Multiscale rheology from bulk to nano using a quartz tuning fork-atomic force microscope

Rheological characteristics exhibit significant variations at nanoscale confinement or near interfaces, compared to bulk rheological properties. To bridge the gap between nano- and bulk-scale rheology, allowing for a better and holistic understanding of rheology, developing a single experimental platform that provides rheological measurements across different scales, from nano to bulk, is desirable. Here, we present the novel methodology for multiscale rheology using a highly sensitive atomic force microscope based on a quartz tuning fork (QTF) force sensor. We employ microscale and nanoscale shear probes attached to the QTF, oscillating parallel to a substrate surface for rheological measurements as a function of the tip-substrate distance with sub-nanometer resolution. Silicone oils with viscosities ranging from 5 cSt to 10 000 cSt are used as calibration samples, and we have successfully derived the bulk rheological moduli. Furthermore, an increase in modulus is observed within the regime of tribo-nanorheology at distances less than 50 nm from the surface. Through such multiscale measurements, it is confirmed that this increase is due to the formation of a layered structure of silicone oil polymers on the solid surface. These results provide a comprehensive understanding of the tribo-rheological properties of complex fluids across different scales.