Current projects:

1. Nonlinear mechanics and memory effect in glassy bio-polymer networks:

Filamentous bio-polymers like actin and collagen are the major constituents of cell-cytoskeleton and extracellular matrix, respectively. Apart from their importance in cell biology, bio-polymers are very interesting from the point of view of material science. Due to their high persistence length, the networks formed by these filaments show striking non-linear response.

Recently, we found that the history of applied shear stress gets encoded in the network architecture in the form of a mechanical-memory. The `memory' of such forcing history is long-lived and decays in a very slow logarithmic fashion. Interestingly, such memory can be quickly erased by stress application in the opposite direction. In condensed matter systems such slow logarithmic relaxation has been observed for a variety of non-equilibrium processes, such as, current relaxation in MOSFET devices, flux creep in superconductors, structural relaxation in colloidal glasses, and compaction in agitated granular materials.

We are presently exploring the connection of such long-lived mechanical memory to the glassy dynamics and 'Kovacs' effect. In particular, we are interested in the spatial distribution of fast and slow relaxation modes in these systems and ways to control them.

For more information on this project please see:

Mechanical Hysteresis in Actin Networks;

S. Majumdar, L.C. Foucard, A.J. Levine, and M.L. Gardel, Soft Matter, 14, 2052-2058, 2018. (Link)

2. Transient jamming dynamics in dense particulate suspensions:

If we add large amount of immiscible solid particles (typically the volume fraction > 0.5) in a Newtonian liquid like water or oil, the resulting mixture becomes significantly non-Newtonian and it is called a dense suspension. Many dense suspensions show remarkable behavior under force: the viscosity can increase by orders of magnitude and the suspension can even develop a solid like yield stress. The microscopic mechanism of such force induced reversible solidification has been a matter of intense debate. The delicate interplay between the inter-particle frictional contact forces and the short range hydrodynamic interactions make these systems particularly interesting. Apart from fundamental interests, understanding flow behaviour of dense suspensions is crucial for many large scale industrial processes.

Recently, we found that such solidification under force is mediated by a fast growing 'jammed' region giving rise to a rapidly propagating 'jamming front'. Understanding the transient dynamics of this 'jamming front' is extremely important in the light of the recent interest in designing flexible shock absorbing materials.

We are currently exploring the correlation between such force induced solidification and particle scale structure formation as a function of microscopic surface properties of the particles and inter-particle lubrication forces.

For more information on this project please see:

1. Dynamic shear jamming under extension in dense granular suspensions;

S. Majumdar, I.R. Peters, E. Han, and H.M. Jaeger, Phys. Rev E, 95, 012603, 2017. (Link)

2. Direct observation of dynamic shear jamming in dense suspensions;

I.R. Peters, S. Majumdar, and H. Jaeger, Nature, 532, 214–217, 2016. (Link)

3. Discontinuous shear thickening in confined dilute carbon nanotube suspensions;

Sayantan Majumdar, Rema Krishnaswamy and A.K. Sood, Proc. Natl. Acad. Sci. U.S.A., 108, 8996-9001, (2011). (Link)

3. Elastic instabilities in worm-like micellar systems:

When a Newtonian liquid is sheared in the annular space between two concentric cylinders, the laminar flow becomes unstable with the formation of Taylor vortices at high Reynolds number (Re). This is known as Taylor-Couette instability. Surprisingly, polymeric / worm-like micellar fluids (self-assembled structures of surfactant molecules) show similar instability at very low Re. This instability is driven by fluid elasticity, rather than inertia. Surfactant worm-like micelles have wide scale applications in oil-recovery, drag reduction, consumer products etc., that often involve highly non-linear flows and deformations in these materials.

Under a constant applied stress, the global shear rate response in these systems show large fluctuations in time. However, how the local strains add up to give rise to such interesting global dynamics is very complex. Recently, using polarized light scattering, we found that the non-linear flow behaviour is intimately related to the organisation / deformations of Taylor vortices.

Currently, we are trying to bridge the gap between the local and global dynamics in these systems. In particular, we want to understand how the microscopic parameters (e.g. screening and micellar lengths) and dynamics (e.g. scission-recombination, reptation) affect the macroscopic non-linear flow behaviour in these systems.

For more information, please see:

1. Nonlinear viscoelasticity of entangled wormlike micellar fluid under large-amplitude oscillatory shear: Role of elastic Taylor-Couette instability; Sayantan Majumdar, and A.K. Sood, Phys. Rev E, 89, 062314, 2014. (Link)

2. Universality and scaling behavior of injected power in elastic turbulence in worm-like micellar gel; Sayantan Majumdar and A. K. Sood, Phys. Rev E (Rapid Comm.) 84, 015302(R) (2011). (Link)