The mechanical characterization of soft particulate materials by nanoindentation based on atomic force microscopy (AFM) is a well-established technique in materials science. However, this technique is very time-consuming for micrometer-sized particles as the indenter has to be centered on the particle apex. As microgels have a broad distribution of Young's moduli, it is important to measure many particles to achieve statistically reliable data. Here, a new approach to nanoindentation is presented where the roles of the sample and the indenter are reversed. The technique is based on fluidic force microscopy (FluidFM): The microgel particle is aspirated to the aperture of a cantilever with an internal channel connected to a microfluidic controller, and the microgel particle is subsequently ramped onto a flat substrate. The experimental validation is carried out with two different types of microgels: ene-functionalized polyoxazoline crosslinked with thiol-functionalized hyaluronic acid (POx-HASH) and polyacrylamide (PAAm). It is demonstrated that in combination with the simplified double contact model, the “inverted” nanoindentation can determine Young's moduli of microgel particles about 5–10 times faster. Moreover, the here-presented technique allows for performing indentation measurements on different substrates. Thereby, it becomes possible to elucidate the role of adhesion in the nanoindentation process.
Specht et al. (Wed,) studied this question.