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Dynamic of Flooded Adhesion in the Presence of Draining Channels
Charles Dhong, Rohini Gupta, and Joelle Frechette
Johns Hopkins University
Baltimore, MD, 21218, USA
[email protected]
The locomotion mechanisms employed by tree frogs
under flooded conditions could offer the ultimate solution
for the need of strong, reversible, reusable, tunable, and
water tolerant adhesives. Central to the adhesion and locomotion of tree frogs are their structured toe pads, which
consist of an array of 10 μm hexagonal epithelial cells separated by interconnected channels that are 1 μm wide and
1-11
10 μm deep.
The mechanisms for tree frog adhesion
under flooded conditions, and by extension the role played
by the structured toe pads, has been the subject of investigations. It is suspected that tree frogs can climb and grip
on wet surfaces (and prevent hydroplaning) by reducing
the hydrodynamic forces through drainage of the fluid in
12,13
the channels present on their toe pads.
While hypothesized, the mechanism for tree frog adhesion, and more
specifically the role played by hydrodynamics and elastic
deformation, has not been clearly demonstrated.
We performed peeling adhesion measurement in viscous fluid between a structured and a smooth surface. The
structured surface consisted of a hexagonal array of cylindrical posts to represent the network of interconnected
channels. The adhesion measurements are consistent with
the facilitated drainage via the channels when the two surfaces are sufficiently close. The adhesion measurements
were also compared to the measurement of the repulsive
hydrodynamic interaction upon approach.
Experimental
We employ a peeling apparatus allowing the control
of the separation between two surfaces prior to adhesion
measurement. The adhesion is measured from a load cell
while the interacting surfaces are separated by a linear
motor. The applied load, fluid viscosity, and loading time
is varied. The structured surfaces were made out of the
SU-8 photoresist. The films were deposited on glass slides
and on a coverslips. The fluids employed were silicone oils
of varying viscosities.
Results and Discussion
We investigated the role of initial loading, loading
time, fluid viscosity and surface structure on adhesion. To
do so we created two surfaces with cylindrical pillars with
varying surface dimensions, as shown in Table 1. The pillars were made out of photoresist SU-8 (rigid) and supported on a layer of Cytop.
Table 1. Surface structure investigated.
Sample
Flat surface
10x3
10x10
Pillar diam.
Pillar height
N/A
10 μm
10 μm
10 μm
10 μm
10 μm
Channel
width
N/A
3 μm
10 μm
A summary of the measured work of adhesion is
shown in Figure 1. The load indicated is the mass that has
been applied to the top surface to make contact with the
bottom one. The loading time is constant in all experiments here at 600 seconds. Three viscosities are shown
here. As seen increasing the load at a given viscosity increases the adhesion for all the surfaces and viscosities
investigated. Similarly, increasing the viscosity increases
the work of adhesion, which is to be expected for viscous
adhesion in the presence of a spring.
Adhesion Energy (J)
Introduction
500
400
300
Smooth
10x3
10x10
W1 = 0.05 kg
W2 = 0.121 kg
W3 = 0.208 kg
200
100
0
W1W2W3
50 cst
W1W2W3
W1W2W3
200 cst 1000 cst
Figure 1. Example of Figure Caption.
Interestingly, if we look more carefully at the effect of
applied load at a given viscosity (Figure 2) we observe that
in the case of the smooth surface increasing the load increases adhesion. This is to be expected as increasing the
load allows the surfaces to reach closer separation before
being pulled apart. The same effect is not observed for the
case of the 10x10 surface, where increasing the load has no
effect on adhesion. We suspect here that the work required
to bring the 10x10 surface in contact with the second surface is significantly decreased due to the presence of draining channels. Therefore boundary contact is achieved prior
to the end of loading time. The reduction in work to bring
the surfaces together is then reflected in the adhesion
measurements as a disappearance of the effect of loading
on adhesion.
12.
Energy (μJ)
250
200
Smooth
10x3
10x10
150
100
50
0
0.05 kg 0.121 kg 0.208 kg
Figure 2. Effect of applied load on the work of adhesion
measured in a fluid with a viscosity of 200 cSt.
Conclusions
We investigated the importance of how surfaces are
brought in contact on the adhesion of structured surfaces
designed to mimic the tree frog toe pads. We found that
the presence of draining channels help bring the surfaces
closer and allow the surfaces to make intimate contact with
a much lower applied load.
Acknowledgements
The authors are grateful for the support by the Office
of Naval Research under N00014110629 and by the ACS
Petroleum Research Fund.
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