Can water be the "adhesive"?

Water is often considered a lubricant because it separates the interface between two solid surfaces, thereby weakening intermolecular forces
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In fact, since the effective range of van der Waals interactions is <0.
6 nm, the van der Waals forces between solid surfaces are weakened as long as there are a few layers of water molecules (~1 nm) separating the interface
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Combined with the fact that solid surfaces are often bumpy and rough, the average clearance is expected to be higher for coatings online at coatingol.
com
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Because of this, underwater reversible bonding has always been a major challenge
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The need for underwater bonding is common in a variety of medical and industrial applications, such as medical patches, tissue engineering, and underwater soft robotics

In nature, some aquatic animals, such as octopuses (Fig.

In view of this, the René Hensel team of the Leibniz Institute for New Materials used two-photon lithography to prepare micro-suction cups, and combined with an in-situ pressure sensor and observation camera to analyze the detailed properties of the suction cup attachment/detachment under water, revealing The hydrodynamics of the suction cup during the underwater attachment process identified three different detachment mechanisms of the suction cup during the underwater detachment process

[The effect of van der Waals force on the suction cup]

The authors used two-photon lithography and submillimeter-scale molding processes to fabricate polyurethane micro-suction cups composed of rhizomes and tapered lips (Fig.

Test results of suction cups in air show that in air, the pull-off stress of substrates with and without holes is similar, implying that suction pressure plays a secondary role

The results of underwater suction cups are just the opposite

To determine any contribution of van der Waals forces, we need to independently measure the suction pressure inside the cup during in situ retraction

Figure 1.

【Strong adhesion mechanism of underwater suction cup】

On the basis of the experimental results, the authors propose the mechanism of strong adhesion of the underwater suction cup: during the process of pulling the suction cup off, suction will be generated inside the suction cup cavity and between the lip and the substrate

Figure 2.

【Separation mechanism of underwater suction cup】

The authors summarized three separation modes of underwater suction cups through experiments (Fig.

Figure 3.

Figure 4.

【summary】

This work explains the detailed mechanism of the attachment of deformable micro-suction cups

1) In a humid environment, a firm adhesion between the two surfaces can be achieved by the presence of water between the suction cup and the substrate
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The adhesion strength of the underwater suction cup is much higher than atmospheric pressure and about an order of magnitude higher than that under dry conditions
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2) The underwater attachment of the deformable micro-suction cup is essentially different from the attachment in the air
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The latter is dominated by van der Waals forces, while the former relies on complex interactions between geometry, elasticity, pull-off velocity and fluid dynamics
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3) Under realistic conditions, the underwater adhesion strength is limited by the elastic properties of the suction cup/rhizome structure, rather than the inherent cavitation properties of water
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Adhesion strength can be further improved by selecting appropriate material parameters to delay buckling of the suction cup lip
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