Friction Forces of Micropatterned Elastomers with Carbon Fibre Fabric Reinforcement on Soft Substrates

Abstract

Gecko-inspired adhesives mimic the external structure of geckos with micropatterned surfaces and the internal structure by fabric reinforcement in soft elastomer adhesive pads. Previous research measured the friction forces of synthetic adhesives, with either an external or internal structure, mainly on hard substrates. Much less is known about the effects on static friction forces on soft substrates of adhesives with a combined external and internal structure and with a contact area beyond a centimetre square. We fabricated 40 by 40 mm adhesive pads (Epad = 2.1 +/- 0.1 MPa) from polydimethylsiloxane (PDMS) elastomer and tested them on two soft PDMS substrates (Esub = 2.6 +/- 0.2 MPa and 1.0 +/- 0.1 MPa). A colloidal lithographic approach was used to fabricate the external structures of the adhesive pads with microscale dimples with and without a terminal layer (TL). The internal structures were fabricated by reinforcement of the adhesive pads with carbon fibre fabric (CFF), which varied in the types of CFF weave and its orientation with respect to the substrate. We found that samples without an external structure generated lower friction on the softer substrates, whereas samples with micropatterned surface generated similar friction between the substrates, presumably due to mechanical interlocking between the external structure and soft substrates. Samples with microscale dimples without TL generated the lowest friction forces among all samples, likely due to limited initial contact with the substrates. Samples with microscale dimples with TL generated similar friction forces as samples without an external structure. Those samples with TL were not able to generate higher friction, due to their fabrication method which restricted the movement of the fibre bundles, hindering the stress redistribution along the sample during the measurements. Samples with an internal structure showed significant higher friction compared to samples without reinforcement, due to a better stress distribution along the samples, but generated similar friction forces among the types of CFF weave.