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Abstract
While the reversible attachment of artificial structures underwater has moved into the focus of many recent publications, the ability of organisms to walk on and attach to surfaces underwater remains almost unstudied. Here, we describe the behaviour of the water-lily leaf beetle Galerucella nymphaeae when it adheres to surfaces underwater and compare its attachment properties on hydrophilic and hydrophobic surfaces underwater and in the air. The beetles remained attached to horizontal leaves underwater for a few minutes and then detached. When the leaf was inclined, the beetles started to move upward immediately. There was no difference in the size of the tarsal air bubble visible beneath the beetles’ tarsi underwater, between a hydrophilic (54° contact angle of water) and a hydrophobic (99°) surface. The beetles gained the highest traction forces on a hydrophilic surface in the air, the lowest on a hydrophobic surface in air, and intermediate traction on both surfaces underwater. The forces measured on both surfaces underwater did not differ significantly. We discuss factors responsible for the observed effects and conclude that capillary forces on the tarsal air bubble might play a major role in the adhesion to the studied surfaces.
Highlights
In adhesion science, the topic of underwater attachment has moved into the focus of many publications in recent years
We describe the behaviour of the water-lily leaf beetle Galerucella nymphaeae when it adheres to surfaces underwater and compare its attachment properties on hydrophilic and hydrophobic surfaces underwater and in the air
There was no difference in the size of the tarsal air bubble visible beneath the beetles’ tarsi underwater, between a hydrophilic (54◦ contact angle of water) and a hydrophobic (99◦) surface
Summary
The topic of underwater attachment has moved into the focus of many publications in recent years. Permanent underwater adhesion using glues is well known for numerous aquatic animals [1,2] and this knowledge was even transferred to the synthesis of biomimetic glues [3,4,5,6]. Reversible artificial adhesive systems based on surface nano- and microstructures for underwater application were recently produced [7,8,9]. Reversible adhesion by animals underwater was the concern of only a few studies. Other researchers focused on the underwater adhesive performance of the hair-like structures of, e.g., diving beetles or mussels, without considering the attachment of the organism as a whole [11,12]. The adhesion to wet substrates as well as the roles of capillary adhesion and nanobubbles still need to be clarified [13]
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