Microdynamics of active particles in defect-rich colloidal crystals

Keara T Saud; Michael J Solomon

doi:10.1016/j.jcis.2023.03.025

Microdynamics of active particles in defect-rich colloidal crystals

J Colloid Interface Sci. 2023 Jul:641:950-960. doi: 10.1016/j.jcis.2023.03.025. Epub 2023 Mar 10.

Authors

Keara T Saud¹, Michael J Solomon²

Affiliations

¹ Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, United States; Biointerfaces Institute, University of Michigan, Ann Arbor, MI, United States.
² Biointerfaces Institute, University of Michigan, Ann Arbor, MI, United States; Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, United States. Electronic address: mjsolo@umich.edu.

PMID: 36989821
DOI: 10.1016/j.jcis.2023.03.025

Abstract

Hypothesis: Because they are self-propulsive, active colloidal particles can interact with their environment in ways that differ from passive, Brownian particles. Here, we explore how interactions in different microstructural regions may contribute to colloidal crystal annealing.

Experiments: We investigate active particles propagating in a quasi-2D colloidal crystal monolayer produced by alternating current electric fields (active-to-passive particle ratio ∼ 1:720). The active particle is a platinum Janus sphere propelled by asymmetric decomposition of hydrogen peroxide. Crystals are characterized for changes in void properties. The mean-squared-displacement of Janus particles are measured to determine how active microdynamics depend on the local microstructure, which is comprised of void regions, void-adjacent regions (defined as within three particle diameters of a void), and interstitial regions.

Findings: At active particle energy E_A = 2.55 k_BT, the average void size increases as much as three times and the average void anisotropy increases about 40% relative to the passive case. The average microdynamical enhancement, <δ(t)>, of Janus particles in the crystal relative to an equivalent passive Janus particle is reduced compared to that of a free, active particle (<δ(t) > is 1.88 ± 0.04 and 2.66 ± 0.08, respectively). The concentration of active particles is enriched in void and void-adjacent regions. Active particles exhibit the greatest change in dynamics relative to the passive control in void-adjacent regions (<δ(t)> = 2.58 ± 0.06). The results support the conjecture that active particle microdynamical enhancement in crystal lattices is affected by local defect structure.

Keywords: Active colloids; Active interactions; Active matter; Colloidal crystals; Defects; Janus particles.