Communication pubs.acs.org/JACS
Vapor-Enabled Propulsion for Plasmonic Photothermal Motor at the Liquid/Air Interface Fanchen Meng,† Wei Hao,† Shengtao Yu,† Rui Feng, Yanming Liu, Fan Yu, Peng Tao, Wen Shang, Jianbo Wu, Chengyi Song,* and Tao Deng* State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: This paper explores a new propulsion mechanism that is based on the ejection of hot vapor jet to propel the motor at the liquid/air interface. For conventional photothermal motors, which mostly are driven by Marangoni effect, it is challenging to propel those motors at the surfaces of liquids with low surface tension due to the reduced Marangoni effect. With this new vapor-enabled propulsion mechanism, the motors can move rapidly at the liquid/air interface of liquids with a broad range of surface tensions. A design that can accumulate the hot vapor is further demonstrated to enhance both the propulsion force as well as the applicable range of liquids for such motors. This new propulsion mechanism will help open up new opportunities for the photothermal motors with desired motion controls at a wide range of liquid/air interfaces where hot vapor can be generated.
Figure 1. Schematic illustration of plasmonic heating-induced photothermal motion for the PGF-motor at the liquid/air interface. The ejection of vapor jet flow propels the motor to move forward. The drawings are originally created by Chengyi Song.
converted heat by the AuNP film was confined at the liquid/air interface and resulted in rapid heating of local liquid with high evaporation efficiency.5 Meanwhile, the explosion of vapor bubbles and the generation of vapor jet flow below the membrane were also observed,6 which inspired us to utilize the generated vapor jet flow as a propulsion force to drive the PGFmotor. In this report, the PGF-motor can achieve speed as fast as 16.75 mm/s in SDS solution with low surface tension of 29.98 mN/m. Directional control has been demonstrated with the PGF-motor. A design of a floating cone-shape tip with PGF inside provides enhanced propulsion and such motor achieves the speed as high as 34.6 mm/s. This new propulsion mechanism provides a different approach in driving motor at the surface of a broad range of liquids. This mechanism can potentially expand the use of such motors in different applications, including cargo delivery,7 electricity generation,8 interfacial catalysis,9 and oil collection.10 Figure 2a shows the experimental setup and the inset in Figure 2b shows the PGF-motor. The rough fibrillar substrate of the paper (Figure S1) enhances the light absorption of the AuNP film due to multiscattering effect, resulting in the broadband solar light absorption that makes it dark purple appearance.4
L
ight-driven motion has been well developed in utilizing optical energy to provide propulsion force for motors floating at the liquid/air interface or inside the liquid.1 Photothermal materials such as carbon or plasmonic nanomaterials have recently been used to drive objects.1c,e,2 Until now, the Marangoni effect induced by the thermal gradient plays a major role in propelling motors without the involvement of chemical reactions.1e,2b Fréchet et al. demonstrated a photothermal-responsive approach to propel a motor via Marangoni effect.1e With the coating of carbon nanotubes capable of locally converting light into heat on their motor, local thermal surface tension gradient was generated, which propelled the motor to move on a liquid surface. In their system, Marangoni effectinduced motion could be quenched by reducing the surface tension upon the addition of surfactant (sodium dodecyl sulfate (SDS)). Such quenching limited the application of the heatdriven motion in cases that involve liquids with low surface tension.3 This paper investigates a new photothermally powered propulsion mechanism that drives motor at the surface of various liquids, including liquids with low surface tensions. In this work, a motor was fabricated by coating a floating paper with gold nanoparticle (AuNP) film (PGF-motor),4 and a new propulsion mechanism was demonstrated to drive the motion of such motor on the surface of various liquids by plasmonic heating-induced vapor jet flow (Figure 1). The © 2017 American Chemical Society
Received: June 16, 2017 Published: August 24, 2017 12362
DOI: 10.1021/jacs.7b06036 J. Am. Chem. Soc. 2017, 139, 12362−12365
Communication
Journal of the American Chemical Society
different intensities. The main peak with the strongest intensity represents the highest temperature reached by illuminating a certain spot at the rear part of PGF-motor. The two much weaker peaks (we denoted them as α and β in Figure 2d) indicate that thermal waves were ejected backward at the liquid/air interface from the center hot zone. Such ejected thermal waves are the result of the ejected vapor jet flow and the backward ejection of the vapor jet flow helped propel the motor forward (SI). On the contrary, only single temperature peak with no distinct thermal waves was observed for the PAPmotor (Figure 2f). When light irradiates the assembled AuNP film, the induced plasmonic heating will rapidly raise the surface temperature of AuNP and also the temperature of the surrounding liquid, which enables the generation of vapor bubbles.11 These vapor bubbles then burst into the mixture of vapor and hot liquid, which provides propulsive force to drive the motion of PGFmotor. For both