Water as a “glue”: Elasticity-enhanced wet attachment of biomimetic microcup structures

Abstract

Octopus, clingfish, and larva use soft cups to attach to surfaces under water. Recently, various bioinspired cups have been engineered. However, the mechanisms of their attachment and detachment remain elusive. Using a novel microcup, fabricated by two-photon lithography, coupled with in situ pressure sensor and observation cameras, we reveal the detailed nature of its attachment/detachment under water. It involves elasticity-enhanced hydrodynamics generating “self-sealing” and high suction at the cup-substrate interface, converting water into “glue.” Detachment is mediated by seal breaking. Three distinct mechanisms of breaking are identified, including elastic buckling of the cup rim. A mathematical model describes the interplay between the attachment/detachment process, geometry, elasto-hydrodynamics, and cup retraction speed. If the speed is too slow, then the octopus cannot attach; if the tide is too gentle for the larva, then water cannot serve as a glue. The concept of “water glue” can innovate underwater transport and manufacturing strategies.

Attachment of bioinspired microfibrils in fluids: transition from a hydrodynamic to hydrostatic mechanism

“…Furthermore, the elastic deformation of a suction cup is limited by the fluid extension, which inherently differs for gases (compressible) and liquids (inextensible). A deformed cup could then break the seal before the hydrostatic pressure limit is reached, which instantly leads to detachment, as recently reported in [13].…”

“…Figure 7 a depicts shrinkage of the circular contact from R0=100 μm to Rb≈80 μm, causing about 20% compressive strain at the circumference of the cap. Such strain possibly leads to an instability, which can potentially rupture the seal and detach the fibril [13]. The red inset in figure 7 a shows the intact (i) seal shortly before it broke (ii).…”

“…All data needed to evaluate the conclusions in the paper are present in the paper and/or the electronic supplementary material [53]. The data generated in this study are presented in figures 1–8 in the main text as well as electronic supplementary material, figures S1–S5.…”

A humidity-resistant bio-inspired microfibrillar adhesive fabricated using a phenyl-rich polysiloxane elastomer for reliable skin patches

“…It is possible to extend the present chemical design to a wider range of applications, such as mushroom-shaped micropillars with higher pull-off forces for multifunctional skin adhesive patches, 17,32,36 directional microwedge adhesives for grasping, 37,38 and microcup structures for wet adhesion. 39,40 Experimental Materials Octamethylcyclotetrasiloxane (D4, monomer, 499.5%, Dow Corning) was dried by calcium chloride powder and distilled under vacuum before use; 2,4,6,8-tetramethyl-2,4,6,8-tetraphenylcyclotetrasiloxane (P2, monomer, 497%, Aladdin), tetramethylammonium hydroxide pentahydrate (TMAH, 97%, Aladdin), and benzoylperoxide (BPO, 98%, Aladdin) were used as received.…”

“…It is possible to extend the present chemical design to a wider range of applications, such as mushroom-shaped micropillars with higher pull-off forces for multifunctional skin adhesive patches, 17,32,36 directional microwedge adhesives for grasping, 37,38 and microcup structures for wet adhesion. 39,40…”