Research Projects

Despite decades of research in polymer solutions, several observations in flows remain unexplained till date. As mentioned in multiple review articles [1-3, 32], one such puzzling observation came from Sridhar and coworkers about two decades back. They subjected high molecular weight polymer solutions to extensional flows. Surprisingly, their measurements imply that the polymer chains are not fully stretched. Even more puzzling is the apparent decrease in stretch of chains with increasing extensional rates. This is in sharp contrast to theoretical and computational expectations from bead-spring models, with a few springs representing the entire chain. Ever since, researchers guessed that the origins of such counter- intuitive observations lie in the presence of self-entanglements and knots in the polymer chains [1-3]. These may become tighter in extensional flows instead of relaxing and prevent chains from reaching the maximum extension. The uncrossability of chain segments might also have a significant role. Moreover, possibility also exists that more knots are generated when an equilibrium ensemble is subjected to flow fields [1]. However, all of these remain hypothesis till date and the conventional computational tools are not sufficient. Broadly, the effects of self- entanglements and knots in the dynamics of polymer chains and rheology in different flow fields is not understood, as reviewed earlier [1]. As observed earlier, the proportions of knots are about 2-3% in shorter chains but become significant or longer ones. These knots are mostly tight, spanning a few Kuhn steps. The current state-of-the-art for polymer chain simulations is the bead-spring model, where each spring represents hundreds of Kuhn steps. Thus, the resolution is not sufficient to capture the effect of such tight knots. Some researchers did attempt to incorporate self-entanglements by introducing spring-spring uncrossability. They observed some effects on rheology but remain inconclusive towards the puzzling experimental observations. Ever since, researchers have asserted the need for finely resolved chain models, down to a single Kuhn step. They guess that due to the massive differences in conformational space between such bead-rod models and coarser bead-spring models, the rheological predictions might be significantly different. Our recent article indeed shows a strong dependence of predictions on the chain model resolution, even in the absence of hydrodynamic interactions (HI) and excluded volume (EV) [56]. Such differences are even amplified in the presence of HI (in preparation). Since then, our group has been actively developing algorithms and in-house GPU codes, with significant progress to speed up computations with such finely resolved chain models with HI and EV. In this proposal, we wish to address the long-standing issue of the role of self-entanglements and knots with chain models resolved to a single Kuhn step.