APS Global Physics Summit student talks: Pronay Dutta, Yeseul Lee, and Kyle Sullivan

Emergent Structures and Re-entrant Transitions in Active-passive Colloidal Mixtures

Speaker:  Pronay Dutta

Authors:  Pronay Dutta and  Manasa Kandula

Dipolar interactions have long been used to control the self-assembly of colloids, but experimental studies on mixtures of active and passive colloids remain limited, with only a few simulations suggesting their potential. We investigate binary mixtures of electric field-driven active colloids and passive dielectric particles and demonstrate that they can produce interesting self-assembled structures , including space-spanning percolated networks.  In this talk, I will discuss the state diagram of steady states in active-passive mixtures, where the dipolar interactions are non-reciprocal and multimodal with intraspecies repulsion and interspecies attraction. The complex directed interactions lead to mesoscale organization and orientational ordering of the active colloids, resulting in new re-entrant transitions in the phase space of active and total area fractions. We identify the critical role played by the orientational ordering of active colloids in dictating the overall stability and dynamics of the observed percolated networks.  Our study provides an experimental demonstration of how active-passive colloidal mixtures can be used to tune the structural and dynamical properties of colloidal gels.

______________________________________________________________________

Are Neutral Polyzwitterions Really Neutral? Charge Symmetry Breaking!

Speaker: Yeseul Lee

Authors: Yeseul Lee and Murugappan Muthukumar

Determination of the net electric charge of a macromolecule dispersed in aqueous media continues to remain elusive. The primary reason for this is the lack of ability to explore the local dielectric constant accompanying water organization in the vicinity of ion-pair formation from a charged monomer and its counterion. In efforts to directly address this fundamental question, the design of our single-molecule experiment is as follows. Choosing a neutral polyzwitterion, where one charged group of the zwitterionic monomer is closer to the chain backbone and the other oppositely charged group of the monomer is distal from the chain backbone, we have conducted single-molecule electrophoresis through a solid-state nanopore. Surprisingly, the neutral polymer is not silent to the electric field, but moves in one direction, but not in the opposite direction. The direction of the molecule’s movement shows that the charge of the zwitterion monomer closer to the backbone is more masked by counterion binding in comparison with the distal charge. We attribute this charge symmetry breaking to a gradient in the local dielectric constant. We suggest a method to quantify this effect by combining our experimental results and our translocation theory based on Fokker-Planck formalism. This discovery that even a neutral polar polymer can function as a polyelectrolyte near an electric field opens a new avenue of understanding of how macromolecules move in crowded environments.

______________________________________________________________________

Self-limiting Assembly of Curvature-frustrated, Shell colloids

Speaker:  Kyle Sullivan

Authors:  Kyle Sullivan, Thomas Videbaek, Madhurima Roy, Mark Stevens, Ben Rogers, and Greg Grason

Geometric frustration in self-assembling systems can propagate local inter-subunit shape misfits to large scale strain gradients yielding finitely sized, self-limiting structures in equilibrium. Recently, models of soft, cylindrical, colloidal shells established a transition from self-limiting to unlimited self-stacking beyond a critical attraction range at zero temperature. However, short-ranged attractions may promote long assembly timescales challenging the feasibility of achieving self-limitation from an initially dispersed state out of equilibrium at finite temperature. Here, we propose a coarse-grained numerical model of frustrated colloidal shells and explore the thermodynamic conditions necessary for successful assembly. We find the regimes of temperature, concentration and attraction range for optimal assembly kinetics with finite temperature molecular dynamics simulations and further investigate the roles of subunit shape and entropy in avoiding kinetic traps which inhibit assembly. In conjunction with ongoing experiments of curved, DNA origami shells, we test theoretical predictions of resulting assembly size distributions and provide critical guidance for further experimental realizations of self-limiting frustrated particle systems.