# Rotation of Liquid Metal Droplets Solely Driven by the Action of Magnetic Fields
## 3 Results and Discussions
This slip layer can reduce the friction like a lubricant, and thus increase the rotational speed. Interestingly, we observed that when the droplet was 100% immersed, the droplet reached its maximum rotational speed when the volume was increased to 0.04 mL, which is smaller than that of other immersion depths (0.06, 0.06, and 0.08 mL). We believe this is due to the fact that the significant increase in rotational speed at 100% immersion flattens the droplets and, therefore, increases the friction between the droplets and the tube. As shown in Figure 2c, NaOH concentration also affects the self-rotational speed of the LMDs. We found that using NaOH solution with a high concentration can lead to a faster rotational speed until the concentration of NaOH solution reaches 0.03 mol/L. When the concentration of NaOH solution exceeds 0.03 mol/L, the rotational speed of the droplets no longer increases with the increase of concentration and remains almost constant. No self-rotation was observed in our experiments when we reduced the NaOH solution concentration to zero. That might be due to the fact that without NaOH, the oxide layer cannot be removed, and the droplet becomes wrinkled [14,17], so the friction between the droplet, the tube, and the solution eventually increases. With the increase of the concentration of NaOH solution, the oxide layer was gradually removed, the friction gradually decreased, and the rotational speed increased. However, when the concentration reaches the threshold (0.03 mol/L), the oxide layer is almost completely removed, and the speed no longer increases with the concentration increase. Considering HCl solution can also be used to remove the oxide layer on the surface of EGaIn droplets [17,47], as shown in Figure 2d, we further studied the influence of HCl concentration on the rotational speed of EGaIn droplets. We observed the increase in rotational speed when increasing the concentration of HCl solution from 0 to 0.5 mol/L. Use in cooling systems is an important potential application of liquid metal, and here we demonstrate the ability of liquid metal self-rotation to accelerate liquid cooling. As shown in Figure 3a, we heated the 0.5 mol/L NaOH solution and 0.08 mL EGaIn in a tube with a heat gun (DH-HG2-2000, Delixi Electric). When the temperature of the solution reached about 40 \({}^{\circ}\)C, a large number of bubbles were separated from the solution similar to boiling. We stopped heating and rotated the motor at 420 RPM and measured the temperature of the solution every 5 seconds. For comparison, the other tube was tested in the same way except that the permanent magnets in the device were removed, that is, the LMD did not self-rotate as the motor rotated. The temperature change is shown in Figure 3b; the group in which the EGaIn droplet self-rotated cooled significantly faster than the group in which the EGaIn droplet did not rotate. After about 520 s, the group of rotated EGaIn cooled to room temperature (\(\sim\)24.2 \({}^{\circ}\)C), and after another 130 s, the other group cooled to room temperature. Figure 3: Cooling system based on a self-rotating LMD. (**a**) Schematic illustration of the cooling setup. (**b**) Temperature of solution vs. time plot. Figure 2: Change of the self-rotational performance under different operating parameters. (**a**) Self-rotational speed vs. speed of the motor plot; blue, red, and yellow curves indicate different magnetic field densities at the center of EGaIn droplets, respectively. (**b**) Self-rotational speed vs. sizes of droplets plot; blue, red, yellow, and purple curves indicate different depths of EGaIn droplet immersion in the NaOH solution, respectively. (**c**) Self-rotational speed vs. concentration of NaOH plot. (**d**) Self-rotational speed vs. concentration of HCl plot. Based on the fact that LMDs are negligibly soluble in most liquids, possess great surface tension, and keep almost unmixed with the solvent [7], LMDs have great potential for application as a mixer.