Is the active suspension in a complex viscoelastic fluid more chaotic or more ordered?

TL;DR

The paper investigates how a viscoelastic polymeric medium influences collective motion of microswimmers (squirmers) using lattice Boltzmann simulations. It reports a pronounced increase in orientational polarization in polymer solutions at intermediate swimmer concentrations, up to factors of $26$ for neutral squirmers and $5$ for pullers, compared with Newtonian fluids. The authors identify a hydrodynamic feedback mechanism: squirmer flows stretch polymers, which align with the swimmers and act as soft confinements, reinforcing swimmer orientation and polarization via hydrodynamic and steric interactions. They validate this by a strong correlation between polarization and a polymer–swimmer alignment parameter, and show that neither squirmer–polymer entanglement nor polymer–polymer entanglement explains the enhancement. These findings provide a framework for controlling active matter via polymer-mediated interactions and highlight the role of polymer deformation in complex fluids.

Abstract

The habitat of microorganisms is typically complex and viscoelastic. A natural question arises: Do polymers in a suspension of active swimmers enhance chaotic motion or promote orientational order? We address this issue by performing lattice Boltzmann simulations of squirmer suspensions in polymer solutions. At intermediate swimmer volume fractions, comparing to the Newtonian counterpart, polymers enhance polarization by up to a factor of 26 for neutral squirmers and 5 for pullers, thereby notably increasing orientational order. This effect arises from hydrodynamic feedback mechanism: squirmers stretch and align polymers, which in turn reinforce swimmer orientation and enhance polarization via hydrodynamic and steric interactions. The mechanism is validated by a positive correlation between polarization and a defined polymer-swimmer alignment parameter. Our findings establish a framework for understanding collective motion in complex fluids and suggest strategies for controlling active systems via polymer-mediated interactions.

Figures (9)

• Figure 1: Simulation snapshot and schematic of a system containing multiple squirmers and polymers. In the snapshot, red spheres represent pullers, and yellow chains represent flexible polymers. Swimmer types---pushers, neutral squirmers, and pullers---are distinguished by their respective $\beta$ values. Hydrodynamics are resolved using the lattice Boltzmann method.
• Figure 2: (a) Schematic illustration of the hydrodynamic feedback mechanism. (b) Dependence of polarization on active stress at various polymer concentrations, with swimmer concentration fixed at $\phi_s = 0.1$. (c) Dependence of polarization on swimmer concentration for different types of swimmers. Solid and dashed lines represent polymer concentrations $\phi_p = 1.0$ and $\phi_p = 0$, respectively.
• Figure 3: (a-c) Cluster size distribution, radius of gyration, and anisotropy parameter $b$ at $\phi_s=0.1$. (a) Cluster size distribution for neutral swimmers ($\beta=0$) and puller ($\beta=0.5$), in the presence and absence of polymers. (b) Radius of gyration for various active stresses, shown for both $\phi_p=1.0$ (solid lines) and $\phi_p=0.15$ (dashed lines) systems. $t_0=3r/B_1$ denotes the characteristic time. (c) Anisotropy parameter $b$ for different active stresses. (d-f) Same analyses conducted at $\phi_s=0.15$.
• Figure 4: (a) Simplified simulation snapshot illustrating the alignment between a polymer and surrounding neutral swimmers within a spherical region of radius $1.5R_g$ at $\phi_s=0.1$ and $\phi_p=1.0$. The large light-blue arrow indicates the polymer’s principal stretching direction, while the small dark-blue arrows represent the swimmers’ orientations. (b) Relationship between the mean polarization $\langle P \rangle$ (solid lines) and mean alignment parameter $\langle A \rangle$ (dash--dot lines) as functions of squirmer concentration for various active stresses at $\phi_p=1.0$.
• Figure 5: Schematic illustration of a polymer interacting with multiple squirmers at $\phi_s=0.15$ and $\phi_p=1.0$. A monomer is considered in contact with a squirmer when their center-to-center distances are less than $d=4.0$. For visualization, monomers within this defined contact distance are colored to match the corresponding squirmer's color.