High Friction from a Stiff Polymer Using Microfiber Arrays

Abstract

High dry friction requires intimate contact between two surfaces and is generally obtained using soft materials with an elastic modulus less than 10 MPa. We demonstrate that high-friction properties similar to rubberlike materials can also be obtained using microfiber arrays constructed from a stiff thermoplastic (polypropylene, 1 GPa). The fiber arrays have a smaller true area of contact than a rubberlike material, but polypropylene's higher interfacial shear strength provides an effective friction coefficient of greater than 5 at normal loads of 8 kPa. At the pressures tested, the fiber arrays showed more than an order of magnitude increase in shear resistance compared to the bulk material. Unlike softer materials, vertical fiber arrays of stiff polymer demonstrate no measurable adhesion on smooth surfaces due to high tensile stiffness.

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

“…Figure 4a shows that the shear force rises dramatically while the probe is being dragged across the sample (between points 1 and 2). For Coulomb friction (F t ZfF n ), this would correspond to a coefficient of friction f O10, as shown previously in Majidi et al (2006). In contrast, experiments on the control in figure 4b show a relatively low shear force, and a coefficient of friction fZ0.2, typical of a stiff polymer on glass.…”

“…Naturally angled cantilever beams show greater compliance in the normal direction than vertical fibres, which must undergo buckling in order to conform to a surface (Autumn et al 2006a;Majidi et al 2006). However, when a shear force is applied to the tip of a vertical beam, it becomes effectively angled, increasing normal compliance and allowing more conformal contact of the array and surface.…”

Compliance of a microfibril subjected to shear and normal loads

“…In the past year, there have been several experimental studies on the frictional behaviour of microfibril arrays against a flat substrate (Majidi et al 2006;Ge et al 2007). The fibril arrays fabricated by Majidi et al (2006) and Ge et al (2007) consist of very stiff fibrils, whereas those fabricated by P. R. Guduru (2007, personal communication) and Shen et al (2008) are made of poly(dimethylsiloxane) (PDMS), a soft elastomer with a shear modulus of the order of 1 MPa.…”

“…Recent interest in bio-inspired adhesives has motivated many researchers to fabricate microfibril arrays (Liu & Bhushan 2003;Peressadko & Gorb 2004;Chung & Chaudhury 2005;Crosby et al 2005;Glassmaker et al 2005Glassmaker et al , 2007Huber et al 2005;Northen & Turner 2005;Yurdumakan et al 2005;Kim & Sitti 2006;Majidi et al 2006;Aksak et al 2007;Gorb et al 2007;Greiner et al 2007; and to study their contact mechanics and adhesion (Jagota & Bennison 2002;Gao et al 2003Gao et al , 2005Persson & Gorb 2003;Hui et al 2004;Persson et al 2005;Spolenak et al 2005a,b;Tang et al 2005;Bhushan et al 2006;Tian et al 2006;Yao & Gao 2006;Chen & Gao 2007). Most of these studies focus on how the interface between the microfibrils and a smooth, hard substrate separates under a normal load.…”

Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution

“…Controlling texture is optimized in many commercial applications, such as tyres [2]. However, there are other routes to increase friction where stiff surfaces are required [3]. When lubrication is necessary, grafting polymers on surfaces may be applied for both oil-based [4][5][6] and aqueous [6][7][8][9] environments.…”