Sliding-induced adhesion of stiff polymer microfibre arrays. I. Macroscale behaviour

Abstract

Gecko-inspired microfibre arrays with 42 million polypropylene fibres cm K2 (each fibre with elastic modulus 1 GPa, length 20 mm and diameter 0.6 mm) were fabricated and tested under pure shear loading conditions, after removing a preload of less than 0.1 N cm K2 . After sliding to engage fibres, 2 cm 2 patches developed up to 4 N of shear force with an estimated contact region of 0.44 cm 2 . The control unfibrillated surface had no measurable shear force. For comparison, a natural setal patch tested under the same conditions on smooth glass showed approximately seven times greater shear per unit estimated contact region. Similar to gecko fibre arrays, the synthetic patch maintains contact and increases shear force with sliding. The high shear force observed (approx. 210 nN per fibre) suggests that fibres are in side contact, providing a larger true contact area than would be obtained by tip contact. Shear force increased over the course of repeated tests for synthetic patches, suggesting deformation of fibres into more favourable conformations.

Sliding-induced adhesion of stiff polymer microfibre arrays. II. Microscale behaviour

“…While Lee et al (2008) showed low peel strength (0.15 N m K1 ) on the whole patch, here a frictional adhesion effect was observed during normal pull-off with a spherical probe, whereby the normal tensile force was proportional to shear force. Repeated LDP testing showed that fibre arrays continue to function after more than 100 experiments, and they gain shear force and pull-off force with each trial.…”

“…Similarly to Lee et al (2008), LDP tests demonstrated increasing shear forces during sliding. While Lee et al (2008) showed low peel strength (0.15 N m K1 ) on the whole patch, here a frictional adhesion effect was observed during normal pull-off with a spherical probe, whereby the normal tensile force was proportional to shear force.…”

“…It is seen from figure 8 that the sample's shear and tensile adhesive forces greatly increase from repeated LDP tests. This increase is possibly due to angling of the fibres (see §4; Lee et al 2008). The maximum normal pull-off force and its corresponding shear force (point 3 in figure 8) for each trial is displayed in figure 9.…”

“…The LDP tests were repeated with sliding velocities between 5 and 100 mm s K1 , and as shown in figure 6, no major differences were observed. The lack of velocity dependence for these polypropylene microfibre arrays is discussed further in Lee et al (2008), and shown for the whole patch in the range from 48 to 240 mm s K1 . Figure 3b depicts the fibres being buckled slightly in step 1 due to the normal indentation of the probe.…”

“…In contrast, part I of this paper (Lee et al 2008) examines the behaviour of a free patch contacting a flat surface, which has an area (4 cm 2 ) more than 400 times larger than spherically probed regions. Although the free patch cannot sustain normal adhesion, the flat geometry and compliant backing used in Lee et al (2008) can sustain estimated fibre shear forces 20 times greater than those used in spherical probe tests. The two parts of this paper show that sliding-induced adhesion is obtained for both micro-and macroscale contacts.…”