Vapor-Driven Transport of Different Types of Objects at the Air–Liquid Interface

Transportation and position control of objects on the surface of liquids is an important part of automation. To drive an object on the surface of a liquid, many methods have been proposed. However, these methods mainly focus on the driving of the object itself, and it is still difficult to precisely control its position. In our study, we propose a new method that uses vapor released from a suspended drop to achieve precise position control and transport of different types of objects at the air–liquid interface. These objects can be a plastic plate, a liquid marble, or an oil drop. The mechanism for controlling objects is that vapor released from a suspended drop causes a surface tension gradient around the object. When the vapor dissolves on the surface of a liquid, the surface tension of the liquid increases. Due to the surface tension gradient, the object moves from the surrounding area to the area below the suspended drop and follows the motion of the suspended drop with the trajectory of a letter. To show that the position of the objects can be precisely controlled by our method, we control the object on the center of a circle, and the maximum offset distance from the center of the circle is less than 3 mm. In addition, we also use vapor released from a suspended drop to transport an oil drop close to an object. After the drop adhered with the object, the object is driven by the oil drop. Compared with other methods that drive the motion of objects by reducing the surface tension of a liquid, our method is easy and the position of objects can be precisely controlled.