Soma Nag

Crack patterns in desiccating clay–polymer mixtures with varying composition

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.

Spreading of fluids on solids under pressure: Slip and stick effects

Spreading of different types of fluid on solids under an impressed force is an interesting problem. Here we study spreading of four fluids, having different hydrophilicity and viscosity on two substrates - glass and perspex, under an external force. The area of contact of fluid and solid is video-photographed and its increase with time is measured. The results for different external forces can be scaled onto a common curve. We try to explain the nature of this curve on the basis of existing theoretical treatment where either the no-slip condition is used or slip between fluid and substrate is introduced. We find that of the eight cases under study, in five cases quantitative agreement is obtained using a positive slip coefficient. The remaining three can be explained with a negative slip coefficient, equivalent to a sticking effect.

Spreading of Non-Newtonian and Newtonian Fluids on a Solid Substrate under Pressure

Strongly non-Newtonian fluids namely, aqueous gels of starch, are shown to exhibit visco-elastic behavior, when subjected to a load. We study arrowroot and potato starch gels. When a droplet of the fluid is sandwiched between two glass plates and compressed, the area of contact between the fluid and plates increases in an oscillatory manner. This is unlike Newtonian fluids, where the area increases monotonically in a similar situation. The periphery moreover, develops an instability, which looks similar to Saffman Taylor fingers. This is not normally seen under compression. The loading history is also found to affect the manner of spreading. We attempt to describe the non-Newtonian nature of the fluid through a visco-elastic model incorporating generalized calculus. This is shown to reproduce qualitatively the oscillatory variation in the surface strain.

Scaling laws for spreading of a liquid under pressure

Abstract We study squeeze flow of two different fluids (castor oil and ethylene glycol) between a pair of glass plates and a pair of perspex plates, under an applied load. The film thickness is found to vary with time as a power-law, where the exponent increases with load. After a certain time interval the area of fluid–solid contact saturates to a nearly steady value. This saturation area, increases with load at different rates for different fluid–solid combinations.