Research

• Length and Time scales in Active and Passive glass Forming Liquids:

Dramatic slowing down of dynamics with decreasing temperature or increasing density in glass-forming liquids is still remained an open unsolved problem even after decades of research. The main puzzle is the dramatic rise of viscosity without any major change in structural properties. In a general continuous phase transition, the slowing down is associated with diverging correlation length scale, and the physics of this phenomenon can be understood simply from the divergence of the length scale within the renormalisation group approach. The mere existence of such a correlation length and its growth is still not understood due to the intrinsic difficulty of defining the order in amorphous systems. Thus, the determination of thermodynamic factors that control structural relaxation and diffusion in glass-forming liquids will be very important to understand this important old problem.  Recent works, including some of our works, suggest the existence of such length scales in the problem. Recent findings of glass-like dynamical behaviour in various biologically relevant systems led to the development of a new field called “active matter” in which the constituent particles experience thermal as well as other non-equilibrium fluctuations (self-propulsion) and these materials in the dense limit produce a new type of glasses known in the literature as active glasses. The emergence of glassiness in these active glasses is of immediate importance because of their possible implications in various biological processes. Some of our recent works suggest that the growth of dynamical and static length scales in these glasses may be enhanced by many factors leading to the possibility of better understanding the physics of glass formation in a diverse set of physically relevant contexts.

• Yielding of Amorphous Solids at Bulk and Nano Scale:

Amorphous material, besides being theoretically interesting, also see their uses extensively in industry. Understanding the failure mechanisms of such materials thus becomes of great practical importance. Using highly parallelised molecular dynamics simulations, our studies try to answer what happens to glasses under various forms of external mechanical loading and what controls the manner in which the glasses eventually break. One of the in-silico experiments that we perform involves providing “self-motility” to some fraction of the particles in a dense amorphous solid and performing tensile testing on the resulting “active glass” that forms. We have found evidence that in certain regimes, introducing activity can decrease the ductility of such systems, which goes counter to intuition. Understanding the mechanism which leads to this and whether the activity can drive a ductile-to-brittle transition would have implications in understanding and even designing various bio-materials. Amorphous materials are prone to catastrophic failure via the formation of shear bands. Shear bands are system-spanning zones of high strain. In another study, we aim to understand the transition from homogeneous to heterogeneous yielding (characterised by shear bands) in amorphous solids under compressive and oscillatory shear. Having an atomistic understanding of how these shear bands formed might give us clues on how to mitigate them. We also plan to extend this analysis to include ultra-stable (or well-annealed) glasses. Another direction of research involves probing cavitation instabilities present in the glassy landscape. These nano-cavities are often a precursor to the sudden failures of materials discussed above.

• Understanding the physics of failure mechanisms in amorphous solids at mesoscale:

In a recent extensive MD study by Paul et al. [Phy. Rev. Res., 2020], it was observed that if one decreases the aspect ratio (ratio of height by width) of metallic glass specimen, the specimen shows a crossover from brittle characteristic (with cavitation) to pronounced necking behaviour, a typical ductile characteristic. A critical aspect ratio wherein this transition is observed is also found to be very generic across dimensions. In the paper, it was also found that there is a critical curvature associated with the critical aspect ratio. As this is a geometric phenomenon, in this part of the project, we want to mimic this aspect ratio effect using continuum simulations. The finite element method (FEM) is employed for solving the continuum (solid mechanics) governing equations. In this project, we want to develop a FE model of amorphous plasticity with information taken from extensive microscopic simulations to mimic the metallic glass closer. Another future goal is to have a unified framework for modelling amorphous plasticity and damage to understand the effect of cyclic shear in these materials. This project will be done in close collaboration with Prof. Pinaki Chaudhuri (IMSc, Chennai) and Dr. Vishwas Venkatesh (IIT-Palakkad).

