Fig. 2. Robotic functions of PBs.

(A) Top views of a PB (upper row) and an LM (lower row) 75 mm³ in volume penetrating the pillar array with the pillar diameter 0.5 mm and the spacing 2.75 mm. (B) Simplified picture of forces acting on a PB during penetration. We assume negligible interaction of PB with the outer pillars. (C) Regime map of collision behavior of 75 mm³ of PBs with the pillar array shown in (A). (D) Bottom views of a PB (upper row) and an LM (lower row), each 75 mm³ in volume, rolling over a glass bead with diameters of 1.8 and 1.2 mm, respectively. (E) Schematic illustration of a PB engulfing a bead. The bead enters the PB with velocity u, leading to an increase in the PB’s surface area by $\mathrm{\Delta }{a}_1\mathit{+}\mathrm{\Delta }{a}_2$. (F) Regime map of PBs (30 to 75 mm³) engulfing glass beads with diameters ranging from 0.63 to 1.80 mm. (G) Side views of initially two PBs (upper row) and two LMs (lower row) that go through the merging process under the acoustic radiation force applied from above. The volume of the premerged PB and LM is identically 75 mm³. (H) Schematic illustration of the parameters governing merging. (I) Regime map of merging conditions for PBs with volumes of 45 to 75 mm³. The transducer height h_t ranges from 60 to 75 mm. (J) Starting from land, a PB (upper row) and an LM (lower row) skim on the water surface and land on the other side. (K) Schematic illustration of a PB skimming across a water surface. (L) Theoretical model of the disengagement force. The nondimensionalized disengagement force, $\widehat{F_{\mathrm{e}}}$, is determined by the Bond number, Bo. Scale bars, 5 mm (A, D, and G) and 10 mm (J).