AFM-based Dynamic Scanning Indentation (DSI) Method for Fast, High-resolution Spatial Mapping of Local Viscoelastic Properties in Soft Materials

An atomic force microscope (AFM) based fast dynamic scanning indentation (DSI) nano-DMA method, which relies only on the commonly available capabilities of commercial AFMs to provide quantitatively accurate high-resolution (∼15 nm) spatial maps of local viscoelastic mechanical properties (E′, E″, and tan φ) in heterogeneous soft adhesive material systems, is described. The versatility of the DSI approach is demonstrated by successfully employing it on three industry-leading commercial AFMs/modules (Asylum's Cypher ES and MFP-3D Infinity AFMs with the FastForceMapping module, and Bruker's Dimension Icon AFM with the PeakForce QNM module). Frequency sweep thermorheological DSI experiments were performed to generate quantitatively accurate nano-DMA master curves spanning an unprecedented frequency range of 5 decades. Quantitative agreement between DSI nano-DMA and bulk DMA measurements is demonstrated for two different homogeneous elastomers (styrene butadiene rubber, SBR, and synthetic natural rubber, SNR). The capability of the DSI methodology in acquiring quantitatively accurate viscoelastic property maps of heterogeneous soft solids was validated through experiments on an SBR-SNR blend sample. Experimental factors affecting DSI data quality (e.g., shift factor and AFM tip size) are also discussed.