Characterizing Dynamic, High-Frequency Friction in Lubricating Complex-Fluid Thin Films Between Viscoelastic Surfaces

To investigate the friction dynamics (time evolution of the friction response, including stiction and stick–slip sliding) between viscoelastic surfaces lubricated with complex-fluid films, a ‘wavelet decomposition’ time-series analysis method was applied to measured friction traces. Data were acquired using an updated ‘Rotating Disk’ attachment for the surface forces apparatus (RD-SFA). We have studied the friction frequency response of PDMS surfaces (sphere-on-flat geometry, 2 cm radius) interacting across various ‘everyday’ fluids (oils, creams, moisturizers, etc.) from 0 to 2500 Hz under high sliding velocities/shear rates. The RD attachment is capable of shearing two surfaces at velocities from mm/s to m/s in controlled temperature, humidity, and vapor composition environments. The friction experiments were performed at varying loads (20–320 mN) and velocities (1–40 mm/s) with a 20-µs sampling time. At such (and especially higher) velocities, ‘wavelet decomposition’ can be used to explore the time evolution of friction dynamics and is the most appropriate method for such tasks given its unique ability to resolve broad-spectrum transient frequency components with good time and frequency localization. This technique is general and enables the unambiguous characterization of any system fluctuations or resonant vibrations associated with stick–slip sliding and other ‘intermittent friction.’ These results illustrate the complex and varied friction dynamics that can arise under different experimental or environmental conditions and have implications for damage, wear, and sensory perception.