Sand swimming is a specialized
locomotion used by several species of lizards and snakes. The following was
posted on the Physics
Central website. Be sure to visit the website and view the videos that
include a Sandfish (Scincus scincus, Family Scincidae) swimming through
sand.
Swimming through sand: The secret of sandfish locomotion
Monday, December 27,
2010
We know how airplanes
glide in the air and how submarines move through water, but we don't know much
about how creatures "swim" through sand. 'Til now...
How an object's shape
affects its generation of lift and drag in both the air and in water is well
understood. Otherwise, we'd be misplacing submarines all the time. But how
objects - animals in particular - create lift and drag in granular materials
like sand is less well understood.
A couple of Ph.D.
students and their professor have been taking a closer look at what happens
when sand-dwelling creatures - like lizards, crabs, snakes and worms - dive below
the surface.
Yang Ding and Nick
Gravish, along with Daniel I. Goldman, their Georgia Tech professor of physics,
have been studying the sandfish lizard, a popular sand-dwelling pet, to see how
it maneuvers in its subterranean environment.
Goldman described the
sandfish as a little lizard that lives in the desert in North Africa. When
startled, it can burrow 10 cm beneath the surface in less than half a second.
Its wedge-shaped head, which biologists believe gives the critter its
lightning-quick burrowing ability, was the project's inspiration.
"We think the
sandfish is the champion of rapid burial," Goldman said.
Another thing the
trio noticed about the lizard, Ding said, is that its belly is really flat.
"We thought that might have an effect," he said.
To test the theory on
both the head shape and the belly, the team dragged three objects of different
shapes through a container filled with tiny glass beads that acted as a sand
analogue. They watched to see whether each object generated any lift - the force
perpendicular to the direction of motion that "pushes" an object up.
The first was a
cylinder. The team dragged it horizontally through the beads (if it were a Coke
can, it would have been dragged from the dash in between the words
"Coca" and "Cola") and measured the forces acting on it.
The cylinder
experienced positive lift; it tended to rise within the beads, headed for the
surface. A square rod was also dragged through the beads and it, too, rose
towards the surface, but just barely. The third object was a half-cylinder. It
experienced negative lift, sinking lower into the beads as it was dragged
along.
Of the three objects,
the half-cylinder most approximates the shape of the sandfish lizard's head.
Since the lizard also experiences negative lift when it enters the sand, the
lab test showed that the half-cylinder was a good starting point for modeling
the lizard's head.
The researchers then
dragged flat plates through the sand. The plates were given roughly the same
angle of attack - or angle away from horizontal - as the leading edge of each
of the objects. To mimic the cylinder, the first plate was at a very small
angle almost perpendicular to the floor. Just as for the cylinder, the plate
experienced positive lift.
The plate was then
dragged forward at a 90 degree angle relative to the floor, and again, as with
the cube-shaped rod, there was next to no lift. Then the plate was dragged at a
wide angle, leaning back from the direction of motion like a lawn chair leans
back from the surf at the beach. This time, as with the half-cylinder, there
was negative lift.
These were exciting
results for the researchers because they realized that they could break up the
shape of any object into flat plates and sum them up in a computer model to see
the forces acting on any object. In addition to showing lift, the models also
helped them to understand how much drag, or force acting opposite the direction
of motion, "tugging" on an object, was being produced.
"We found that
we can basically understand the forces by decomposing them in flat
plates," Gravish said. "You can build whatever object you want to see
what forces it undergoes in granular materials."
A database of how
objects respond when traveling through granular materials can be created simply
by finding the sum of simple materials - the plates. Since there are no
equations to describe locomotion in granular materials, the find was
particularly exciting.
"What you really
want to do in all this business is figure out the principles of what's going
on," Goldman said. The results of this research have opened the door for
the physicists to do just that.
On an earlier
research project, Goldman's CRAB Lab used high-speed x-ray imaging to observe
the lizard's movement when submerged. They found that it doesn't use its legs
when swimming through sand, instead tucking them by its side and slithering
like a snake.
Using the data
garnered from watching lizards swim and the new lift and drag research, the
CRAB Lab got down to serious business and built a sandfish lizard robot they
hope to debut at the 2011 International Conference on Robotics and Automation.
They envision creating a rubble-swimmer that could aid with search-and-rescue
missions after disasters like the earthquake in Haiti or the 9/11 collapse of the
Twin Towers.
Ding, Gravish and
Goldman's paper, "Drag induced lift in granular media," is due to
appear in Physical Review Letters
Dec. 31.
Posted by Echo Romeo