Phase transition kinetics in colloidal systems

Solid-solid transformations are technologically important in shape-memory materials, metals, and alloys. A deep understanding of the microstructural changes and the underlying kinetic mechanisms is still lacking. The kinetics of fluid-gel transitions are difficult to study because of the lack of a convenient control parameter. In our group, we use electric-field-induced dipolar interactions as a switch to induce phase transition quenches, allowing us to study kinetics, and to probe reversibility and irreversibility. [PhD work of Shivani Semwal. Collaborators: Ivan Saika-Voivod (MUN), Alfons van Blaaderen (Utrecht), Priti Mohanty (KIIT-Bhubaneswar), Peter Schurtenberger (Lund), Surajit Sengupta and Saswati Ganguly (TIFR-Hyderabad)]

Saswati Ganguly, Priti Mohanty, Peter Schurtenberger, Surajit Sengupta, Anand Yethiraj, “Contrasting the dynamics of elastic and non-elastic deformations across an experimental colloidal martensitic transition”, Soft Matter, 13, 4689 – 4697 (2017)

Macromolecular crowding

The living cell is a crowded macromolecular environment. While crowding has been viewed primarily as an excluded volume hard-sphere effect, proteins are charged and have hydrophobic and polar groups, so non-specific soft interactions are important. In our research, we have used diffusion NMR to study diffusion in macromolecular complex fluids [see our review: Barhoum, S., Palit, S., & Yethiraj, A. (2016). Progress in Nuclear Magnetic Resonance Spectroscopy, 94, 1-10.].

Swomitra Palit et al., "Combining diffusion NMR and SANS enables precise measurements of polymer chain compression in a crowded environment", Phys. Rev. Lett, 118, 097801 (2017)

The bottom-up approach: Combining diffusion NMR with small-angle neutron scattering (SANS) and rheology, we study structure and microscopic dynamics in a polymer-colloid model system for crowding. [PhD work of Swomitra Palit (2018) and Venketesh Thrithamara. Collaborators: Stefan Wallin (MUN), Yun Liu (NIST)]

Swomitra Palit, Lilin He, William A. Hamilton, Arun Yethiraj, Anand Yethiraj, “Combining diffusion NMR and SANS enables precise measurements of polymer chain compression in a crowded environment”, Phys. Rev. Lett, 118, 097801 (2017).

The top-down approach: To contrast with our model-system approach, we are also examining the dynamics of a disordered protein (alpha-synuclein) in a realistic biological crowding environment made of lysed bacterial cells. [PhD work of Yanitza Trosel. Collaborator: Valerie Booth (MUN)]

Tunable Hydrodynamics

PhD work of Somayeh Khajehpour and Atul Varshney. Collaborators: Shankar Ghosh (TIFR-Mumbai) and Atul Varshney (IISER-Bhubaneswar)

We stumbled upon a beautiful model system for tunable hydrodynamics by
re-discovering the electrohydrodynamics of oil drops in an immiscible leaky dielectric oil, originally discovered by G. I. Taylor [Melcher, J. R., & Taylor, G. I. (1969). Annual Review of Fluid Mechanics, 1, 111-146.]. There were two key differences in our point of view: (a) While previous work on electrohydrodynamics studied individual drops, we examined collective behaviors and interactions of many drops; (b) In our work, we have employed high-speed imaging and used electric field frequency as a tuning parameter to control the hydrodynamic lengthscale. The outcome of this new point of view was rich dynamical phenomena as a function of field strength and frequency, with ordered drop arrays at high frequency, chaotic flows at low frequency, and multiscale turbulence at DC arising from anomalous super-diffusion in the fluid. The design of new materials using these novel interactions remains un-explored.

Nanoscale active matter

We are making active nanoparticles that could serve as active crowders and can be studied by new(-ish) optical techniques such as fluoresecence dynamic difference microscopy or fluoresence correlation spectroscopy. [M.Sc. Anas Alhasanat, with the help of undergraduate MUCEP students. Collaborators: Francesco Piazza & Josef Hamacek (Centre Biophysique Moléculaire, Orléans)]