Measuring creep response of soft samples by magnetic force microscopy

In this work, the creep response of soft gel and cell samples after aplying a step in loading force by means of magnetic fields has ben directly measured by Atomic Force Microscope. By analysing the creep data with the standard linear solid model, we can quantify the viscous and the elastic properties of soft samples independently. Cell, in comparison with similar soft gels, are much more viscous, as has ben qualitatively observed in conventional AFM force curve data before. With the new method the spring constant and the viscous damping coefficient of friction from the creep response data have been quantified. Two different modes for applyig a force step have been proposed: (1) by raiing the sample height an indirect force step is being applied, or (2) by employing magnetic cantilevers a direct force step can be applied. Both lead to similar responses, whereas the latter seems to be defined since it resembles closely a constant strain mode. The former is easier to implement in most instruments, thus may be preferable from a practical point of view.