High Friction from a Stiff Polymer Using Microfiber Arrays

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.

Figure 1

SEM of an array of $20\mu \mathrm{m}$ long, $0.6\mu \mathrm{m}$ diameter polypropylene fibers etched from a polycarbonate membrane; scale bar represents $10\mu \mathrm{m}$.

Figure 2

Plot of normal pressure vs shear resistance for polypropylene fiber arrays and controls; (▴) radius $R=0.3\mu \mathrm{m}$, (●) $R=0.6\mu \mathrm{m}$, (■) $R=2.5\mu \mathrm{m}$, (○) unprocessed control, (□) processed control; (left) $\mathrm{\text{loading area}}=1.27{\mathrm{cm}}^2$, $\mathrm{\text{sample size}}=15$; (right) $\mathrm{\text{loading area}}=0.033{\mathrm{cm}}^2$, $\mathrm{\text{sample size}}=5$; error bars represent 1 standard deviation in the data; solid lines represent theoretical predictions from Eqs. (1, 2, 3) for $R_t=3R$.

Figure 3

Force response of $R=0.3\mu \mathrm{m}$ fiber array indented by a spherical probe ($\mathrm{\text{radius}}=5.17\mathrm{cm}$); greater compliance at low $\Delta$ may be a result of fiber sparsity near the top of the array due to length variation; (dashed line) theoretical predictions based on ideal column buckling for various buckling modes; (dash-dotted line) cantilever bending model for various angles of inclination from vertical; arrows indicate loading direction.

Figure 4

Applied normal load and shear resistance vs time for an array of $0.3\mu \mathrm{m}$ radius polypropylene fibers. Dotted lines from left to right indicate (i) transition from vertical approach to horizontal drag at $5\mu \mathrm{m}/\mathrm{s}$, (ii) pause after drag, and (iii) vertical retraction.