Interfacial rheological properties of adsorbed protein layers and surfactants: a review.

Protein adsorption at the oil/water interface: characterization of adsorption kinetics by dynamic interfacial tension measurements

Visser [J. Colloid Interface Sci. 34, 26 (1970)] has used hydrodynamic techniques, i.e., rotating cylinders, to study the removal of submicron particles from various substrates. He developed empirical relationships which relate the force of adhesion to the viscous drag experienced by the particle, which is assumed to be embedded in a steady, viscous sublayer. Although this technique is a valuable experimental tool, the question of how a particle is moved is evaded. Detailed investigations of the turbulent boundary layer (reported in the fluid mechanics literature) have shown that the viscous sublayer is anything but steady, and is continually disrupted by turbulent “bursts.” These bursts, which are not unlike miniature tornados, may cause instantaneous lift forces sufficient to detach a particle. This model of an unsteady sublayer has been used to predict the possible lift forces acting on the particle. These can arise either from impulsive motions or from the generation of a quasi-steady updraft over a particle by a burst. From these predicted lift forces, a removal criterion is obtained which for a given fluid reduces to τωd4/3 ⩾ constant where τω is wall shear stress and d is particle diameter. This result is shown to be in general agreement with previous empirical studies. The rate of removal can be predicted by combining this analysis of lift forces with literature data on the size and frequency of bursts. Although this latter information is taken from studies in pipe-flow and flow over a flat-plate, the predicted rates are in qualitative agreement with the experimental data from rotational instruments.

Mechanism of detachment of colloidal particles from a flat substrate in a turbulent flow

Complex fluid-fluid interfaces are common to living systems, foods, personal products, and the environment. They occur wherever surface-active molecules and particles collect at fluid interfaces and render them nonlinear in their response to flow and deformation. When this occurs, the interfaces acquire a complex microstructure that must be interrogated. Interfacial rheological material properties must be measured to appreciate their role in such varied processes as lung function, cell division, and foam and emulsion stability. This review presents the methods that have been devised to determine the microstructure of complex fluid-fluid interfaces. Complex interfacial microstructure leads to rheological complexity. This behavior is often responsible for stabilizing interfacial systems such as foams and emulsions, and it can also have a profound influence on wetting/dewetting dynamics. Interfacial rheological characterization relies on the development of tools with the sensitivity to respond to small surface stresses in a way that isolates them from bulk stresses. This development is relatively recent, and reviews of methods for both shear and dilatational measurements are offered here.

M. van den Tempel

Papers

Relaxation processes at fluid interfaces

Interaction forces between condensed bodies in contact