Hydrodynamic interactions mask the true heterogeneity of a microscopic collective

TL;DR

Hydrodynamic interactions in microscopic collectives can obscure true heterogeneity, making systems appear more diverse than they intrinsically are, which complicates accurate inference of individual agent properties.

Contribution

This study reveals how dominant hydrodynamic interactions mask true heterogeneity in microscopic systems, affecting the inference of agent characteristics from observed data.

Findings

Hydrodynamic effects can make a homogeneous system appear heterogeneous.

Interactions influence collision angles, making them more isotropic.

Perceived heterogeneity depends on the balance between hydrodynamic interactions and intrinsic variability.

Abstract

Coordinated movement and self-organisation of active self-driven agents is common in nature and is seen across different scales, from herds of animals to collective motion in bacteria. Often, these systems are heterogeneous in composition, with different agents having different intrinsic motilities. Inferring these intrinsic characteristics and quantifying the level of heterogeneity in a collective system is crucial to understanding the observed emergent phenomena. However, when interaction effects dominate, i.e. the observed movement of an agent is strongly influenced by its interacting neighbours, inferring the intrinsic characteristics of agents becomes a challenge. We consider a collective system of agents that undergo purely physical interactions like collisions and long-range hydrodynamic interactions, which resembles a system of microswimmers immersed in a fluid medium. We incorporate heterogeneity into the system through variations in agent motility and examine how the perceived heterogeneity, inferred from measured speeds, depends on the strength of hydrodynamic interactions and the true intrinsic variability. The interplay between short-range collisions, long-range hydrodynamic interactions, and intrinsic heterogeneity makes the inference problem non-trivial. When hydrodynamic effects dominate, true heterogeneity is effectively masked, making even a homogeneous collective appear heterogeneous. The competing effects of collisions, which slow agents down, and hydrodynamic interactions, which enhance their motion, further complicate reliable inference. Hydrodynamic interactions also modify collision angles, rendering them more isotropic. Overall, the findings show highlight experimentally measured properties of microscopic collectives may not accurately reflect their true characteristics.