Abstract
Synthetic fibrillar adhesives inspired by nature, most commonly by the gecko lizard, have been shown to strongly and repeatedly attach to smooth surfaces. These adhesives, mostly of monolithic construction, perform on par with their natural analogues on smooth surfaces but exhibit far inferior adhesive performance on rough surfaces. In this paper, we report on the adhesive performance of functionally graded microfibrillar adhesives based on a microfibre with a divergent end and a thin soft distal layer on rough surfaces. Monolithic and functionally graded fibre arrays were fabricated from polyurethanes and their adhesive performance on surfaces of varying roughness were quantified from force–distance data obtained using a custom adhesion measurement system. Average pull-off stress declined significantly with increasing roughness for the monolithic fibre array, dropping from 77 kPa on the smoothest (54 nm RMS roughness) to 19 kPa on the roughest (408 nm RMS roughness) testing surface. In comparison, pull-off stresses of 81 kPa and 63 kPa were obtained on the same respective smooth and rough surfaces with a functionally graded fibre array, which represents a more than threefold increase in adhesion to the roughest adhering surface. These results show that functionally graded fibrillar adhesives perform similar on all the testing surfaces unlike monolithic arrays and show potential as repeatable and reusable rough surface adhesives.
Highlights
Geckos have the ability to cling to both rough and smooth surfaces robustly as evidenced by the wide variety of the surfaces that they climb on in their natural habitat
They concluded that geckos were able to stay attached to the surface with 618 nm root mean square (RMS) roughness the longest, which is slightly different than the findings presented in [5]
Visual inspection of the force–distance data suggests that FG-7 μm sample is affected significantly less from substrate roughness used in this study in terms of pull-off stress compared with the monolithic sample
Summary
Geckos have the ability to cling to both rough and smooth surfaces robustly as evidenced by the wide variety of the surfaces that they climb on in their natural habitat. Measurements carried out by Huber et al [5] with individual seta demonstrates the ability of gecko to cope with local roughness When they measured the pull-off force of single seta from rough surfaces with root mean square (RMS) roughness ranging from 20 nm to 1.1 μm, a minimum pulloff force of 10 nN was obtained for adhering surfaces of RMS roughness 100–300 nm. Pugno & Lepore [6] studied the adherence of Tokay geckos in a similar fashion where they measured the time of adhesion to surfaces of varying roughness and used the time of adhesion as a relative measure of adherence They concluded that geckos were able to stay attached to the surface with 618 nm RMS roughness the longest, which is slightly different than the findings presented in [5]. While these studies use different methods to characterize the performance of the gecko on rough surfaces, their findings using rough surfaces of sub-micrometre to several micrometres RMS roughness suggest that the gecko seta is very adept at mitigating the effects of local roughness on adhesion
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