Fig. 4: Evaluation of surface and edge adhesion.

a Plot of pickup test results versus relative contact surface area and relative contact edge length. Closed circles indicate successful pickups and open circles indicate unsuccessful pickups. The blue, green, and red colors correspond to the pickup by (i) surface contact alone, (ii) mainly-edge contact, and (iii) edge and surface contact, respectively (see discussion in the main text). The green line was calculated using Eq. (1). The yellow-shaded region is the region in which pickup occurs (the pickup region), and the white region is the region in which pickup is impossible. The gray-shaded region (iv) is the experimentally unfeasible region. b–e Schematics of the pickup regions: b surface-dominant adhesion, c edge-dominant adhesion, d, e both surface and edge adhesion. d is the case where 2D flakes can be picked up by surface contact alone, and e is the case where 2D flakes cannot be picked up by surface contact alone.