High-Throughput Mechanical Characterization of Single Microgel Particles by Fluidic Force Microscopy

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.