Molecular behavior of fluid gels - the crucial role of edges and particle surface in macroscopic properties.

Fluid gels exhibit unique properties during oral processing and thus are well known in gastronomy as well as for use in dysphagia patients. Agarose fluid gels, which are produced by gelation under shear, in particular, show elastic solid-like behavior at rest but a fluid-like behavior once critical stress is exceeded. In a previous study this special behavior is addressed to the "hairy" structure of the microgel particles - dangling gel parts and chains on the particle surface - which plays a crucial role in the rheological, mechanical and tribological properties of the gels. In this paper, atomic force microscopy (AFM) was used to investigate the underlying microscopic structures and develop a consistent physical model, which relates the irregular particle structures and their heterogonous shape to the experimental observation of the previous studies. One crucial point is the inner structure of the gel particles, which show a dense area in the center, whereas towards the periphery the network and thus the elastic properties change. Agarose gels by forming helices and meshes, which defines the basic length scale for their elastic response in bulk. These properties in turn depend on the concentration and preparation conditions. The present study is meant to address the still prevalent lack of understanding regarding a direct structure-property relationship of these novel fluid gels. Controlling the properties of such fluid gels may play a crucial role in the texture modification of foods and beverages for dysphagia.