A system has been developed for studying the dynamic behaviour of gas bubbles in liquid media and is currently being used to investigate the behaviour of a single nitrogen bubble in molten LDPE. It involves the formation of an initial gas-liquid interface from a capillary by direct injection of the gas into the melt, subsequent growth until an equilibrium state is reached and the application of ambient pressure ramps which result in bubble expansion or contraction. The response depends on the rate of change of the ambient pressure. Experiments have been conducted at various temperatures above the crystalline melting temperature, Tm, of LDPE in the range 120°C-150°C. The numerical data acquired are in the form of pressure and temperature values, and these data are superimposed on stored video images of the bubbles for further processing on an image analyser whereby bubble sizes and shapes are determined.
The results obtained reveal the effect of melt viscoelasticity on the dynamics of bubbles in highly viscous fluid media. Furthermore, the changes in bubble shapes with time due to buoyancy effects are also shown. Clearly, the bubble response in the melt is governed by a combination of viscous and elastic effects. However, in the absence of suitable viscous solutions to the problem, we have attributed the behaviour to a time-dependent elastic response, but only as a first order approximation.