Peak forces in high-resolution imaging of soft matter in liquid.

The maximum force exerted by the tip of a force microscope on the sample surface is a critical factor that determines the spatial resolution and the degree of invasiveness of the measurement, in particular, on soft materials. Here we determine the conditions needed to image soft matter in the 30-500 MPa range while applying very small forces. Imaging at sub-50 pN in the elastic regime can only be achieved under strict conditions in terms of force constant values (below 0.1 N/m) and free amplitudes (below 2 nm). The peak force depends on the operational parameters, probe properties, the elastic and/or viscoelastic response of the sample, and the contact mechanics model. Images of heterogeneous samples are never taken at a constant peak force. Under the same operational conditions, smaller forces are obtained on the more compliant materials. We also find that the viscoelastic response reduces the peak force with respect to the purely elastic regions. Our findings are summarized in three-dimensional maps that contain the operational conditions for imaging at low forces.