Polymers and colloids in flows : from dynamics to self-healing

Abstract

Polymers and colloids are important building blocks of life as well as many modern technologies. Driven by ow, polymers and colloids can express very complex yet interesting behavior. This thesis aims at a fundamental understanding of the dynamical properties of dierent polymer-colloid mixtures in flows using computer simulations. A special motivation comes from the blood clotting process. Our blood is a complex uid made of polymeric proteins and colloid-like cells. Controlled by ow, a blood-clotting protein (the so-called von Willebrand factor or vWF) can change shapes from a compact structure to an extended morphology. This polymeric protein later on forms composites with the colloidal cells (platelets) and completes the initial blood-clotting task. In this thesis, we build minimalist simulation models trying to capture the essential physics behind blood clotting. We first examine the behavior of single polymers in passive owing colloidal suspensions. Our results show that the presence of colloids has a pronounced eect on the unfolding and refolding cycles of collapsed polymers (which is believed to be a good model for vWF), but has negligible effects for non-collapsed polymers. Further inspection of the conformations reveals that the strong ow around the colloids and the direct physical compression exerted on the collapsed polymers diffusing in between colloidal shear bands largely facilitate the initiation and unraveling of the collapsed chains. We believe these results are important for rheological studies of (bio)polymer- (bio)colloid mixtures, and give insight on the activation of von Willebrand factor in owing cell suspensions. We then study interacting polymer-colloid mixtures in flows. In blood clotting, the formation of plug, which is essentially a polymer-colloid (vWF-platelet) composite, is believed to be driven by shear ow, and contrary to our intuition, its assembly is enhanced under stronger flow conditions. Here, inspired by blood clotting, we show that polymer-colloid composite assembly in shear flow is a universal process that can be tailored to obtain dierent types of aggregates including loose and dense aggregates, as well as hydrodynamically induced log-type aggregates. The process is highly controllable and reversible, depending mostly on the shear rate and the strength of the polymer-colloid binding potential. Our results have important implications for the polymer-colloid binding potential. Our results have important implications for the assembly of polymer-colloid composites, an important challenge of immense technological relevance. Furthermore, flow-driven reversible composite formation represents a new paradigm in non-equilibrium self-assembly. We also study binary colloidal mixtures and self-associating polymers, both of which are very relevant to blood clotting. Platelet margination refers to the phenomenon that for flowing red blood cell and platelet mixtures in vessels, the platelets will migrate to vessel walls. Using a simple binary colloidal suspension model, we show that the nonhomogeneous red blood cell distribution as well as the shear dependent hydrodynamic interaction is key for platelet margination. We believe this separation process is important not only in the biophysics of blood clotting, but also in applied science such as drug delivery or coatings. Catch-bonds refer to the counterintuitive notion that the average bond lifetime has a maximum at a nonzero applied force. They have been found in several ligand-receptor pairs including vWF/platelet GP1b-alpha. Here we use coarse-grained simulations and kinetic theory to demonstrate that a multimeric protein, with self-associating domain pairs, can display catch-bond behavior in ow. Our biomimetic design shows how one could build and tune macromolecules that exhibit catch-bond characteristics. We finally include an appendix that describes an unrelated project that is to solve for the block copolymer propagator in polymer field theory using Lattice Boltzmann method originally developed for hydrodynamics. Comparing to the conventional pseudo-spectral method, the Lattice Boltzmann approach is slightly inaccurate yet has many extra benefits including the optimal parallel computing eciency and the ability for grid refinements and arbitrary boundary conditions.