Active Screws: Emergent Active Chiral Nematics of Spinning Self-Propelled Rods

Abstract

Several types of active agents self-propel by spinning around their propulsion axis, thus behaving as active screws. Examples include cytoskeletal filaments in gliding assays, magnetically-driven colloidal helices, and microorganisms like the soil bacterium $\it{M. xanthus}$. Here, we develop a model for spinning self-propelled rods on a substrate, and we coarse-grain it to derive the corresponding hydrodynamic equations. If the rods propel purely along their axis, they form an active nematic at high density and activity. However, spinning rods can also roll sideways as they move. We find that this transverse motion turns the system into a chiral active nematic. Thus, we identify a mechanism whereby individual chirality can give rise to collective chiral flows. Finally, we analyze experiments on $\it{M. xanthus}$ colonies to show that they exhibit chiral flows around topological defects, with a chiral activity about an order of magnitude weaker than the achiral one. Our work reveals the collective behavior of active screws, which is relevant to colonies of social bacteria and groups of unicellular parasites.

Figures (7)

• Figure 1: Classes of active chiral particles.\ref{['Fig chiral-self-propelled']}, Chiral self-propelled particles propelling on the plane and spinning around the axis perpendicular to it. \ref{['Fig spinners']}, Spinners do not translate. \ref{['Fig rollers']}, Rollers spin around an in-plane axis perpendicular to their self-propulsion. \ref{['Fig screws']}, Active screws spin and self-propel along the same axis.
• Figure 2: Motion and interactions of active screws.\ref{['Fig screwing']}, An active screw spinning at a rate $\dot\phi_i$ with a pitch $\ell_\parallel$ propels along its axis at a speed $v_i^{0,\parallel} = \ell_\parallel \dot\phi_i$. \ref{['Fig rolling']}, An active screw can also propel sideways by rolling at a speed $v_i^{0,\perp}=\ell_\perp \dot\phi_i$. \ref{['Fig interactions']}, The interactions between active screws include forces, torques that change their orientations, and torques that change their spinning rate.
• Figure 3: Activity-induced nematic order. Phase diagram displaying the nematic order strength $S$ (see SM). Following Ref. Das2024, the numerical phase boundary (white) is identified from the points of steepest ascent of the measured $S$. The theoretical phase boundary (red) is predicted by setting $a_Q = 0$ in \ref{['eq nematic-growth-rate']}. The nematic phase is chiral in the presence of rolling.
• Figure 4: Chiral flows around topological defects.\ref{['Fig plus-schematic']}-\ref{['Fig minus-schematic']}, Defect schematics show the order parameter (color) and the director field (lines). \ref{['Fig plus-experiments']}-\ref{['Fig minus-experiments']}, Experimental flow fields from Ref. Han2025. \ref{['Fig plus-theory']}-\ref{['Fig minus-theory']}, Fit of active chiral nematic theory (see \ref{['fit']} of SM and Ref. Han2025).
• Figure S1: Nematic order in simulations. Phase diagrams showing activity-induced nematic order both without reversals (\ref{['Fig nematic-no-reversals']}) and with reversals at a rate $f_\text{rev} =1$ (\ref{['Fig nematic-reversals']}).