High Friction from a Stiff Polymer Using Microfiber Arrays C. Majidi, 1, * R. E. Groff, 1 Y. Maeno, 2 B. Schubert, 1 S. Baek, 1 B. Bush, 3 R. Maboudian, 3 N. Gravish, 4 M. Wilkinson, 4 K. Autumn, 4 and R. S. Fearing 1 1 Department of Electrical Engineering & Computer Science, University of California, Berkeley, California 94720, USA 2 Adhesive Tape Research Department, Nitto Denko Corporation, Umeda, Osaka 530-0001 Japan 3 Department of Chemical Engineering, University of California, Berkeley, California 94720, USA 4 Department of Biology, Lewis & Clark College, Portland, Oregon 97219, USA (Received 15 May 2006; published 18 August 2006) 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. DOI: 10.1103/PhysRevLett.97.076103 PACS numbers: 46.55.+d, 62.25.+g, 68.35.p, 81.40.Pq High-friction, low-adhesion materials are important for applications such as automobile tires and shoes. Since surface roughness limits contact area, soft materials have typically been used to obtain high friction [1]. Dry friction of stiff polymers (E  1 GPa)[2,3] and rubbers [1,4,5] on glass is a well studied area, with rubber friction coefficients an order of magnitude or more greater than stiff polymer. Alternatively, stiff materials in ordered fiber arrays [6,7] can have an effectively high compliance, permitting high contact area on rough surfaces. By appropriate choice of fiber array geometry, frictional and adhesion properties can be controlled. Let the coefficients  and ^  denote the ratio of shear resistance to applied load for smooth surfaces and fiber arrays, respectively. Recent work with vertically aligned multiwalled-carbon-nanotubes (VACNT) has shown fric- tion coefficients of ^   0:795 on glass (a 9 increase over free nanotubes) with 50 m long fibers [8] and ^   2:2 on a 20 m radius gold sphere [9]. For long fibers and high enough surface energy [10], VACNT can make side contact under high preloads (20 N=cm 2 ) and show both tensile (11:7N=cm 2 ) and shear adhesion (7:8N=cm 2 ) [11]. Normal adhesion of VACNT samples is particularly pro- nounced at the nanoscale, where tests performed with a scanning probe microscope indicate a pull-off strength of 20 nN over an area of 0:001 m 2 [12]. In this Letter, we show that a relatively stiff thermoplastic fiber array can provide high shear resistance without adhesion over a macroscopic area of 1:27 cm 2 . Shear resistance in a poly- propylene fiber array is increased by more than a factor of 10 compared to flat polypropylene film. Fiber arrays were synthesized by casting one layer of 25:4 m thick polypropylene film (TF-225-4, Premier Lab Supply Inc.) into a 20 m thick polycarbonate filter (ISOPORE, Millipore Inc.) of 0.3, 0.6, or 2:5 m pore radius. The polycarbonate filter was pressure filled in a vacuum oven for 25 minutes at 200  C and then dissolved in methylene chloride. The array of fibers (Fig. 1) show height variation and only a slight amount of clumping [13] due to a relatively low surface energy compared with other polymers. Static friction measurements were performed on a tradi- tional pulley apparatus, where the sample was loaded in shear by a string run over a pulley to a hanging weight. The polypropylene sample was placed on an acetone-cleaned glass slide and subject to a constant normal load by a brass weight on a rigid flat platform. The shear load was in- creased until first sliding was observed. Experiments were performed on arrays of 0.3, 0.6, and 2:5 m radius poly- propylene fibers as well as two types of controls. One control was the unprocessed 25:4 m thick polypropylene film and the other was processed film that underwent the same fabrication steps as the fiber arrays with the exception FIG. 1. SEM of an array of 20 m long, 0:6 m diameter polypropylene fibers etched from a polycarbonate membrane; scale bar represents 10 m. PRL 97, 076103 (2006) PHYSICAL REVIEW LETTERS week ending 18 AUGUST 2006 0031-9007= 06=97(7)=076103(4) 076103-1  2006 The American Physical Society