Fig. 2.

(A) Illustrations of the probe–sample interface during a typical indentation test consisting of six sequential phases. The vertical stage with the spherical probe was actuated toward the sample (the approach phase). After physically touching the meniscus sample surface (the contact phase), the probe continued to indent the sample at a prescribed rate and target displacement (the indentation phase). For most of the tests, the probe was retracted immediately after indentation (the unload phase). For the tests with 1 μm/s, the vertical stage remained at the target displacement while the probe continued to deform the sample (the equilibration phase), until equilibrium was achieved (the equilibrium phase), followed by the unload phase. (B) Representative plots of normal force (F_n) and z-stage position during phases 1 to 6 of a typical test. (C) Normal force plotted as a function of indentation depth for the same test. Equilibrium and effective moduli were obtained from the force and depth data captured at the phases 5 and 3 using the Hertz model.