Superhydrophobic photothermal graphene composites and their functional applications in microrobots swimming at the air/water interface

Highlights

• A novel material with photothermal and superhydrophobic properties is reported.

• Microrobots with superhydrophobic and photoresponsive properties are prepared.

• A microrobot with multi-modal motion driven by infrared laser is realized.

• A water strider-like robot can swim and jump under the control of laser and magnet.

Abstract

Photoresponsive superhydrophobic microrobots based on Marangoni effect are attracting more and more attention. Graphene has high photothermal properties and nanostructures. It is an excellent material for manufacturing superhydrophobic microrobots moving at the air/water interface. However, it is difficult to fabricate superhydrophobic microrobots with complex shapes moving at the air/water interface using graphene without chemical modification. Herein, Graphene/polydimethylsiloxane composite materials were prepared, which exhibited excellent superhydrophobic and photothermal properties. These properties were systematically investigated under various conditions. The developed material exhibits good manufacturability and can be easily made into a variety of small-scale complex structures and different shapes of microrobots with superhydrophobic and photothermal properties. With different structures, it is proved that the microrobots can exhibit a variety of motions, including linear, rotary, and oscillatory, under the action of infrared light. Combined with mechanical analysis, the three motion forms can be integrated into a single microrobot. Finally, a water strider-like robot is fabricated, that can glide and 180° roll-over jump on the water surface, exhibiting good infrared light driving performance and magnetic controllability. This work not only presents a method to prepare superhydrophobic microrobots based on graphene, but also provides a reference for the development of multifunctional microrobots, and further promotes the application of microrobots in liquid detection and oil recovery.

Introduction

Diversity of animals and their adaptability to the environment have provided a reference for the development of robot technology. Based on the various capabilities of creatures in the nature, researchers have developed a wide range of bionic robots, such as fish-like [1], snake-like [2], scorpion-like [3], insect-like [4], [5], and water strider-like robots [6], [7]. These robots have broad application prospects in the fields of scientific detection and environmental monitoring owing to their unique functions and motions, which have garnered an increasing amount of attention from the researchers. A microrobot capable of efficiently moving on the water surface (at the air/water interface) can complete various tasks in the ocean, river, or microfluidic surface, and has been widely investigated in the past decades. However, improving the efficiency, long-time and long-distance movement of such robots continues to be a challenge.

Studies have reported that superhydrophobic technology can improve the motion efficiency of microrobots [8], [9]. Furthermore, with the advancement of material science and manufacturing technology, several microrobots with superhydrophobicity, moving on the water surface have been manufactured. For example, some researchers have added superhydrophobic materials to the bodies of the microrobots [10], [11]. These robots can slide on the water surface under the effect of electrical stimulation. However, because the power source of these robots is electrical energy, they always need wires connected during movements. Therefore, they are not capable of achieving long-distance movement and control, which restricts their application scope. In recent years, to realize the wireless driving of microrobots, some researchers have used the Marangoni effect and added reagents at the end of the microrobots. With the diffusion of reagents in water, a surface tension gradient is formed on the water surface. Under the action of a surface tension gradient, the microrobot can move rapidly on the water surface [12], [13], [14]. Although this method can realize the rapid movement of microrobots, during movement, the reagent discharges into the water, which can negatively affect the water environment. Light is a type of clean energy [15], and has negligible impact on the water environment; light energy can therefore be used to accurately and remotely control the movement of microrobots [16], [17], [18]. The movement of microrobot caused by the surface tension gradient induced by light is considered an effective method for driving microrobots on the water surface [19], [20], [21], [22]. The driving unit of these microrobots is a photothermal materials. Because light can be converted to other forms of energy, photothermal materials can transform light into heat energy, thereby generating a heat gradient on the surface of water and forming a surface tension gradient for the realization of the efficient microrobot movement. However, these microrobots do not exhibit superhydrophobic characteristics and are subject to the resistance of water while moving. Therefore, only those microrobots exhibiting superhydrophobicity and high photoresponse characteristics must be used for this purpose.

To enable the integration of superhydrophobic and photothermal characteristics into microrobots, a significant amount of research has been conducted on photothermal materials. Graphene, carbon nanotubes, and other carbon-based materials have excellent photothermal conversion ability [23], [24], in other words, they can easily convert light into heat [25]. Photothermal materials are also widely used in the research and preparation of various photothermal composites [26], [27], [28]. Furthermore, they can be used as functional materials for superhydrophobic microrobots owing to their micro and nano-structures [29], [30], [31]. For example, Chen et al. used two-dimensional transition metal carbides as photothermal materials and hydrophobic reagents for chemical treatment. Combining these with a filter paper, a superhydrophobic photothermal driven microrobot was prepared [32]. Zhu research group prepared ultra-long hydroxyapatite nanowires by chemical treatment and modification and used them to prepare superhydrophobic photothermal microrobot via the filtration method [33], [34]. In addition, Zeng et al. used chemically modified multi-walled carbon nanotubes to prepare a superhydrophobic microrobot, which can float on the water surface and realize movement under the action of infrared light [35]. Compared to carbon materials that require chemical modification to exhibit superhydrophobicity, it is important to note that graphene has several unique and advantageous properties, including high strength, good flexibility, excellent thermal properties, and easy processing. In particular, graphene has micro and nano-structures, which can be used to prepare photoresponsive superhydrophobic materials. However, it is difficult to construct a superhydrophobic layered structure with graphene without chemical regent modification [36], [37]. Therefore, combination of graphene with superhydrophobic properties for the fabrication of microrobots exhibiting effective movements on the water surface has not been widely investigated.

In this work, we combined the photothermal properties of graphene and polydimethylsiloxane (PDMS) to fabricate composite materials with photothermal effect and superhydrophobic property. And a variety of microrobots sliding on the water have been fabricated using composite materials. Moreover, the composite materials exhibit moisture resistance for a variety of liquids; hence, it can be used as an anti-pollution coating, and the pollutants on the surface of materials can be removed using water. In addition, the composite materials have good machinability, thorough shape design, and preparation of a complex structure is possible. With these advantages, a variety of microrobots with different structures can be prepared. The resulting microrobots can perform a variety of motions owing to their different structures. Under the action of infrared light, they can realize linear, rotary, and oscillatory motion at the air/water interface. In addition, microgears with bidirectional rotation were prepared using superhydrophobic materials, which could rotate rapidly at the air/water interface under infrared light. Microgears have potential applications in controlling the velocity and direction of fluid in microfluidic chips. A water strider-like robot was fabricated using composite materials. Its motion direction can be driven and controlled remotely by infrared light. Furthermore, combined with magnetic technology, this robot can accelerate the swim and roll-over jump on the water surface. The prepared microrobots can be used in the fields of liquid detection and liquid/oil recycling. It also provides a novel idea and reference for the development and application of optical controlled multi-mode microrobot.