Experimental rheology of model colloidal dispersions

Experimental rheological aspects of colloidal dispersions are now being investigated with improved skills and with better defined model systems. New methods allow more detailed investigations. The relationship between the rheological macroproperties, microproperties and processes is the focus of the most recent investigations. Better insight has been gained concerning this relationship

Linear viscoelasticity of emulsions : I. The effect of an interfacial film on the dynamic viscosity of nondilute emulsions

The dynamic viscosity of nondilute monodisperse emulsions is calculated by using a cell model. Two possibilities for describing the mechanical properties of the interfacial film between the internal and the external phase are considered: (A) the film is assigned a two-dimensional linear viscoelastic behavior and (B) the film is treated as a shell with finite thickness containing a Newtonian liquid. The resulting expressions for the dynamic viscosity show that model B has two relaxation times and model A has at least two or more. If a Voigt-Kelvin model is used to describe the interfacial rheology, model A will also have just two relaxation times. The results obtained may be used to interpret measurements on emulsions in terms of microscopic parameters of these emulsions

Effective viscosity of dispersions approached by a statistical continuum method

The problem of the determination of the effective viscosity of disperse systems (emulsions, suspensions) is considered. On the basis of the formal solution of the equations governing creeping flow in a statistically homogeneous dispersion, the effective viscosity is expressed in a series expansion in terms of correlation functions. The contribution of the interfacial tension to the effective viscosity is also considered and finally bounds for the effective viscosity are indicated

On expansion parallax distances for planetary nebulae

The distances to individual wind-driven bubbles such as Planetary Nebulae (PNe) can be determined using expansion parallaxes: the angular expansion velocity in the sky is compared to the radial velocity of gas measured spectroscopically. Since the one is a pattern velocity, and the other a matter velocity, these are not necessarily the same. Using the jump conditions for both shocks and ionization fronts, I show that for typical PNe the pattern velocity is 20 to 30% larger than the material velocity, and the derived distances are therefore typically 20 to 30% too low. I present some corrected distances and suggest approaches to be used when deriving distances using expansion parallaxesComment: 7 pages, 6 figures, accepted for publication in A&