Shear-induced aggregation and break up of fibril clusters close to the percolation concentration

Abstract

IntroductionStructural changes of complex fluids under steady shear flow were investigated for different type of materials, such as suspensions, associating polymers and polymer solutions [1,2,3,4,5,6]. Under specific conditions, depending on the material, a shear-thickening regime is observed followed by shear-thinning behaviour [3,4,6]. Shear-thickening is often the result of shear-induced formation of intermolecular structures (clusters), leading to an increase in the effective volume fraction [1,2,3,5,7]. Clusters convected by shear flow may aggregate upon contact, due to attractive interactions of particles on the periphery of the clusters. Cluster-cluster collisions that occur along the streamlines of shear flow lead to the formation of larger clusters [7]. In polymer solutions, the size of these clusters depends upon the concentration [4]. Shear-induced stretching of the structures can also contribute to a shear-thickening effect [4,8,9]. Increasing the shear rate increases the stress on these structures, and once a critical stress is exceeded, they break up, giving rise to shear-thinning behaviour [3,4,6]. Abstract To probe the behaviour of fibrillar assemblies of ovalbumin under oscillatory shear, close to the percolation concentration, c p (7.5%), rheo-optical measurements and Fourier transform rheology were performed. Different results were found close to c p (7.3%), compared to slightly further away from c p (6.9 and 7.1%). For 6.9 and 7.1%, a decrease in complex viscosity, and a linear increase in birefringence, Dn¢, with increasing strain was observed, indicating deformation and orientation of the fibril clusters. For 7.3%, a decrease in complex viscosity was followed by an increase in complex viscosity with increasing strain, which coincided with a strong increase in Dn¢, dichroism, Dn¢¢, and the intensity of the normalized third harmonic (I 3 / I 1 ). This regime was followed by a second decrease in complex viscosity, where Dn¢,Dn¢¢ and I 3 /I 1 decreased. In the first regime where the viscosity was decreasing with increasing strain, deformation and orientation of existing clusters takes place. At higher oscillatory shear, a larger deformation occurs and larger structures are formed, which is most likely aggregation of the clusters. Finally, at even higher strains, the clusters break up again. An increase in complex viscosity, Dn¢, Dn¢¢ and I 3 /I 1 was observed when a second strain sweep was performed 30 min after the first. This indicates that the shear-induced cluster formation and break up are not completely reversible, and the initial cluster size distribution is not recovered after cessation of flow.

In Vitro Digestion of β-Lactoglobulin Fibrils Formed by Heat Treatment at Low pH

“…Recently, it was found that β-Lg can form fibrils on heating treatment at low pH, on heat treatment under high pressure, or in the presence of organic solvents ,− . Many studies on the mechanism of fibril formation and on the building blocks of fibrils have been carried out ,− . Akkermans et al claimed that rather than denatured intact proteins, the building blocks of β-Lg fibrils are peptides obtained through acid hydrolysis of β-Lg molecules on heat treatment.…”

Re-formation of Fibrils from Hydrolysates of β-Lactoglobulin Fibrils during in Vitro Gastric Digestion

“…It has been found that heat treatment results in long, straight, flexible amyloid-like fibrils at a pH lower than the isoelectric point (e.g., pH 2–3), whereas only short, curly aggregates are formed at high pH (higher than the isoelectric point, e.g., pH > 7). − During the past decade, many studies to understand the mechanisms of fibril formation and to identify the building blocks of fibrils have been carried out. − Using atomic force microscopy, Radford and co-workers attempted to achieve detailed understanding of the formation of amyloid fibrils from β 2 -microglobulin at different protein concentrations, pH values, and ionic strengths. This group found two distinct competitive pathways for fibril formation as well as heterogeneity of the amyloidal assembly .…”