How do fire ants control the rheology of their aggregations? A statistical mechanics approach

Abstract

<jats:p>Active networks are omnipresent in nature, from the molecular to the macro-scale. In this study, we explore the mechanical behaviour of fire ant aggregations, closely knit swarms that display impressive dynamics culminating with the aggregations’ capacity to self-heal and adapt to the environment. Although the combined elasticity and rheology of the ant aggregation can be characterized by phenomenological mechanical models (e.g. linear Maxwell or Kelvin–Voigt model), it is not clear how the behaviour of individual ants affects the aggregations’ emerging responses. Here, we explore an alternative way to think about these materials, describing them as a collection of individuals connected via elastic chains that associate and dissociate over time. Using our knowledge of these connections—e.g. their elasticity and attachment/dissociation rates—we construct a statistical description of connection stretch and derive an evolution equation for the corresponding stretch distribution. This time-evolving stretch distribution is then used to determine important macroscopic measures, e.g. stress, energy storage and energy dissipation, in the network. In this context, we show how the physical characteristics and activities of individual ants can explain the elasticity, flow and shear thinning of the aggregation. In particular, we find that experimental results are matched if the detachment rate between two individuals increases with tension in the connection.</jats:p>

Treadmilling and dynamic protrusions in fire ant rafts

“…These cohesive swarms are cross-linked by reversible ant-to-ant bonds [10][11][12] which may dissociate from highly stressed states and re-associate into lower energy configurations without sustaining damage. In the last 10 years, researchers have begun to investigate the mechanical properties of these aggregated swarms, which demonstrate nonlinear viscoelastic responses because of the reversibility of their inter-ant bonds [9,13,14]. However, another remarkable feature of fire ants that contributes to their complex response is activity [15,16].…”

Collective protection and transport in entangled biological and robotic active matter

“…For robotics, the creation of a coherent swarm of simple robots has been a dream of roboticists for years, and our robotic system is part of an emerging trend in leveraging mechanics and physics to perform collective tasks in a decentralized way (34,35) rather than the traditional algorithm-based and centrally-controlled approach to swarms (30,(52)(53)(54)(55)(56)(57)(58). For biology, the worm blobs hold exciting potential to inspire adaptive active materials as well as advance our understanding of emergent biomechanics of living collectives (15)(16)(17)(18)(19). We note that, to the best of our knowledge, the only other entangled assemblage capable of emergent motility occurs at cellular scales, where the amoeboid cells of the slime mold D. discoideum form a motile slug synchronized by cAMP waves (59).…”

“…The latter type of entangled active matter aggregates enables the formation of large mechanically functional structures (bivouac, rafts, bridges etc.) that enable both new functionalities not accessible to the individual as well as enabling survival benefits to the collective, specially in harsh and adverse environmental conditions in which it is impossible for individuals to survive on their own (15)(16)(17)(18)(19).In engineered systems, the emergent dynamics of active matter collectives has been explored in particles ranging in 21 size from micrometers (active colloids) to centimeters (robots) 22 (20-24). Specifically for collective swarm robotics, the major-23 ity of the past work has focused on mathematical modeling 24 (25-29).…”

“…The latter type of entangled active matter aggregates enables the formation of large mechanically functional structures (bivouac, rafts, bridges etc.) that enable both new functionalities not accessible to the individual as well as enabling survival benefits to the collective, specially in harsh and adverse environmental conditions in which it is impossible for individuals to survive on their own (15)(16)(17)(18)(19).…”

Interplay of elastic instabilities and viscoelasticity in the finite deformation of thin membranes

“…In this context, results from this work may be used to control, fine-tune and eventually harvest the rubber and buckling instabilities in practical applications. Finally, a fundamental understanding of the interplay between elastic instabilities and viscosity may also be of relevance on a wide variety of synthetic and biological materials, such as non-Newtonian fluids [67], the cell walls of plants and fungi [68] as well as cell sheets [69] and aggregations of insects [70].…”