ACMD Seminar: Braiding Dynamics of Topological Defects in Confined Active Nematics

Daniel Beller
Assistant Professor, Dept. of Physics & Astronomy, Johns Hopkins University

Tuesday, June 17, 2025, 3:00-4:00 PM ET (1:00-2:00 PM MT)

In-person at Bldg. 101 LR-D* with VTC to Boulder 1-4072
Online at: Zoom Gov (email seminar chairs for link to talk)

Abstract: Active nematics exhibit “topological chaos” in the bulk driven by the motions of topological defects. Strong confinement can lead instead to ordered flows together with periodic motions of defects. Recently, a periodic motion of three +1/2 disclinations known as the golden braid was demonstrated in the interior of a cardioid, with the cardioid’s cusp effectively pinning a -1/2 topological charge. Can this principle be generalized to produce ordered flows with even greater numbers of +1/2 defects by encoding their excess charge into the boundary conditions? We investigate this question with simulated active Beris-Edwards nematodynamics together with tools from braid theory. We show that when excess topological charge is programmed into the anchoring conditions or boundary geometry, we recover the golden braid motion of three defects and we predict a silver braid motion of four defects. We rationalize these findings by demonstrating how to predict time-averaged vortex structure from the boundary charge. Based on a simple counting argument for vortices and defects, our theory predicts that no such ordered flow will emerge for five or more +1/2 defects; our simulations corroborate this prediction by exhibiting only aperiodic defect swaps. Finally, we discuss implications of our findings for the production of topological entropy by active nematics.

Bio: Daniel Beller is an assistant professor in the Department of Physics and Astronomy at Johns Hopkins University. He and his research group use theoretical and computational approaches to study a range of soft matter and biological systems with emergent, complex spatial structure. Previously, he was an assistant professor in the Department of Physics at the University of California, Merced. He obtained his Ph.D. in Physics at the University of Pennsylvania, and he worked as a postdoctoral researcher at Harvard and Brown Universities.

Host: Leroy Jia

Note: This talk will be recorded to provide access to NIST staff and associates who could not be present to the time of the seminar. The recording will be made available in the Math channel on NISTube, which is accessible only on the NIST internal network. This recording could be released to the public through a Freedom of Information Act (FOIA) request. Do not discuss or visually present any sensitive (CUI/PII/BII) material. Ensure that no inappropriate material or any minors are contained within the background of any recording. (To facilitate this, we request that cameras of attendees are muted except when asking questions.)

*Safety Precaution: The hallway leading from the Courtyard to the exit closest to B-111 and B-113 will be used by contractors to move debris, machinery, and other supplies, as well as will be heavily trafficked by the contractors throughout the process. Be aware of the safety precautions posted during this time.

Note: Visitors from outside NIST must contact Meliza Lane at least 24 hours in advance.