Soft materials under non-equilibrium conditions

In the past, most studies have focused on the equilibrium properties of biopolymers, but more recently it has become evident that their non-equilibrium behavior plays an important, if not predominant role in their function. It is thus of fundamental interest to understand the dynamics of driven biological or bio-inspired polymers since it will provide insight into the interplay of the relevant forces determining the overall behavior of these systems. Furthermore, a thorough understanding of polymers under unsteady conditions may lead to the development of more sophisticated techniques with many opportunities for application, such as DNA sequencing. We are currently interested in the interplay between hydrodynamic forces arising due to the presence of the flow and intrinsic equilibrium forces of soft materials. We are particularly interested in:

  1. Flow-induced unfolding of single polymer chains

    Inspired by the study of von Willebrand Factor, a large biopolymer known to unfold from a globular structure to elongated structure at high flow rates, we have performed theoretical and coarse-grained simulation studies of polymer chains under a variety of flow conditions. Our studies focus on how flow induces unfolding events, and how this knowledge could be used to induce targeted unfolding in different locations in the body.

  2. Single chain unfolding in the presence of colloids

    A large volume fraction of the blood stream consists of colloidal particles like blood cells, which can influence both flow behavior and polymer unfolding. To understand the unfolding of polymers like von Willebrand Factor in a biological context, the group has used massively parallel GPU simulations to understand the unfolding of polymer chains in the presence of colloids.

  3. Influence of dynamic binding on single chain kinetics

    Typical simulation studies of polymer chains use Lennard-Jones interactions between monomers to induce a globular state. By instead incorporating a novel reaction model for dynamic binding between monomers, it is possible to decouple the equilibrium state of the polymer globule from the dynamics of globule formation. Using this model, the group is studying the kinetics of polymer collapse under both flow conditions and direct pulling.

  4. Propulsion based on supermagnetic colloids

    Recently, we have designed a new self-assembled microswimmer for future microrobotic applications that uses surface-based hydrodynamics for both locomotion and propulsion. However, the full potential of this device is still to be discovered and more studies are needed to understand the most facile route to surface flow manipulation, Eventually, we aim to design more complex walkers that could in principle revolutionize current methods for treating different problems that range from in vitro fertilization to kidney stone disposal, and could be incorporated into microfluidic technologies.

Selected publications

Dynamics of polymers in flowing colloidal suspensions
H. Chen and A. Alexander-Katz
Phys. Rev. Lett. 107, 128301 (2011).

Equilibrium structure and dynamics of self-associating single polymers
C.E. Sing, A. Alexander-Katz
Macromolecules 44(17), 6962-6971 (2011).

Dynamics of collapsed polymers under the simultaneous influence of elongational and shear flows
C.E. Sing, A. Alexander-Katz
J. Chem. Phys. 135, 014902 (2011).

Controlled surface-induced flows from the motion of self-assembled colloidal walkers
C. E. Sing, L. Schmid, M. F. Schneider, T. Franke, A. Alexander-Katz
Proc. Natl. Acad. Sci. USA 107(2), 535-540 (2010).

We can predict the geometry of a single chain under strong shear flows with analystical theory, and demonstrate the presence of an upwards tether force due to hydrodynamics.

We develop fundamental models to describe the dynamic behaviors of collapsed polymers in the presence of, for example, elongational flows.

By incorporating Bell-model type interactions into traditional polymer chain models, we can derive polymers into a collapsed conformation, only now including dynamic binding information.

Department of Materials Science and Engineering MIT