Humidity induced softening leads to apparent capillary effect in gecko adhesion

  • PDF / 85,488 Bytes
  • 6 Pages / 595 x 842 pts (A4) Page_size
  • 101 Downloads / 140 Views

DOWNLOAD

REPORT


1274-QQ06-04

Humidity induced softening leads to apparent capillary effect in gecko adhesion Bin Chen*a, Huajian Gao# *Engineering Mechanics, Institute of High Performance Computing, A*STAR,138632, Singapore. # Division of Engineering, Brown University, Providence, RI, 02912, USA. ABSTRACT Humidity can lower the stiffness of beta keratin, which is the main component of spatula pads terminated at the bottom of a gecko’s toe. To see if this softening can influence gecko adhesion, numerical simulation of the vertical peeling of a spatula pad adhered on a rough surface is performed, which shows that the reduction in material stiffness indeed leads to substantial increases in the pull-off force "(Chen, B. and Gao, H. International Journal of Applied Mechanics, 2010). This work provides an alternative explanation of experimental observations on the capillary effect in gecko adhesion. a

Corresponding author: Tel.: (65) 64191583; Email address: [email protected]

INTRODUCTION Among numerous conjectures on the underlying mechanism of gecko adhesion, such as glue, suction and interlocking, the van der Waals and capillary interactions have survived the experimental test so far. While it has been shown that the van der Waals force plays the primary role in gecko adhesion [2,3], humidity was also shown to have a strong effect [4-6]. Although the experiments have convincingly shown that humidity does play important roles in gecko adhesion, interpretation of the underlying physical mechanisms remains controversial. Fig. 1a shows the spatulae structure, the finest terminal branches of setae under gecko’s toe. It is known that capillary forces due to liquid bridges would require a sufficient amount of water between gecko’s spatula and substrate. Considering the fact that the spatula is hydrophobic, significant capillary condensation is not expected to occur until a relative humidity (RH) level exceeding 90% [5]. In fact, the ellipsometry data [5] confirmed that only an adsorbed monolayer of water is present. Meanwhile, another important experiment [7] showed that the adhesion energy remains constant and is independent of relative humidity up to 6070% RH even for hydrophilic surfaces with contact angles less than 10 degree, which was consistent with other reports of the capillary effect on adhesion at nanoscale [8,9]. These experiments have casted substantial doubt on the existing interpretation

of the observed humidity effect on gecko adhesion in terms of capillary condensation. Therefore, it remains a puzzle how relative humidity can cause a continuous rise in the pull-off force of a single spatula pad as observed in experiments [5]. This paper is a summary of our recent numerical simulations [1] which showed that the moisture induced stiffness reduction of spatula leads to substantially higher adhesion force with magnitude comparable to those observed in experiments. Based on these simulations, we are proposing an alternative interpretation of the effect of humidity on gecko adhesion, without the controversial assumptio