Fig. 3

Measurement of the equivalent stiffness k of the measurement system. The PDMS block in Fig. 2 was replaced to the metal block for the measurement of k. The loading–unloading of the gross displacement Z was tested in the speed of motorized stage at 0.1 μm/s, and both of the loading–unloading are plotted. The result shows that a hysteresis in the measurement system is small enough to be negligible. Therefore, the equivalent stiffness was determined to k = 10.5 kN/m from the gradient as shown