Determination of elastic properties of single aerogel powder particles with the AFM

Atomic force microscopy (AFM) allows direct measurement of local elastic sample properties by force spectroscopy. The AFM tip indents into a soft sample and the resulting relation between loading force and indentation is used to determine the elastic properties of the sample. In order to calculate the indentation an analytical expression for the sensor response vs. z-piezo displacement based on the Hertz model of mechanical contact is derived. This model is fitted to data obtained on a silica aerogel sample using a least-squares method. The aerogel powder particles were analysed individually for their surface structure and elastic behaviour. Prior to the indentation experiments, the aerogel surface was characterised by AFM in the tapping mode. The results were validated by the comparison of data obtained by using two types of cantilevers of very different stiffnesses (spring constant k=0.2 and 54 N/m) and by assessing the reversibility of the indentation process. Tip indentations smaller than 200 nm were usually reversible, whereas indentations of 2000 nm caused irreversibilities.