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Scanning electron microscopy images of pillar types. Both pillars share a base width of and a height of . The pillar in (a) is thick at the base and tapers to a tip. The pillar in (b) is thick at the base and has uniform thickness along the length of the beam.
Diagram of mounted pillar and force-sensing cantilever. The black arrows indicate the relative motion of the pillar and cantilever. The blue and green arrows indicate the direction of positive forces exerted on the cantilever. This convention is used in the force plots.
Adhesion test data. There are two phases to this test. To the left of the vertical dotted line, the pillar is approaching the cantilever. To the right of the line, the pillar is being retracted at an angle. From to the initial contact point, there is a signal drift likely due to charge on the PDMS sample. Between the initial contact and peak compression point, the cantilever stage approaches the sample applying positive normal force to the cantilever. After the peak compression, the stage is moved in both the normal and shear directions to generate forces in both axes. In this case, change in the slopes of the forces show an adhesive failure. The forces at this interface failure are recorded for plotting in the limit surface.
Limit surfaces for tapered and nontapered single pillars. Each data point reports the forces preceding a single failure event. Trials are run at 19 pull angles ranging between 0° (shear) and 90° (vertical). Each of these angles is tested at two different levels of peak compressive load. Parabolic and linear fits are included as a guide to the eye.
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