Fig. 1

Mechanism of AFM force ramp, experimental design, data acquisition and interpretation. The LDPE sample was mounted onto a metal puck. A sharp cantilever tip indented into the LDPE sample and retracted away when a preset applied force was reached (a). A laser shined from the top, hit and deflected off from the back side of the cantilever. The deflection signal was received by a position sensitive photodiode (PSPD). The cantilever motion was driven by the attached Z piezo. Depending on the choice of technique, the drive signal could be a triangular wave (FV), a corner-rounded triangular wave (FFV) or a sine wave (PFQNM). The PFQNM force measurement was schematically depicted in b, Force versus time plot clearly illustrated the tip underwent a snap-in contact when brought close the sample surface and a snap-out of contact when retracted away from the sample surface. The Sync Distance was a time constant at which the height sensor reached the lowest position. The force versus Z displacement curve (F-Z) was recorded by the AFM and further converted into the force versus tip-sample separation (F-D) curve (c). The DMT modulus was extracted by fitting the contact part of retraction curve with DMT model. The integration over the hysteresis loop was referred to as energy dissipation