Free-Colloidal Probe Lateral Force Microscopy (fCP-LFM) for Nanotribology of Sliding and Rolling Contacts

While sliding friction at the nanoscale is routinely measured by colloidal probe lateral force microscopy (CP-LFM), measuring rolling friction at this scale remains challenging due to the difficulty of resolving lateral forces and particle rotation under well-defined normal loads. Here, we present an extension of a recently published platform that uses free-colloidal probes to enable quantitative measurements of both sliding and rolling friction at individual particle-substrate contacts. Our approach combines a custom-made colloidal probe with a holder for free particle rotation with lateral force detection and optical particle tracking. The modular holder design is compatible with particles of different sizes and materials, enabling studies of many systems of interest. We functionalize the holder surface using a lubricious poly(ethylene-glycol) (PEG) brush to minimize the internal friction of the free-moving particle within and extend the regime in which rolling friction can be measured. In addition, we present a scalable synthesis route for smooth, polymeric particles with tunable size (3-8 $\mu$ m) and anisotropic fluorescence, enabling direct tracking of particle rotation without modifying native surface properties. Together, these developments establish free-colloidal probe LFM, or fCP-LFM, as a robust and adaptable tribological platform for quantifying friction at the microscale under different modes of relative motion, suitable for systematic studies of nanoscale particle-surface interactions across a wide range of materials and conditions.

The online version contains supplementary material available at 10.1007/s11249-026-02147-8.