Liquid metal slingshot

This Rapid Communication presents the self-propelled and controllable motion of liquid metal droplets, which is driven by surface tension and does not depend on any external force field. Its motion speed and direction can be controlled just as a projectile fired by a slingshot: The liquid metal droplet is the projectile, and a substrate surface serves as the bowstring. This motion provides a potential mechanism for the application of liquid metals in soft robotics, drug delivery, chip cooling, and other fields.

Figure 1

The morphology evolution of a liquid metal droplet with time in case 1. The volume of a single droplet is 2.5 μl, and the radius is 842 μm. The velocity of the droplets when they depart the surface is about 0.160 m/s. Two groups of red arrows in the figure respectively indicate the expansion of the liquid bridge and the contraction of the droplets.

Figure 2

Variation of surface energy and projection kinetic energy vs time. The dotted lines in red and black are the theoretical values from Eqs. (4) and (5), respectively. The simulated results are very close to the theoretical results.

Figure 3

Variation of projection velocity and dimensionless velocity with droplet radius. All data are from the experiment. The dotted line average representatives the average values of dimensionless velocities on different substrate surfaces

Figure 4

The effect of bowstring shape on projection velocity and direction. Two equal-sized liquid metal droplets are placed back and forth to coalesce and induce the firing. The angle between the two bowstrings in case 2 and case 3 is 120° and 90°, and the bisector of the two bowstrings is perpendicular to the substrate. The angle between the two bowstrings in case 4 and case 5 is 120° and 150°, and the angle between the bisector and the substrate is 120° and 60°.

Figure 5

(a) The relationship between projection kinetic energy, bowstring energy, and bowstring shape. The calculation of kinetic energy takes the average projection velocity of three experiments for each case. (b) The relationship between the projection direction and the shape of the bowstring. Case 1 is the case shown in Fig. 1. The radii of droplets are 842 μm in all cases and the substrate material is Al-FAS.