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Tribology of Hierarchical Surface Texture Mimicking the Hexagonal Pattern Evolved in Tree Frogs

Alexey Tsipenyuk
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Alexey Tsipenyuk

on 21 December 2013

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Transcript of Tribology of Hierarchical Surface Texture Mimicking the Hexagonal Pattern Evolved in Tree Frogs

Potential applications
Recently,
Continental
has developed a winter tire with honeycomb profiles.
enhanced wear performance on dry roads
less aquaplaning and better braking on wet roads
better grip, and more traction on ice
Tree Frog toe pads
Measurements
Fabrication
Tribology of Hierarchical Surface Texture Mimicking the Hexagonal Pattern Evolved in Tree Frogs
All natural surfaces are to some degree rough, and this roughness occurs at many different length scales.
To get over this problem, at least one of the
surfaces must be extremely soft
, so that the softer material conforming to the contours of the other can
increase actual contact
.
Such materials
do not stick
to each other because the
forces
between neutral molecules are
negligible
at separations of only a few atomic distances and the
roughness prevents sufficiently close contact
between the surfaces.
Roles for toe pad microstructures
Alexey Tsipenyuk
,
Asst. Prof. Michael Varenberg
allows the pads to conform to the shape of large-scale irregularities
Questions?
To the naked eye, the frogs' toe pads appear patterned with flat-topped, hexagonal cells surrounded by grooves filled with liquid (watery mucus).
... or perhaps even a nonslip shoes.
Frogs of several families adapted for living and moving about in trees are known as
tree frogs
. All are characterised by expanded
adhesive disks on the tips of their toes
and fingers allow them to climb up any surface. Tree frogs can adhere and move on rough (hard) vertical surfaces during heavy rain where the surfaces are
flooded with water
.
On closer inspection, however, the tops were not flat at all but rather were covered by tightly packed "nanopillars", each with a small dimple in the end, which
generate powerful friction
against the surfaces they contact.
Tree frogs
do not have to overcome the sticking forces
exerted by their toe pads
every time they make a step
. They have a very efficient
detachment mechanism
by which the toe pads are gently
peeled off the surface
, a process
requiring minimum force
and the frog’s
jumping

ability

remain

unaffected.
conform to surface irregularities
make the pad softer
absorb excess water
rapid optimisation of the fluid layer
actual contact with the surface (friction forces)
Understanding the adhesive properties of tree frog feet could lead to
better tire design
...
fluid secretion
mucus gland
mm
um
nm
spread

mucus

evenly
over the pad surface
under wet conditions,
remove

surplus

water
Brazilian tree frog (Meyers et al., 2011)
Litoria caerulea (Federle et al. 2006)
SU-8 is a commonly used epoxy-based, near-UV, negative photoresist. It can be used to pattern high aspect ratio (> 20) structures. When exposed, SU-8's long molecular chains cross-link causing the solidification of the material.
Complete data set of the maximum friction force values as a function of area density. Only first level of the hierarchical hexagonal-microstructure with 10 μm in height and 50 μm in diameter.
20% of area density
80% of area density
Advanced low noise signature and energy efficient shaft bearings for submarine industry.
Complete data set of the maximum friction force values as a function of area density. The base pillars have a radius of 50μm and a height of 10μm. The top pillars have a radius of 4μm and a height of 2μm.
Full transcript