Generally, interactions of oil drops at the air–liquid interface mainly have two features, namely, attraction and repulsion. However, in our study, we find that the oil drops at the air–liquid interface have other interacting features, that is, the atomic-like motion and the “capture” motion. For the atomic-like motion, oil drops attract each other at a long distance, but repel when they are about to come into contact with each other. For the “capture” motion, a big oil drop can actively “capture” oil droplets like a zooplankton. In our research, we analyze interfacial forces among the oil drops. Based on the experiments and analyses, we demonstrate that the atomic-like motion of oil drops is mainly due to the lateral capillary force and the surface tension force, and the “capture” motion is mainly due to the unbalanced impact force of flow fluid around the drops. In addition, based on our results, we use the oil drops to perform many functions at the air–liquid interface. For example, the oil drops can drive an object with linear and rotational motion. When a carbon tetrachloride drop is suspended above the air–liquid interface, it can be used to control an oil droplet to pass through serpentine grooves and obstacles. In addition, the suspended carbon tetrachloride drops also can be used to rank multiple droplets with a special shape. Based on the results, our study makes it possible to use oil drops to transport materials, drive objects, and even collect droplets at the air–liquid interface.

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A water nanodroplet spontaneously moving on a solid surface having a continuous wettability gradient from a hydrophobic to hydrophilic region.

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The interacting forces on the objects at the air− liquid interface are important for researching self-assembly of objects. Due to the interacting forces among objects, the objects self-assemble at the air−liquid interface and organize into ordered structures. In general, for the interacting of oil drops, they can attract coalescence or repel dispersion. However, according to our former research, we find that interactions of oil drops can be motion like interactions of gas atoms. The oil drops cannot coalesce to form a big oil drop or separate to form ordered structures. They attract at a long distance but repel when they almost contact. In this study, according to our simulation, we demonstrate that the atomic-like motion of oil drops is mainly caused by the surface tension gradient and the lateral capillary force. Based on the results, our research will facilitate the fundamental understanding about interactions among oil drops. Apart from giving detailed demonstration about the interacting forces among oil drops, our research may also put forward research about self-assembly, oil emulsification, and microfluidics.