pure water and SDS solutions with low concentration (e.g., 3g-SDS solution), the PGF-motor moved at the speed of less than 3 mm/s. The maximum velocity increased when SDS concentration exceeded 20 g/100 g water (Figure 2b). There are less vapor bubbles, and thus much less vapor jet flows generated for pure water and solutions with low SDS concentration than for solutions with high SDS concentration. In the concentrated SDS solutions, the solutions are in the supersaturated state and the undissolved SDS solute aggregates into giant particles.12 These dispersed particles along with the lower surface tension enhanced the heterogeneous nucleation and growth of vapor bubbles around the heating spots.13 When more vapor jets were produced, the speed of the motor was thus increased substantially. In addition to aqueous solution, we also studied the motion of the PGF-motor at the ethanol/air interface. As shown in Figure 3a,b, when the laser light was positioned at the rear part
Figure 2. (a) Schematic illustration of experimental setup. (b) Relation between the maximum velocities of PGF-motor and the SDS solutions with different concentrations. The inset shows the PGFmotor (scale unit: centimeter). (c−f) IR images of PGF-motor (c) and PAP-motor (e) on the surface of 30g-SDS aqueous solution and the corresponding temperature distributions for PGF-motor (d) and PAPmotor (f) along the red lines. (Scale bar = 1 cm)
Figure 2b shows the maximum velocities of heat-driven motion of PGF-motor on pure water and SDS solutions with SDS concentration ranging from 3 to 30 g per 100 g of water. When the PGF-motor was placed on pure water under light illumination, it moved slowly at a speed of 1.81 mm/s. After adding SDS at 3 g/100 g H2O, the speed of motor increased to 1.87 mm/s. The surface tension of SDS solution kept decreasing after the concentration of SDS exceeded 3 g/100 g water, but the reduction of surface tension was getting smaller as the concentration increased (Table S1). After the aqueous SDS solution reached its saturated concentration (∼20 g SDS/ 100 g water), the motor achieved the speed larger than 10 mm/ s (Movie S1). The observed movement of motor in the experiment was not due to the Marangoni effect because such motion would be quenched by the addition of SDS surfactant into water.1e Another local thermal gradient-induced force, thermophoretic force, most likely did not play any major role in the observed motion because the direction of the resultant thermophoretic force (SI), acting at the rear part of PGFmotor, was opposite to the movement direction of the motor.1f We thus hypothesize that the motion of PGF-motor is mainly propelled by a new driving mechanism that is different from Marangoni effect and thermophoretic effect. To gain further understanding of the propulsive force for the motion of PGF-motor, we studied the motion of PGF-motor and a motor based on the plain airlaid paper (PAP-motor) on highly concentrated SDS solutions (30g-SDS). The PAP-motor did not move on the surface of SDS solution, whereas the PGFmotor moved rapidly on the surface of SDS solution at a speed of 16.75 mm/s. The maximum temperatures reached ∼91 °C for the PGF-motor (Figure 2c,d) and 40 °C for the PAP-motor (Figure 2e,f) (Movie S1). The temperature profile along the movement direction (red straight line in the middle image of Figure 2c) of PGF-motor in Figure 2d exhibits triple peaks with
Figure 3. (a and b) Thermal mapping image of the moving PGFmotor at the ethanol/air interface under laser illumination. The force analysis and trajectories of clockwise (c, d) and counterclockwise (e, f) rotational motion. 12363
DOI: 10.1021/jacs.7b06036 J. Am. Chem. Soc. 2017, 139, 12362−12365
Communication
Journal of the American Chemical Society of the motor, thermal waves were observed and the PGF-motor moved forward and its velocity reached 11.65 mm/s (Movie S2), which was comparable to the speed achieved in concentrated SDS aqueous solution. A similar series of thermal waves ejected backward from the rear part of the motor were observed, indicating that the vapor jet flow was ejected from the hot zone into the surrounding ethanol. The studies of motors with different sizes and the light intensity effect are presented in the SI. For more precise control of the motion direction of the PGFmotor, a different shape design strategy of the PGF-motor was adopted to achieve rotational motion other than linear motion. The parameters of size and shape design of the PGF-motor are shown in Figure S2. The motor could swerve in different directions as long as laser light was located at the edge of front triangle of motor. Figure 3c,e illustrates the detailed force analysis for the rotational motion. Recoil force F1, originated from the ejection of vapor jet flow from the hot zone, was perpendicular to the edge of the front triangle of motor, resulting in the rotational torque T. During the movement process, torque T rotated motor around the center of mass under the balance of buoyancy and gravity. The clockwise and anticlockwise rotations were attained by positioning the laser light at different sides of front triangle. The trajectories of controllable movement of the motor are shown in Figure 3d,f (Movie S3). For the PGF-motors studied above, we notice most of the generated vapor was dissipated upward into air and was not collected and converted into propulsion force. Such motors also moved slowly (
