Ring Shear Tester as an in-vitro testing tool to study oral processing of comminuted potato chips.

Oral processing of solid foods is an extremely dynamic and complicated activity that involves multiple processes in tandem such as comminution, mixing, dilution, hydration and enzymatic breakdown that gradually transform the food from a morsel or a bite to a bolus that is ready for swallowing. It is hypothesised that just after "first bite" and initial particle reduction and hydration of solid brittle foods, the response to deformation of food particles is analogous to studies on the flowability and cohesion of wetted powders, which are effectively characterised using a Ring Shear Tester (RST). We examine this hypothesis and determine whether the RST measures properties of solid snack foods (potato chips or crisps, PCs) that are relevant to their dynamic sensory response, which includes capturing the effect of hydration on comminuted PCs. The RST is found to differentiate PCs obtained from different manufacturing sources (e.g. baked versus fried), and its measurements of cohesion and friction can be considered in context of the structure and composition of the PCs as well as oral processing. Remarkably, RST measurements for this small set of PC samples correlate with several sensory attributes that arise during mastication, which includes Sharpness and Ease of Clearance. This study highlights the potential of the RST as a new tool for oral processing research.

Interfacial Assembly of Surfactant-Decorated Nanoparticles: On the Rheological Description of a Colloidal 2D Glass.

We address the rheology of assemblies of surfactant-decorated silica nanoparticles irreversibly adsorbed at the gas/liquid interface. Positively charged surfactant molecules (such as CTAB) bind to silica nanoparticle surfaces, and the resulting particle-surfactant complexes adsorb at gas/liquid interfaces. The surfactant molecules control the wettability of such decorated nanoparticles and their adsorption. The interparticle forces can be tuned by changing the surfactant concentration Cs. Increasing Cs, in addition to a decrease of the particles wettability, leads to an increase of the area fraction of particles at the interface. Oscillatory shear measurements (strain- and frequency-sweep) have been performed. Here, we explore the effect of the surfactant concentration Cs. At high enough Cs, the interface is highly packed, and an overall solidlike response is observed, with 2D glass properties.

Hard and soft colloids at fluid interfaces: Adsorption, interactions, assembly & rheology.

Soft microgel particles inherently possess qualities of both polymers as well as particles. We review the similarities and differences between soft microgel particles and stiff colloids at fluid-fluid interfaces. We compare two fundamental aspects of particle-laden interfaces namely the adsorption kinetics and the interactions between adsorbed particles. Although it is well established that the transport of both hard particles and microgels to the interface is driven by diffusion, the analysis of the adsorption kinetics needs reconsideration and a proper equation of state relating the surface pressure to the adsorbed mass should be used. We review the theoretical and experimental investigations into the interactions of particles at the interface. The rheology of the interfacial layers is intimately related to the interactions, and the differences between hard particles and microgels become pronounced. The assembly of particles into the layer is another distinguishing factor that separates hard particles from soft microgel particles. Microgels deform substantially upon adsorption and the stability of a microgel-stabilized emulsion depends on the conformational changes triggered by external stimuli.

Equation of state and adsorption dynamics of soft microgel particles at an air-water interface.

Understanding the adsorption dynamics of soft microgel particles is a key step in designing such particles for potential applications as stimuli-responsive Pickering stabilizers for foams or emulsions. In this study we experimentally determine an equation of state (EOS) for poly (N-isopropylacrylamide) (PNIPAM) microgel particles adsorbed onto an air-water interface using a Langmuir film balance. We detect a finite surface pressure at very low surface concentration of particles, for which standard theories based on hard disk models predict negligible pressures, implying that the particles must deform strongly upon adsorption to the interface. Furthermore, we study the evolution of the surface pressure due to the adsorption of PNIPAM particles as a function of time using pendant drop tensiometry. The equation of state determined in the equilibrium measurements allows us to extract the adsorbed amount as a function of time. We find a mixed-kinetic adsorption that is initially controlled by the diffusion of particles towards the interface. At later stages, a slow exponential relaxation indicates the presence of a coverage-dependent adsorption barrier related to crowding of particles at the interface.