The viscosity and yield stress of flocculated suspensions were reviewed in terms of fractal structure and strength of flocs. The fractal structure is formed by the sequential collision process of clusters. However, the cluster rearrangement process is important with the number of contacts between clusters. Floc strength is determined by the product of cohesive force per contact and the number of contacts between clusters. This simple expression will determine the size of flocs at given shear rate, thus determining effective volume of flocs. Shear thinning behavior is expressed by putting this value into Mori-Ototake viscosity equation. The asymptotic behavior of shear stress at zero shear limit is interpreted as yield stress. The obtained yield stress is expressed as a function of volume fraction of colloid. The functional form is different from that obtained for the flocculated sediment with vane in rather concentrated region. Importance for further analyses on the loose, soft and unstable sediment with large deformable flocs is implied.
An emulsion is a mixture of two immiscible liquids such as oil and water, in which one is dispersed in the other as droplets. Generally, surfactants are used to form a stable emulsion. Surfactants, having both hydrophobic carbon chains and hydrophilic polar headgroups, position themselves on the interface between oil and water to stabilize an emulsion. Do you kwon that counterions, a part of surfactants, play an important role in stabilizing an emulsion? The counterions affect the properties of emulsion, such as the phase behavior and the shape. "Screening effect" is one of the important roles of counterions; electrostatic repulsions between polar headgroups must be screened to a certain extent by the counterions in order to form a stable emulsion. Here, these counterions' roles are reviewed by taking surfactant AOT-based W/O (water in oil) emulsions for instance.