The lifting Hele-Shaw cell (LHSC) is used to study adhesion as well as viscous fingering. In the present paper we report a series of observations of development of the interface for different viscous fluids, both Newtonian and non-Newtonian, in a LHSC operated at a constant lifting force. Glass and perspex are used as the plates in two different sets of experiments. The objectives are 1) to measure the time required to separate the plates as a function of the lifting force and 2) to note the force above which viscous fingering appears. We find that for the Newtonian fluids, the plate separation time follows a universal power law with the lifting force, irrespective of fluid and substrate. The non-Newtonian fluids too, with proper scaling obey the same power law. The appearance of fingering, however, depends on the properties of the fluid as well as the substrate. We suggest a modified form of the capillary number which controls the onset of fingering; this new quantity, termed the "fingering parameter" involves the dielectric constants of the substrate and fluid in addition to the viscosity and surface tension.
Crack patterns in desiccating clay suspensions are drastically altered by the addition of polymers. In this paper we report a systematic study of the effect of varying the composition of a clay-polymer composite on the formation of crack patterns. Experiments as well as computer simulations have been done. Details of the morphology and fractal dimension of the experimental patterns are observed and the simulation is done on a two-dimensional spring network model. We find a transition from a completely fragmented fractal pattern at high clay content to a continuous film at about 50% clay content. The results of the simulation are in good qualitative agreement with the experiments. The study is expected to be of importance for clay-polymer composites. These can be designed to give improved mechanical and electrical properties for practical applications.
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