able to predict the extra and deviatoric surface stresses well, up to moderate deformations.

Interfacial rheological properties play a key role in the stability of a wide range of high-interface materials and thin films. For many systems, it is desirable to understand the response of the interface to a change of composition in the surrounding bulk phases. Stimuli, such as changes in pH or electrolyte concentration, can have a major effect on the structure and properties of the interfacial layer, or induce adsorption and desorption phenomena. Shear rheology is a particularly sensitive measure of such changes, as it only probes the extra stresses in the interface, regardless of possible variations in interfacial tension. In the present work, a widely used geometry for interfacial rheometry, the double-wall ring, is modified to enable subphase exchanges. The trade-off between the speed of exchange and the stress exerted by the flow in the subphase onto the interface is carefully considered. The optimal geometrical design is found by employing Computational Fluid Dynamics (CFD). A geometry with inlets positioned at the bottom and outlets near the interface minimizes the mixing time. Experiments on interfaces with colloidal particles and proteins, subjected to changes in electrolyte concentration and pH, respectively, are used to evaluate the performance of the setup.