A. Prins

Bubble growth in carbonated liquids

Abstract The formation and growth of a bubble in a liquid is a subject that has been widely investigated. In the present study, bubble growth in carbonated liquids was investigated. A model cavity was developed at which bubble growth occurred as soon as the liquid became sufficiently oversaturated with gas. A theory based on Ficks first law was developed to describe this bubble growth at the cavity and to predict the moment of detachment from the latter. This theory was compared with two other available theories by F.C. Frank (Proc. R. Soc. London, 201A (1950) 586), and F. van Voorst Vader, F. Erkens and M. van den Tempel (Trans. Faraday Soc., 60 (1964) 1170), describing the transport of molecules to a liquid surface driven by a concentration gradient. It appeared that by considering the geometrical changes that take place at the boundary layer during bubble growth, in combination with the penetration theory and by applying Ficks first law, it is possible to describe the observed bubble growth at a cavity more successfully than by means of the two theories of Frank and van Voorst Vader. Nevertheless, our theory can only be used if no convection is involved. The moment of detachment depends on the perimeter of attachment, the dynamic surface tension of the liquid and the wetting properties of the cavity. The developed theory was successfully applied by using a computer program to calculate the times needed for very small consecutive increases in bubble radius. In this way, transient phenomena such as bubble growth and detachment could be predicted.

Effect of copper ions on the drainage stability of foams prepared from egg white

In this paper we investigate the effect of copper ions on the stability of foams prepared from egg white. We compare the time of formation and stability of foams prepared from fresh egg whites, with and without added copper ions. We find that the foams prepared with copper ions take more time to form, but are more stable. The effect increases upon dilution of the egg whites, which shows that the bulk phase is not responsible for the increase in stability. Microscopy shows that the initial bubble size distribution of the foam is not affected by the addition of copper ions. Measurements with a ring trough show that the surface tension of the liquid vapor interface of the foam films is also unaffected. The results of the ring trough measurements show that the increase in drainage stability is caused mainly by an increase in the surface dilatational elasticity of the interface. There is also an increase in the surface dilatational viscosity, but only at frequencies less than 0.1 Hz. The increase in the surface dilatational elasticity affects the drainage stability of the foam during the initial seconds of the drainage process, whereas the increase in the surface dilatational viscosity affects the long-term drainage stability.

The use of overflowing cylinder technique to measure stagnant surface behaviour

Abstract In the overflowing cylinder technique applied to aqueous surfactant solutions, the expansion rate of the surface is governed by surface tension gradients which develop autonomously in the surface. These surface tension gradients can either accelerate the expansion rate or they can slow down the expansion rate depending on the experimental conditions and on the nature of the surfactants used. The slowing down of the surface movement causes also the slowing down of the liquid in close contact with the surface. The overflowing cylinder technique enables to quantify such stagnant surface behaviour and opens the possibility to relate this to special aspects of foaming behaviour. In this contribution which is devoted to the commemoration of Paul Joos, the results of the overflowing cylinder technique will be related to the draining behaviour of a foam column.

Foaming behaviour and dynamic surface properties of liquids

A growing interest has occurred in the physical mechanisms that are involved in the the various aspects of foaming behaviour such as foam making (overrun and bubble size distribution) and foam keeping (drainage, coalescence and Ostwald ripening). It has recently been shown that dynamic surface properties, both close and far from equilibrium, play an important role in these physical mechanisms. Recently, well-defined physical methods have also been made available to measure these surface properties. With regard to foaming behaviour, further investigations have to be encouraged in order to develop experimental techniques which are able to distinguish between the different physical aspects of foaming behaviour. As yet, however, a dynamic surface property that fulfils all the requirements for a good foaming behaviour such as a high overrun, a small bubble size, a slow drainage, no coalescence and no Ostwald ripening still does not exist. Further knowledge of the physical background of foaming behaviour can help to find a compromise between the formulation of foaming liquids and the way of foam making to realise the required foaming behaviour.

Surface dilational behaviour of aqueous protein solutions: History effects

Abstract Aqueous protein solutions have dynamic surface properties that are found to depend on the experimental technique by which they are measured. Especially aqueous solutions of globular proteins have dynamic surface properties that are sensitive for the history of the surface. By means of a Langmuir trough (Lt) technique and an overflowing cylinder (Oc) technique, these differences in surface behaviour can be quantified. It appears that much higher surface dilational viscosities are measured by using the Lt technique than with the Oc technique. These results are interpreted in a qualitative way by taking into account the different histories of the liquid surfaces in the two techniques and the more or less globular nature of the proteins used. The consequence of the observed phenomena for practical application is discussed in view of the behaviour of bubbles in a moving liquid. It is concluded that both techniques, although they give different results, can be applied to different practical situations.

A constant stress, parallel plate viscometer without bearing for very low shear stresses

A simple, constant stress viscometer without bearing is described. The rotating part consists of a ferromagnetic disc which floats on the liquid. It is driven and centred by a fast rotating magnetic field. The shear stress can be varied continuously between 5*10-5 and 0.5 Pa by changing the distance between the disc and the rotating magnet.