The Mechanics of Fire Ants. Mechanics of Soft Matter April 5 th, 2018 Moira Bruce, Karan Dikshit & Rob Wagner

Transcription

1 The Mechanics of Fire Ants Mechanics of Soft Matter April 5 th, 2018 Moira Bruce, Karan Dikshit & Rob Wagner

2 Outline Introduction to Fire Ants The Mechanical Behavior of Fire Ants Floating Rafts Cohesion Surface Tension Continuums of Fire Ants Rheological Tests Creep Tests Courtesy of Mlot, et al. [1]

3 Introduction: Ants as a Soft Matter Material Mesoscopic Building Blocks: Fire Ants Aggregations Occur; When Ants are Placed in Water In Other Controlled Environments E.g., Confinement Fire ants cluster together to form a continuous medium [2]. The medium behaves like both a fluid and a solid, depending on the time scale and load conditions. (b)-(d) Ants exhibit solidlike, elastic response (short timescale). (e)-(g) Ants exhibit fluid-like, viscous response (long timescale).

4 Ant Aggregations: Bond Types Types of ant bonds are formed using mandibles, tarsal claws, and adhesive pads Max Tensile Force 620 ± 100 dyn Leg-Leg connection 195 ± 7 dyn Leg-Body tensile force 69 ± 52 dyn A) Ant tarsal claw. B) Leg-to-adhesive pad bond C) Mandible-to-leg bond D) Claw-to-adhesive pad bond. [3] Ants linking their tarsi together. [3] A) Aggregation of ants acting as a raft. B) Fire ant using mandibles to attach. C) Fire ants using tarsal hooks to attach.[1]

5 Cohesion & Surface Tension Cohesion Aggregations exhibit finite Young s modulus and yield strength Ringelmann Effect: Yield stress does not scale linearly with crosssectional area: ants in group work 50% less than alone Density Packing fraction, ants control spacing (ρ raft = 0.2 g/ml) Surface Tension Wetting Surface Tension of raft: 10 3 dyn/cm (10x surface tension of H 2 O) Viscosity Have viscosity of 10 4 P (similar that of high-viscosity Si oil) Aggregations of ants with forced contact times of 5s, 60s, 240s. [4] Partial Wetting: water upon ants. [1] Tensile strength between aggregations vs forced contact times. [8]

6 Fire Ants Fluid-like Behavior: Rheology [2] Falling Sphere Stress plateaus at 70 Pa Attributed to force-dependent detachment rate Shear-thinning observed Δη => viscosity is adjusted by ants to maintain stress Viscosity data is compiled using different techniques. Top: photo of falling sphere viscometry set-up. Bottom: graphic of rheometry setup. [2] (a) Shear stress versus shear strain rate. (b) Viscosity versus shear strain rate. [2]

7 Fire Ants Fluid-like Behavior: Creep Creep measures time evolution of strain under constant stress. Various constant stresses applied Regions of linear strain-time relation (i.e. constant strain rate) observed. At low stress, regions of low strain observed Means ants are storing elastic energy instead of flowing Creep behavior (i.e. strain vs. stress) of fire ant aggregations at various applied stresses. (a) 40 Pa, (b) 70 Pa, (c) 100 Pa and (d) 200 Pa. [2]

8 Fire Ants Viscoelastic Behavior: Frequency Sweep Oscillatory Strain: γ t = γ 0 sin ωt Measure lag in stress response to find; Elastic (or Storage) Modulus, G Dissipation (or Loss) Modulus, G G G ~ω 0.39 for ants => dynamic relaxation time that is a function of frequency Ants maintain similar energy dissipation and storage values at all ω => k d = f( γ) ሶ (a) γ(t) and σ(t). (b) G and G plotted for various amplitudes of strain. (c) G and G plotted for a range of ω at 4 different live aggregation densities. (d) G and G versus ω for Maxwell model (note: intersection => single relaxation time). (e) G and G plotted for a range of ω at 3 different dead aggregation densities (G > G => dead ants are always elastic)

9 Conclusion Fire Ant Aggregations: a Soft Matter Material that Exhibit; 1. Hydrophobicity, cohesion, and surface tension 2. Elastic Properties at Short Timescales & High Densities 3. Shear-thinning, Fluid-like Properties at Long Timescales 4. Viscoelastic properties (i.e. Both Energy Storage & Dissipation) with a force-dependent relaxation rate All attributed to their ability to detach and reattach into lower stress configurations at a force-dependent rate. Image courtesy of < ants-form-giant-rafts-to-survive-floods >

10 Questions? Image courtesy of <

11 Citations 1. Mlot, Nathan J., Craig A. Tovey, and David L. Hu. Fire Ants Self-Assemble into Waterproof Rafts to Survive Floods. Proceedings of the National Academy of Sciences 108, no. 19 (May 10, 2011): Tennenbaum, Michael, Zhongyang Liu, David Hu, and Alberto Fernandez-Nieves. Mechanics of Fire Ant Aggregations. Nature Materials 15, no. 1 (January 2016): Foster, Paul C., Nathan J. Mlot, Angela Lin, and David L. Hu. Fire Ants Actively Control Spacing and Orientation within Self-Assemblages. Journal of Experimental Biology 217, no. 12 (June 15, 2014): D. L. Hu, S. Phonekeo, E. Altshuler, and F. Brochard-Wyart. Entangled Active Matter: From Cells to Ants. The European Physical Journal Special Topics 225, no. 4 (July 1, 2016): Jones, Steven G., Niki Abbasi, Abhinav Ahuja, Vivian Truong, and Scott S. H. Tsai. Floating and Sinking of Self-Assembled Spheres on Liquid-Liquid Interfaces: Rafts versus Stacks. Physics of Fluids 27, no. 7 (July 1, 2015): Mlot, Nathan J., Craig Tovey, and David L. Hu. Dynamics and Shape of Large Fire Ant Rafts. Communicative & Integrative Biology 5, no. 6 (November 1, 2012): Ni, Rui, and Nicholas T. Ouellette. On the Tensile Strength of Insect Swarms. Physical Biology 13, no. 4 (2016): Phonekeo, Sulisay, Tanvi Dave, Matthew Kern, Scott V. Franklin, and David L. Hu. Ant Aggregations Self-Heal to Compensate for the Ringelmann Effect. Soft Matter 12, no. 18 (May 4, 2016): Phonekeo, Sulisay, Nathan Mlot, Daria Monaenkova, David L. Hu, and Craig Tovey. Fire Ants Perpetually Rebuild Sinking Towers. Royal Society Open Science 4, no. 7 (July 1, 2017): Tennenbaum, Michael, and Alberto Fernandez-Nieves. Activity-Driven Changes in the Mechanical Properties of Fire Ant Aggregations. Physical Review E 96, no. 5 (November 9, 2017):