• Origin of Ferroelectric nematic phase:

Experimentally it is found that the temperature dependence of electroviscous coefficient shows a diverging behaviour near the nematic to ferro-nematic phase transition temperature and is two orders of magnitude larger than the usual nematic liquid-crystal. There are other experimental results in which viscosity is found to increase up to 600% near nematic to ferroelectric phase transition in the presence of the electric field. We aim to develop an understanding of this ferroelectric nematic phase using MD simulation to study the rheological properties of these liquid crystals in the presence of an electric field. Shearing the nematic phase in the presence of an electric field is another goal that will be studied in this project. We want to study the system’s response to increasing shear rate in the presence of the electric field and to confirm if shear can give rise to flexo-electric polarization, which was observed very recently in experiments. Depending on the result, it might be interesting to study these effects in other liquid phases like smectic, ferroelectric nematic and ferroelectric smectic. This will be done in close collaboration with Prof. Surajit Dhara, a soft condensed matter experimentalist from the University of Hyderabad.

• Anomalous Low Frequency Vibrational Modes in Amorphous solids:

Amorphous solids possess an excess of low-frequency modes above the Debye prediction, termed the “Boson peak”. It has been suggested that the large specific heat of glasses and the plastic failure of amorphous solids are intimately related to these low-frequency non-phononic vibrational modes. However, a complete understanding of the structural and statistical properties of these vibrational modes has been elusive and remains an important current topic of interest in the field of disordered solids. We are studying amorphous solids prepared in open boundary conditions that are stable under all small perturbations. Our initial observation suggests that open boundary solids of sufficiently large system size possess more stability in their vibrational spectrum compared to solids prepared under periodic boundary conditions (PBC). We found that an amorphous solid with zero bulk and shear stress shows w⁵ behaviour if the solids have mechanical stability; otherwise, it shows w⁴ behaviour. In this project, we want to develop a theoretical understanding of the emergence of universal w⁵ vibrational density of states in stable amorphous solids with zero bulk and shear stress rather in competition with the usual Debye-like density of states. We also want to study the role of these modes, which are believed to be quasi-localized spatially, on the mechanical properties of these solids. This project will be done in collaboration with my TIFRH colleague, Dr. Kabir Ramola. 

• Understanding Active Matter and Active Glasses within Hamiltonian Formalisms

Nature builds complex structures via Self- assembly. Famous examples are the complex structures formed by a flock of birds and a school of fishes through topological interactions with many individuals. Flocking and forming compact groups of collectively moving individuals is a hallmark of animal group behaviour. Examples are ubiquitous: the development of multicellular organisms, bacterial colonies, large-scale insects swarm, flocks of birds etc. A similar collective motion happens in engineered systems like driven colloids and grains. These research directions lead to a new field of study known as active matter, with significant attention on active glasses. Active glass characterizes the extreme dynamical slowdown without any detectable structural change in a dense assembly of self-propelled particles (SPP). Recent experiments show that signatures of glassy dynamics, such as non-exponential relaxation, caging, and dynamical heterogeneity, are essential in many biological and biology-inspired systems. Examples include a cellular monolayer, cell-cytoplasm, colonies of bacteria and ants, vertically vibrated rods, and other artificial active systems. These recent surges in research in this broad field of non-equilibrium statistical mechanics also led to the discovery of many interesting novel phenomena whose understanding is still lacking. In this project, we want to develop a model system that, in thermal equilibrium, might have behaviour similar to active matter, which is inherently non-equilibrium in nature. This model can also be a good model to describe the collective motion of bacterial colonies (E. Coli). This part of the proposal will be done in close collaboration with my German collaborator, Prof. Juergen Horbach from Heinrich Heine University, Dusseldorf, Germany.

• Developing Enhanced Sampling Methods:

In most standard enhanced sampling methods for example, meta-dynamics one requires a good reaction coordinate to be defined before these methods can be used to obtain useful results. If the choice of the coordinate is not good, the results can be completely useless. In practice one often chooses such reaction coordinates by trial and error for one particular problem and this choice changes depending on the problem. Thus, there is no generic formalism to choose the reaction coordinate. Non-affine fields found in the context of mechanical deformation of amorphous systems can be such a generic method as preliminary results suggest that non-affine directions is connected to the smallest barrier in the system. It is generally accepted that at low temperatures, the smallest barriers are the one, which plays crucial roles in the dynamics. We would like to develop enhanced sampling methods based on these non-affine fields and finally try to understand many biological processes like protein folding and protein-ligand binding which are very slow to be accessed by normal computational methods (possibly collaborating with Prof. Jagananth Mondal).