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, 102 (45), 16293-6

Evidence for Capillarity Contributions to Gecko Adhesion From Single Spatula Nanomechanical Measurements

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Evidence for Capillarity Contributions to Gecko Adhesion From Single Spatula Nanomechanical Measurements

Gerrit Huber et al. Proc Natl Acad Sci U S A.

Abstract

The hairy attachment system on a gecko's toes, consisting of one billion spatulae in the case of Gekko gecko [Ruibal, R. & Ernst, V. (1965) J. Morphol. 117, 271-294], allows it to adhere to nearly all surface topographies. The mechanistic basis for gecko adhesion has been intensely investigated, but the lowest hierarchical level, that of the spatula, has become experimentally accessible only recently. This report details measurements of the adhesion force exerted by a single gecko spatula for various atmospheric conditions and surface chemistries. Through judicious choice and modification of substrates, the short- and long-range adhesive forces are separated. In contrast to previous work [Autumn, K., Sitti, M., Liang, Y. C. A., Peattie, A. M., Hansen, W. R., Sponberg, S., Kenny, T. W., Fearing, R., Israelachvili, J. N. & Full, R. J. (2002) Proc. Natl. Acad. Sci. USA 99, 12252-12256], our measurements clearly show that humidity contributes significantly to gecko adhesion on a nanoscopic level. These findings are crucial for the development of artificial biomimetic attachment systems.

Figures

Fig. 1.
Fig. 1.
The lizard G. gecko with one foot adhering to a glass plate (foreground) and setal structures of its attachment organs (background).
Fig. 2.
Fig. 2.
SEM image of a single seta glued to an AFM cantilever. (Insets) Lamellar structure at lower magnification (Upper Right) and four single spatulae isolated at the setal tip by focused ion beam micromachining (Lower Center).
Fig. 3.
Fig. 3.
Spatular pull-off force of specimen 1 vs. the contact angle θW of a water drop on four types of Si wafers and on glass. Wafer families N and T differ by the thickness of the top amorphous Si oxide layer. The “phob” type was obtained from “phil” type wafers by deposition of octadecyl-trichlorosilane. The relative humidity during the experiment was 52%. For comparison, the pull-off forces on a glass substrate (θW = 58.4°) measured at comparable humidity (glass square data point taken from Fig. 4) is included. Additionally, pull-off forces while completely submerged under water [open circles labeled with glass(w) and N-phil(w)] are displayed.
Fig. 4.
Fig. 4.
Spatular pull-off forces on glass and N-phob vs. humidity at ambient temperature. The straight line corresponds to the calculations following Eq. 4. (Inset) The increase Δh in water-film thickness on a Si wafer with increasing humidity as measured by ellipsometry.

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