Emergent tetratic ordering in autophoretic rods mediated by torque.

Area of Science:

• Soft Matter Physics
• Active Matter Systems
• Nanotechnology

Background:

• Synthetic microswimmers, like chemically driven nanorods, exhibit complex collective behaviors driven by various interactions.
• Understanding emergent pattern formation is crucial for designing advanced active materials.

Purpose of the Study:

• To investigate the influence of torque-mediated interactions on the clustering behavior of self-propelled autophoretic gold-rhodium (Au-Rh) nanorods.
• To elucidate the role of hydrodynamic, electrokinetic, and phoretic forces in self-assembly dynamics.

Main Methods:

• Integration of experimental studies, finite element modeling (FEM), and Brownian dynamics (BD) simulations.
• Characterization of nanorod clustering at varying particle fractions and fuel concentrations.
• Quantification of interaction torques using FEM.

Main Results:

• Transient V-shaped clusters observed at low particle fractions (<1%) and high fuel concentrations (5 wt% H2O2).
• Stable dimers, trimers, and higher-order clusters formed at increased particle fractions (>1%).
• Emergence of tetratic clusters (orthogonal alignment) at intermediate densities (approx. 10%) under low fuel conditions (1 wt% H2O2).
• FEM determined a net torque of 8.23 × 10^-20 Nm driving rotational motion and clustering.

Conclusions:

• Hydrodynamic and phoretic torques are critical drivers of collective behavior and self-assembly in synthetic microswimmers.
• The study provides insights into designing active materials with tunable self-assembly properties.
• Findings advance the understanding of emergent phenomena in non-equilibrium systems.