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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials
Vapor-Driven Transport of Different Types of Objects at the Air-Liquid Interface Dong Liu, Awais Mahmood, Ding Weng, and Jiadao Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b05718 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019
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The Journal of Physical Chemistry
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Vapor-Driven Transport of Different Types of Objects at the
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Air-Liquid Interface
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Dong Liu1, Awais Mahmood1, Ding Weng1 and Jiadao Wang1*
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1State
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China.
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ABSTRACT
Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, People’s Republic of
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Transportation and position control of objects on the surface of liquids is an important part of
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automation. To drive an object on the surface of a liquid, many methods have been proposed.
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However, these methods mainly focus on the driving of an object, and it is still difficult to precisely
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control position of it. In our study, we propose a new method that uses vapor released from a
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suspended drop to achieve precise position control and transport of different types of objects at the
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air-liquid interface. These objects can be a plastic plate, a liquid marble, and an oil drop. The
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mechanism for controlling objects is that vapor released from a suspended drop causes a surface
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tension gradient around the object. When the vapor dissolves on the surface of a liquid, surface
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tension of the liquid increases. Due to the surface tension gradient, the object moves from the
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surrounding area to the below of the suspended drop and follows the motion of the suspended drop
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with trajectory of letters. To show that position of the objects can be precisely controlled by our
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method, we control the object on the center of a circle and the maximum offset distance from the
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center of the circle is less than 3 mm. In addition, we also use vapor released from a suspended
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drop to transport an oil drop near to an object. After the drop adhered with the object, the object is
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driven by the oil drop. Compared with other methods that driving motion of objects by reducing
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surface tension of a liquid, our method is easy and position of objects can be precisely controlled.
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INTRODUCTION
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To control and transport objects at the surface of liquids, many methods have been propose1-3,
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such as using light4, magnetic fields5, thermos-capillary effect6, chromocapillary effect7, and
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chemical reactions8. The basic theory of propelling objects at the air-liquid interface is to create a
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surface tension gradient9 around the objects. Although these methods have lots of advantages,
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some drawbacks still exist. For example, precisely control the position of an object is difficult,
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special treatment of the controlled object is required, the device is complicated, and the type of
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objects to be controlled is single. In our study, vapor released from a suspended drop was used to
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control and transport different types of objects at the air-liquid interface, and the detailed
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mechanisms were also researched. The object can be a plastic plate, a liquid marble, and an oil
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drop, which significantly expands the types of objects that can be controlled by the same method.
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The mechanism to control and transport of objects is due to changing of surface tension by a
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suspended drop. When vapor released from a suspended drop is absorbed by a liquid, its surface
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tension increases.10 Due to a surface tension gradient around the object caused by the released
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vapor, a surface tension force is generated on the objects11, and the objects move from surrounding
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area to the below of the suspended drop. In addition, we also measured the control accuracy of our
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method for the three types of objects. According to measurement, when the objects were controlled
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on the center of a circle, the maximum offset distance from the center of the circle was less than 3
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mm. Furthermore, we also used our method to control a decanol drop, and transported it near to a
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plastic plate. After the decanol drop adhered with the plate, the plate was driven. Therefore,
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compared with other methods of decreasing surface tension of the working position, our method
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can precisely control the position of the object and some simple applications can be implemented.
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EXPERIMENTAL SECTION
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The device to control and transport an object at the air-liquid interface is shown in Figure 1a. In
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Figure 1a, diameter of the culture dish is 60 mm and its height is 20 mm. The liquid is deionized
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(DI) water and it is added with a small amount of decanol liquid (2 μL). In addition, materials of
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the suspended drop are carbon tetrachloride (CCl4) liquid, and it is suspended below a transparent
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plastic plate. Because the contact angle of CCl4 liquid on surface of the plastic plate is small (the
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contact angle can be changed due to evaporation of the liquid, but it is smaller than 30 °), the
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suspended CCl4 drop is difficult to fall off from the plastic plate. Volume of the suspended CCl4
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liquid is 10 μL, and it can be used for more than 2 minutes from observation. To suspended a CCl4
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drop below a plastic plate, many methods can be used. In our study, we injected carbon
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tetrachloride liquid directly into the bottom of the transparent plate with a microinjector. The
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specific steps to control an object are as follows. First, deionized water was added to a culture dish
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until the height of the liquid was 1-3 mm lower than the edge of the culture dish. Then, a small
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amount of decanol liquid (2 μL) was added on the liquid (the mixture solution should be stirred
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for 1 minute) and the object to be controlled was placed on the surface of the liquid. In final, a
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transparent plastic plate suspended with a CCl4 drop (10 μL) was placed on the culture dish. When
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we moved the suspended CCl4 drop to above of the object, the object would be controlled by it. If
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we moved the suspended drop to other position, the object would follow the trajectory of the
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suspended drop. By this method, different types of objects were controlled. In addition, when the
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distance between the suspended CCl4 liquid and the surface of the solution is greater than 3 mm,
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the motion of the object will be not distinct.
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Three types of objects were used in our study, that is plastic plates, liquid marbles12-13, and
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decanol drops (dyed with a biological stain, Supporting Information). To transport and control the
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objects, conditions of them at the air-liquid interface are firstly researched. When density of the
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plate is smaller than the solution, the plate can float at the air-liquid interface. However, for a plate
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with a larger density than the liquid, due to buoyancy and surface tension,14 the plate can also float
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on the surface without sinking.15 Hence, for a plate with a larger density than the liquid solution,
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they also can be transported and controlled by our method. The liquid marble is a type of liquid
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drop stabilized by solid powders at the surface.12 Due to special properties of liquid marbles, they
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have many potential applications, such as material transport12, miniature reactors16, and sensors17.
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In addition, when density of liquid marbles is larger than that of the solution, liquid marbles can
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also exist on the surface without sinking due to properties of the outer powder and interfacial
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forces.18 Hence, for a thin plate and a liquid marble on the surface of a liquid solution, their
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conditions on a liquid surface are shown in Figure 1b and Figure 1c. However, when a small
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amount of oil liquid is dropped at the air-liquid interface, the dropped liquid can be spreading or
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forming a lens shape, and its behavior depends on the spreading parameter.19 The spreading
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parameter of an oil drop on the surface of a liquid solution can be expressed as
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S al ( ao ol ) (Figure 1d). When the spreading parameter S>0, the dropped liquid will
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spread at the air-liquid interface; when the spreading parameter S
