Measuring Rolling Friction at the Nanoscale

Abstract

Colloidal probe microscopy, a technique whereby a microparticle is affixed at the end of an atomic force microscopy (AFM) cantilever, plays a pivotal role in enabling the measurement of friction at the nanoscale and is of high relevance for applications and fundamental studies alike. However, in conventional experiments, the probe particle is immobilized onto the cantilever, thereby restricting its relative motion against a countersurface to pure sliding. Nonetheless, under many conditions of interest, such as during the processing of particle-based materials, particles are free to roll and slide past each other, calling for the development of techniques capable of measuring rolling friction alongside sliding friction. Here, we present a new methodology to measure lateral forces during rolling contacts based on the adaptation of colloidal probe microscopy. Using two-photon polymerization direct laser writing, we microfabricate holders that can capture microparticles, but allow for their free rotation. Once attached to an AFM cantilever, upon lateral scanning, the holders enable both sliding and rolling contacts between the captured particles and the substrate, depending on the interactions, while simultaneously giving access to normal and lateral force signals. Crucially, by producing particles with optically heterogeneous surfaces, we can accurately detect the presence of rotation during scanning. After introducing the workflow for the fabrication and use of the probes, we provide details on their calibration, investigate the effect of the materials used to fabricate them, and report data on rolling friction as a function of the surface roughness of the probe particles. We firmly believe that our methodology opens up new avenues for the characterization of rolling contacts at the nanoscale, aimed, for instance, at engineering particle surface properties and characterizing functional coatings in terms of their rolling friction.

Figure 1

Fabrication of the probe and experimental procedure. (A) Design and dimensions of the holder used for microparticle capture. (B) Schematic of 3D printing of the holders by 2PP-DLW and optical micrographs of the prints. (C) Transfer of the holders onto a sacrificial glucose layer on top of a glass slide and optical micrograph after transfer. (D) Probe assembly by gluing a single holder onto a tipless AFM cantilever with UV glue and optical micrograph of the probe. (E) Representative SEM image (false-colored) of a particle (red), captured by the holder (blue). (F) Friction loop obtained by performing standard lateral force microscopy. (G) Angular displacement of the probe particle in the scanning direction as a function of time. Rolling is detected if the angular displacement exceeds 2° between consecutive frames. (H) Friction loop, color-coded to visualize and separate the rolling and sliding motion of the particle.

Figure 2

Comparison of two lateral-force calibration methods for the 3D-printed probes (wedge-calibration versus test-probe method). (A) 3D model of the wedges and corresponding height profiles, obtained from AFM linescans. (B) Optical micrograph of the wedges, fabricated using 3D 2PP-DLW. (C) Representative friction loop obtained on the wedge (top) and separated into the flat and sloped part (bottom). (D) Side-view of the large colloidal probe (silica, 90 μm) used for the test-probe method. (E) Top-view of the test probe against a sharp edge. (F) Histogram of repeated measurements of the lateral sensitivity. The inset depicts a representative deflection-vs-distance curve. (G) Comparison between the test probe-method and the wedge method in terms of sensitivity α.

Figure 3

Effect of internal holder roughness. (A) SEM image of a holder, fabricated using the IP-Dip photoresin. (B) SEM image of a holder, fabricated using the IP-S photoresin. (C) AFM topography scan of a hemisphere representing a negative of the holder cavity from (A). (D) AFM topography scan of a hemisphere representing a negative of the holder cavity from (B). (E) Comparison of AFM height profiles, extracted from (C,D). (F) Comparison of the friction coefficient of a rough particle on a substrate with similar asperities, measured with holders fabricated with IP-Dip and IP-S, respectively. The inset shows a representative plot of friction force versus normal force.

Figure 4

Friction loops of a fixed (orange) and a free (blue) RB particle, displaced across the patterned substrate at an applied normal force of 15 nN. The optical micrograph shows the change in roughness on the substrate (top). The inset shows an optical micrograph of the colloidal probe after converting it to a conventional fixed colloidal probe by gluing a RB particle inside the cavity of the holder.

Figure 5

Raspberry (RB) particles (12 μm microparticle) with different roughness, expressed as nanoparticle diameter. (A) 100, (B) 200, (C) 300, (D) 400, and (E) 500 nm. (F) Effective friction coefficient μ of RB particles with different nanoparticle diameters (100–500 nm) on a rough substrate decorated with 100 nm nanoparticles. (G) Evolution of the rotation of the RB particles in (F), expressed as a percentage relative to pure rolling without slip, as a function of the applied normal force.

Figure 6

Rotation of raspberry (RB) particles (12 μm microparticle with 300 nm nanoparticles) on a rough substrate (100 nm nanoparticles) as a function of normal force over a broad range of normal forces applied by two cantilevers of different stiffness.