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Elasticity in disordered medium

Colloidal thin films of varying rigidity detaching from a substrate under an electric field induced stress are studied by video microscopy. For soft films, the process of detachment shows single-particle dynamics, analogous to desorption. For rigid films, a collective delamination spanning hundreds of particles occurs. A competition among the rigidity of the film, the interaction with the substrate, and the external stress leads to a correlation length over which the film delaminates at a critical stress. The phenomenon is described as a dynamical transition in a disordered elastic medium.

Atul Varshney, P. Sharma, A. Sane, S. Ghosh and S. Bhattacharya “Desorption to Delamination:Dynamics of Detachment in a Colloidal Thin Film”. Physical Review Letters 105, 154301(2010) [poster]

Desorption Delamination
Desorption of Polystyrene Spheres Delamination of Silica Film

Amorphous to amorphous transition in particle rafts

Space-filling assemblies of athermal hydrophobic particles floating at an air-water interface, called particle rafts, are shown to undergo an unusual phase transition between two i.e., a low density 'less-rigid' and a high density 'more-rigid' amorphous states as a function of particulate number density (\Phi). The former is shown to be a capillary-bridged solid and the later a frictionally-coupled one. Simultaneous studies involving direct imaging as well as measuring its mechanical response to longitudinal and shear stresses show that the transition is marked by a subtle structural anomaly and a weakening of the shear response. The structural anomaly is identified from the variation of the mean coordination number, mean area of the Voronoi cells and the spatial profile of the displacement field with \Phi. The weakened shear response is related to local plastic instabilities caused by the depinning of the contact-line of the underlying fluid on the rough surfaces of the particles.

Atul Varshney, A. Sane, Shankar Ghosh and S. Bhattacharya “Amorphous to amorphous transition in particle rafts”. Physical Review E 86, 031402(2012) [supplementary], arXiv

Formation of Particle rafts Formation of particle-rafts
Folds formation Folds formation in particle rafts prepared using 0.3mm sized hydrophobic silica particles
Variation of (a) longitudinal modulus K_A and (b) shear modulus G as a function of mean coordination number Z. Particle size dependency on G (shown in inset) in 'less-rigid' (triangles) and 'more-rigid' (rectangles) states of the rafts. ModulliVsZ


Elastic Properties in Nanocomposite

We find that the elastic modulus of nanocrystalline ZnO shows a non-monotonic dependence on the crystallite size and suggest that the non-monotonicity arises from an interplay between the elastic properties of the individual grains and the intergranular region.

V. Panchal,S.Ghosh, S. Gohil, N. Kulkarni and P. Ayyub “Non-monotonic size dependence of the elastic modulus of nanocrystalline ZnO embedded in a nanocrystalline silver matrix” . J. Phys.: Condens. Matter 20, 345224(2008)