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Viscoelastic Ants
December 17, 2015
In the days long before
YouTube videos, my weekly dose of quality
humor came from
Monty Python's Flying Circus. Even today, snippets of this show, such as the
The Lumberjack Song[1] and
The Spanish Inquisition[2] are still popular. One
catchphrase on Monty Python was "
and now for something completely different." That describes the topic of this article, the
mechanics of
fire ants.
Materials scientists often examine the
mechanical properties of
natural materials, such as
wood and
limestone, and occasionally they'll examine the
natural products of
animals, such as
spider silk and
nacre (mother of pearl). While
mechanical testing on parts of animals, such as
bone, is sometimes done, why would someone want to assess the mechanical properties of a whole
living organism?
Fire ants get their name from their
sting, which injects an
alkaloid venom,
Solenopsin, that causes a sensation similar to being
burned by
fire. Just as for
bee stings, the sting of the fire ant can kill sensitive people, and there are significant populations of these ants in fourteen
US states.[3]
Masses of fire ants will change shape in response to the
environment. Fire ants will link their bodies to form
self-assembled aggregations such as
rafts to float in
floods, and they will also link their bodies to build
bridges across gaps. Just like a
liquid, these aggregations can drip and spread, and they can also withstand
applied loads. A
physicist and
mechanical engineers at the
Georgia Institute of Technology (Atlanta, Georgia) decided to investigate the liquid-like and
solid-like mechanical properties of fire ants, and their findings appear in
Nature Materials.[4-6]
Fire ant (Solenopsis invicta) worker. This South American species established itself in the southeastern United States in the 1930s.
(Specimen from Brackenridge Field Laboratory, Austin, Texas, via Wikimedia Commons.)
The Georgia Tech research team put thousands of ants into a
rheometer, an
apparatus used to measure the
shear strength of liquid and semi-solid materials (see figure).[6] The ants were
sheared at speeds from a very slow rate of about 0.0001 rev/min to higher rates up to about 100 rev/min. Live ants had mechanical properties similar to those of dead ants at high speeds. When the ant aggregate is forced to
flow, live ants break their linkages with other ants and "
play dead," causing the
viscosity to dramatically decrease.[6]
Fire ant rheometer
(Illustration by the author using Inkscape. Ant image via Wikimedia Commons.)
Alberto Fernandez-Nieves, an
associate professor of
physics at Georgia Tech, summarizes the rheometer results as follows:
"It's not unlike ketchup... The harder you squeeze, the easier it flows. But with ants, this happens much more dramatically than with ketchup."[6]
Ants in a rheometer.
At high shear rates, the ants "play dead," an action that decreases the viscosity of the mass.
(Georgia Tech image.)
Says
David Hu, an associate professor of
mechanical engineering at Georgia Tech,
"Ants seem to have an on/off switch in that they let go for sufficiently large applied forces... Despite wanting to be together, they let go and behave like a fluid to prevent getting injured or killed."[6]
A similar
phenomenon is seen when an object, such as a
penny, is dropped into a pile of ants. In that case, ants will flow around the
coin as it sinks, and the sinking takes a considerable time to happen. When such a pile is poked quickly, the mechanical response is like that of a
spring, and the pile quickly returns to its original shape.[6] Such a mechanical behavior is known as
viscoelasticity, and the rheometer measurements also show
viscoelastic behavior.[4]
If a penny is dropped into a pile of ants, the insects release their hold on each other in response to the pressure. When the coin passes through, the ants reconnect.
(Georgia Tech image.)
I discussed viscoelasticity in a previous article (
Silly Putty, August 6, 2014).
Silly Putty, a
trademarked material produced by
Crayola, is an example of a viscoelastic material. The original Silly Putty composition was just
polydimethylsiloxane, [C2H6OSi]n, a
silicone oil, reacted with
boric acid.[7-8] The viscoelasticity of the polydimethylsiloxane gave the material the mechanical property that it could be slowly worked, like a
clay, but it would act as an
elastic solid when the rate of applied force is large.
In other findings, when the ant
density increases, the
elastic modulus rises. This is likely a consequence of ant crowding and subsequent
jamming.[4] The
shear-thinning behavior appears to happen at a stress load just below where individual ants would be torn apart.[4]
Experiments with dead ants did not show any liquid-like behavior.[6]
There might be an application of such ant dynamics in
self-healing materials.[6] Says Hu,
"If you cut a dinner roll with a knife, you're going to end up with two pieces of bread... But if you cut through a pile of ants, they'll simply let the knife go through, then reform on the other side. They're like liquid metal - just like that scene in the Terminator movie."[6]
This
research is supported by the
U.S. Army Research Laboratory and the
U.S. Army Research Office Mechanical Sciences Division,
Complex Dynamics and Systems Program.[6]
References:
- Monty Python's Lumberjack song, YouTube Video, January 19, 2006. A German language version can be found here (Lumberjack-Song in German, YouTube Video, August 4, 2006).
- Monty Python's Flying Circus - Complete Spanish Inquisition, YouTube Video, January 18, 2009.
- "States affected by: Imported Fire Ant," USDA, Animal and Plant Health Inspection Service.
- Michael Tennenbaum, Zhongyang Liu, David Hu, and Alberto Fernandez-Nieves, "Mechanics of fire ant aggregations," Nature Materials, October 26, 2015, doi:10.1038/nmat4450.
- Supplementary Information for ref. 4 (PDF File).
- Ants: Both Solid-like and Liquid-like - Ants act like ketchup when forces applied, Georgia Institute of Technology Press Release, October 26, 2015.
- Mcgregor Rob Roy and Warrick Earl Leathen, "Treating dimethyl silicone polymer with boric oxide," US Patent No. 2,431,878, December 2, 1947.
- James G E Wright, "Process for making puttylike elastic plastic, siloxane derivative composition containing zinc hydroxide," US Patent No. 2,541,851, Feb 13, 1951.
- Additional fire ant images and videos, via Dropbox.
Permanent Link to this article
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