Research
On the beach, local interactions between individual grains of sand can determine if your castle collapses or stands. Small changes in the local composition of articular cartilage can change the mechanical properties by orders of magnitude, making the difference between a healthy joint or arthritis. Soft and living matter is rich with examples of howmicroscale interactions and organization influences macroscale properties. The opposite is also true - macroscopic forces and fields can alter the microscopic organization. The application of shear stress can result in the formation of particle force networks in shear thickening suspensions. Macroscopic chemical gradients can drive microscopic organization of microbial communities in nature. Understanding this micro-macro coupling is a fundamental challenge at the intersection of physics, biology, and engineering.
During my PhD and postdoc, I have made progress in uncovering this coupling within different soft matter systems. In my PhD, I studied how the interparticle interactions and microstructure alter the macroscopic flow properties of dense colloidal suspensions. In my postdoctoral studies, I am using the techniques and tools acquired by the study of non-living systems during my PhD to study living systems, specifically bacterial motility and growth.
Morphodynamics of Bacterial Communities in 3D
Previous studies of bacterial growth on two-dimensional surfaces have shown that local and internal stresses can cause wrinkling, alter cell orientation, and change the overall colony shape. These morphological changes have important biological implications, enhancing nutrient and oxygen transport in biofilms, modifying the local environment, and even reducing bacterial diversity. However, studies of isolated stresses in 2D biofilms do not capture the complexities of global stresses in three-dimensional environments, where we expect heterogeneous stress distributions, local deformations, and anisotropic strains. The challenge of creating a transparent medium that supports bacterial growth in three dimensions has limited our understanding of how mechanical stresses affect bacterial growth in 3D. I developed techniques for creating transparent three-dimensional environments suitable for bacterial growth and designed structured bacterial colonies in 3D. I have identified key parameters that determine shape, size, and spatial organization of two species of bacterial colonies growing in three dimensions.
Publications:
M.Ramaswamy, A. Martinez Calvo, C Trenado Yuste, N. Wingreen, S. S. Datta,
Morphodynamics of bacterial communities proliferating in three dimensions.
Chemotaxis of Bacterial Colonies
Chemotaxis is the movement of organisms in response to a chemical gradient. The process of chemotaxis has ramifications for diverse areas from human health to bioremediation using microbes. My current projects in this area include investigating the changes in bacterial chemotactic waves as it travels from a liquid media into a porous, soil-like environment, and studying the chemotaxis of bacterial colonies towards ephemeral nutrient sources.
Universal Scaling in Shear Thickening Suspensions
Nearly all dense suspensions undergo dramatic and abrupt increases in the viscosity when sheared at high stresses. Such shear thickening transitions occur when the dominant interactions between the suspended particles shift from hydrodynamic to frictional. I have been working to interpret abrupt shear thickening as a precursor to a rigidity transition, and give a complete theory of the viscosity in terms of a universal crossover scaling function from the frictionless jamming point to a rigidity transition associated with friction, anisotropy, and shear. I am now working to use the same formulation to understand the data from other shear protocols. This reformulation opens the door to importing the vast theoretical machinery developed to understand equilibrium critical phenomena to elucidate fundamental physical aspects of the shear thickening transition.
Publications:
M. Ramaswamy, I. Griniasty, J.P. Sethna, B. Chakraborty, I. Cohen, A universal scaling framework for controlling phase behavior in thickening and jamming suspensions, (under review, Physical Review Letters).
M. Ramaswamy, I. Griniasty, D.B. Liarte, A. Shetty, E. Katifori, E. Del Gado, J.P. Sethna, B. Chakraborty, I. Cohen, Universal scaling of shear thickening transitions, Journal of Rheology, 67(6), 1189-1197 (2023).
Tuning Shear Thickening Behaviour in Colloidal Suspensions
Shear thickening suspensions, where the viscosity increases dramatically with shear rate, are crucial in a number of applications including cements, 3D printing and soft robotics. However, they also cause a number of problems by clogging pipes, jamming nozzles and breaking instruments. Over the last few years, I have worked extensively on tuning this shear thickening behaviour via external mechanical techniques like orthogonal flows and acoustics. These techniques are not only crucial in the many applications of these suspensions but have also led to new insights into the dynamics of force chain formation and breaking, and the structure of the force chain network.
Publications:
P. Sehgal, M. Ramaswamy (co-first authors), C. J. Ness, E.Y.X. Ong, I. Cohen, B.J. Kirby, Viscosity Metafluids, (accepted, Physical Review Research).
E. Y. X. Ong, A. R. Barth, N. Singh, M. Ramaswamy, A. Shetty, B. Chakraborty, J. P. Sethna, and I. Cohen, Jamming memory into acoustically trained dense suspensions under shear, Physical Review X, 14, 021027 (2024).
R. Niu, M. Ramaswamy (co-first authors), C. Ness, A. Shetty, and I. Cohen, Tunable solidification of cornstarch under impact: How to make someone walking on cornstarch sink, Science Advances, 6.19, (2020).
P. Sehgal, M. Ramaswamy (co-first authors), I. Cohen, B.J. Kirby, Using acoustic perturbations to dynamically tune shear thickening in colloidal suspensions, Phys. Rev. Lett, 123, 128001 (2019).
Confinement Effects in Colloidal Suspensions
Previous research on confined suspensions has shown that these suspensions display a wide range of structures that could alter the viscosity. However it is extremely difficult to probe the viscosity of these suspensions, as commercially designed rheometers are incapable of reaching these small gaps. I used a custom built rheoscope to connect the various structures to the measured viscosity and compared experiments with simulations to disentangle the various contributions to the suspension stress. These measurements illustrate the complex relationship between structure and rheology, and suggest new techniques to tune the viscosity of confined suspensions. I am now working to perform similar experiments in the shear thickening regime, and understand the effect of the confinement length scale on the suspension viscosity. These experiments will help us tune the behaviour of confined suspensions in applications ranging from automobile components to household items.
Publications:
M. Ramaswamy, N.Y.C. Lin, B.D. Leahy, C.Ness, A.M. Fiore, J.W. Swan, I. Cohen, How confinement-induced structures alter the contribution of hydrodynamic and short-ranged repulsion forces to the viscosity of colloidal suspensions, Phys. Rev. X, 7, 041005 (2017).
Measuring the Tensorial Components of Suspension Stress
While there are many studies on both the macroscopic changes in the viscosity and the microscopic suspension properties that cause shear thickening, little is known about the mesoscale – how the forces in these systems are transmitted and the force network that is set up in these suspensions. Using a novel shear protocol I have measured the stress response due to the three dimensional force network in these shear thickening suspensions. I am working on comparing these results to a tensorial model, and using high precision featuring techniques to simultaneously image these force networks. These results are key to further understanding and and more efficiently tuning the behavior of shear thickening suspensions.
