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Learning from nature : how mussels stick to rocks, and what it can teach us about material design
Mussels permanently adhere to surfaces through a circular plaque that is
attached to the animal body via a long thin thread ; forming a
mushroom-shaped geometry. A plaque just a few millimeters in diameter with
a 250-micron diameter thread can withstand large pull forces of a several
Newtons without debonding. While the strength of individual chemical bonds
plays a role in determining the adhesive strength, the contact mechanics
associated with the mushroom shape is also critically important. To better
understand the role of mechanics and geometry on the adhesive strength of
mussels, we study the detachment of the mussel holdfast from glass using a
custom built load frame. This device is capable of pulling on samples along
any orientation and measuring the resulting force, while simultaneously
imaging both the bulk deformation of the plaque and the debonding
glass-plaque interface. Using this device, we measure the bond strength,
observe debonding initiation, and relate the load force to the bulk
deformation of the plaque. We find that the shape of the mussel holdfast
improves the bond strength by an order of magnitude compared to other
simple geometries and that mechanical yielding of the mussel plaque further
improves the bond strength by nearly two orders of magnitude as compared to
the strength of the interfacial chemical bonds. Mechanical tests are
complemented by ultrastructural studies using electron microscopy and
neutron scattering. These reveal the internal structure of the plaque to be
a dense interconnected network of pores, that may play important roles in
stress distribution and energy dissipation through geometric rearrangements
or fluid flow between pores. These results show that adhesive strength can
be tuned without need for interfacial chemical modification, and suggest
new pathways for synthesis of biomimetic adhesive structures.
