Moutushi Dutta Choudhury

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.

Unstable crack propagation in LAPONITE® gels: selection of a sinusoidal mode in an electric field

We report observation of wavy cracks and naturally patterned fracture surfaces in drying LAPONITE® paste. Desiccation cracks are shown to follow undulating, corrugated paths even when the speed of crack propagation is lower than the sound velocity in the medium by two orders of magnitude. Fast Fourier transform of the wavy crack path shows that it is a superposition of several sinusoidal modes and their harmonics. When the paste is exposed to a DC electric field during drying, by imposing a 50 V potential, some of the modes are suppressed. Increasing the voltage to 100 V results in survival of only one pure sinusoidal mode of wavelength ∼292 μm. We suggest that an effective mixed mode loading develops as a result of faster evaporation at the upper surface of the paste, and this is responsible for the instability leading to the wavy contour of the crack. The present study provides an insight into the mechanism of wavelength selection under an electric field of sufficient strength. We also show that unstable crack propagation may have similarity with the mechanism that exists in an auxiliary experiment: breaking of a perspex sheet.

Crack Patterns in Drying Laponite–NaCl Suspension: Role of the Substrate and a Static Electric Field

We report the formation of crack patterns in drying films of Laponite-NaCl solution. Crack patterns that develop upon drying aqueous Laponite-NaCl solution change drastically as the amount of NaCl is varied in the solution. In this work, we have investigated the effect of NaCl on drying films of aqueous solution of Laponite under two conditions: (i) when the film is bounded by a wall, as in Petri dish experiments and (ii) when the film does not have any boundary, as in experiments with droplets. In order to obtain insights into the effect of the substrate, the experiments have been done with two different substrates of different hydrophobicities, polypropylene and glass. The formation of crack patterns has been explained on the basis of the wetting and spreading properties of the solution on these substrates and the effect of salt on colloidal aggregation. In this work, we have shown that the presence of salt in aqueous Laponite solution can induce crack patterns depending on the nature of the substrate. Another important aspect of this work is the role of NaCl in crack inhibition in desiccating films of aqueous Laponite, in the presence of static electric field. This effect can be utilized to suppress undesirable crack formation in many applications.

Droplet Drying Patterns on Solid Substrates: From Hydrophilic to Superhydrophobic Contact to Levitating Drops

This review is devoted to the simple process of drying a multicomponent droplet of a complex fluid which may contain salt or other inclusions. These processes provide a fascinating subject for study. The explanation of the rich variety of patterns formed is not only an academic challenge but also a problem of practical importance, as applications are growing in medical diagnosis and improvement of coating/printing technology. The fundamental scientific problem is the study of the mechanism of micro- and nanoparticle self-organization in open systems. The specific fundamental problems to be solved, related to this system, are the investigation of the mass transfer processes, the formation and evolution of phase fronts, and the identification of mechanisms of pattern formation. The drops of liquid containing dissolved substances and suspended particles are assumed to be drying on a horizontal solid insoluble smooth substrate. The chemical composition and macroscopic properties of the complex fluid, the concentration and nature of the salt, the surface energy of the substrate, and the interaction between the fluid and substrate which determines the wetting all affect the final morphology of the dried film. The range of our study encompasses the fully wetting case with zero contact angle between the fluid and substrate to the case where the drop is levitated in space, so there is no contact with a substrate and angle of contact can be considered as 180°.