High-Speed Atomic Force Microscopy for In-Situ Full-Field Measurement of Large Deformations

Abstract Background Atomic Force Microscopy (AFM) is widely used for high-resolution imaging and for probing the contact mechanics and multiphysics of materials at the nanoscale. However, its application to quantitative, full-field finite deformation studies of soft materials remains unexplored. Objective This work focuses on accurate high-speed, high-resolution AFM imaging of polydimethylsiloxane (PDMS) specimens in order to compute full-field strains with nanoscale spatial resolution. Methods The AFM cantilever dynamics were optimized for accurate high-speed imaging through independent control of the cantilever resonance frequency and spring constant, and by active Q-control. A miniature mechanical testing device was developed to fit inside a compact AFM, thereby enabling simultaneous mechanical loading and nanoscale imaging via a symmetric specimen stretching mechanism. Full-field in-plane strain fields from PDMS specimens subjected to finite deformations were extracted via Digital Image Correlation (DIC) from AFM images obtained during incremental specimen stretching. Results This experimental methodology successfully enables low error AFM imaging of PDMS at 100 Hz scan rate, signifying a 100-fold reduction in image acquisition time compared to previous studies using AFM to measure strain fields. The imaging repeatability, assessed with the aid of DIC, was accurate within 0.5% error in mean strain. Full-field strains derived from high-speed AFM images of PDMS specimens tested in situ under an AFM, agreed very well with macroscale optical measurements, including Poisson’s ratio calculations obtained up to the point of specimen failure. Conclusion The results of this study illustrate the feasibility of high-speed AFM as a quantitative tool for nanoscale full-field strain analysis, offering new opportunities for probing the large deformation mechanics of soft materials.