Transient Reciprocating Motion of a Self-Propelled Object Controlled

Jun 20, 2014 - Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan. •S Supporting ... wire in a H2O2 solution1 or a liquid...
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Transient Reciprocating Motion of a Self-Propelled Object Controlled by a Molecular Layer of a N‑Stearoyl‑p‑nitroaniline: Dependence on the Temperature of an Aqueous Phase Satoshi Nakata,*,† Tomoaki Ueda,† Tatsuya Miyaji,† Yui Matsuda,† Yukiteru Katsumoto,†,⊥ Hiroyuki Kitahata,‡ Takafumi Shimoaka,§ and Takeshi Hasegawa§ †

Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan Department of Physics, Graduate School of Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan § Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ‡

S Supporting Information *

ABSTRACT: The mode-bifurcation of a self-propelled system induced by the property of a N-stearoyl-p-nitroaniline (C18ANA) monolayer developed on an aqueous phase was studied. A camphor disk was placed on a C18ANA monolayer, which indicated a characteristic surface pressure−area (π−A) isotherm. A camphor disk transiently exhibited reciprocating motion at a higher surface density of C18ANA. The amplitude of the reciprocating motion increased with an increase in the temperature of the aqueous phase below 290 K, but reciprocating motion varied to irregular motion over 290 K. The temperature-dependent reciprocating motion is discussed in terms of the π−A curve for C18ANA depending on the temperature. The interaction between C18ANA molecules was measured by Fourier transform IR spectrometry and Brewster-angle microscopy. As an extension of the study, the trajectory of reciprocating motion could be determined by writing with a camphor pen on the C18ANA monolayer.

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INTRODUCTION There are several self-propelled systems, e.g., a gold−platinum wire in a H2O2 solution1 or a liquid droplet or solid at immiscible interfaces.2 However, most systems indicate unidirectional motion depending on the initial condition and their direction of motion is influenced by thermal fluctuation or controlled by external force. In contrast, living organisms, such as flagella motors of bacteria, can change the direction, velocity, and nature of motion while sensing the environment under almost isothermal and nonequilibrium conditions,3 like taxis. That is, the autonomy of animate systems is clearly higher than that of inanimate systems.4−18 In addition, living organisms indicate nonlinear phenomena, such as oscillation, bifurcation, synchronization, and so on. Therefore, introduction of nonlinearity enhances the autonomy of inanimate systems to mimic the motions of living organisms.7,9 A camphor system19−21 is one of the simplest and most readily available self-propelled systems because it is easy to prepare the internal21−24 (e.g., shape of solid camphor, use of the derivatives, coupling with chemical reactions) and external fields25−27 (addition of chemical compounds, shape of the water channel). Multiple camphor disks on water indicate synchronized swimming and collective motion.28,29 We have also qualitatively reproduced the natures of self-motion by using a numerical calculation.22,23,26−28 © 2014 American Chemical Society

Recently, we found that the nature of motion of a camphor disk changed when the camphor disk was placed on a monolayer of N-stearoyl-p-nitroaniline (C18ANA).30−32 Here, the surface pressure−area (π−A) isotherm of the C18ANA monolayer has a local minimum and local maximum values, i.e., the system is highly nonlinear. In particular, reciprocating motion was generated one-dimensionally around the local minimum. In this study, temperature dependency of transient reciprocating and irregular motions of a camphor disk on a C18ANA monolayer was investigated. This temperaturedependent change in the motion is discussed in terms of the π−A curve of C18ANA. The interaction between C18ANA molecules was evaluated by Fourier transform IR spectroscopy (FTIR) and Brewster-angle microscopy (BAM). As an extension of this study, the trajectory of reciprocating motion could be determined by drawing a character with a camphor pen on the C18ANA monolayer. Received: February 3, 2014 Revised: June 19, 2014 Published: June 20, 2014 14888

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EXPERIMENTAL SECTION

As a self-propelled element, (+)-camphor purchased from Wako Chemicals (Kyoto, Japan) was used without further purification. Water to prepare a sample solution was first distilled and then purified with a Millipore Milli-Q filtering system. A camphor disk (diameter, 3 mm; thickness, 1 mm; mass, 5 mg) was obtained with a pellet die set for FTIR. The camphor motion was monitored with a digital video camera (SONY HDR-CX590, Japan; minimum time-resolution, 1/30 s) and then analyzed by software (ImageJ, National Institutes of Health, U.S.). The isotherm of the surface pressure (π) under the decrease in the surface area (A) was obtained with a surface pressure meter (Kyowa Interface Science Co. Ltd., HMB, Saitama, Japan). 200 mL of a 2 mM CaCl2 aqueous solution as an aqueous phase was poured into the trough of the surface pressure meter. To prepare a N-stearoyl-p-nitroaniline (C18ANA) monolayer on the aqueous phase, a C18ANA solution dissolved in chloroform was carefully dropped on the aqueous phase with a micro syringe. The amount of C18ANA developed on the aqueous phase was 84 nmol, and the surface area was changed from 0.021 to 0.005 m2. C18ANA was synthesized based on previous reports.30−32 The experiments were performed in a water bath connected to a circulation thermostat (Yamato Scientific Co., Ltd., CLH300, Japan) to control the temperature change within 1 K. The temperature on water was measured with an infrared camera (Ti45, Fluke Co., U.S.). IR spectra were recorded on a Nicolet 6700 Fourier transform IR spectrometer equipped with a MCT detector. The resolution of IR spectrum was 2 cm−1, and each spectrum was obtained as a result of 512 scans. To measure IR spectra of a C18ANA crystal, the attenuated total reflection (ATR) technique was employed, in which the cell was made of a horizontal ZnSe crystal (RI = 2.403) with an incident angle of 45°. A homemade thermoelectric device was used to control the temperature of the sample cell with an accuracy of ±0.1 K. The cell was sealed to prevent the evaporation of the sample. The subtraction of the background solvent spectrum and the nonlinear least-squares method were performed on the software coded by one of the authors (Y. K.). BAM images of C18ANA monolayers on water at 298 K were measured using a Nima Technology (Coventry, England) MicroBAM Brewster-angle microscope with a spatial resolution of 8 μm obtained by a 30 mW 662 nm laser. This equipment was installed over the Langmuir trough by a custom-made metal block provided by Altech (Tokyo, Japan).

Figure 1. Surface pressure−area (π−A) isotherm for a C18ANA monolayer at different temperatures of the aqueous phase. The compression rate of A was 0.052 nm2 molecule−1 min−1.

Figure 2. (a) Definitions of R, θ, and L to analyze camphor motion and (b) time-course of snapshots of camphor motion at T = (b-1) 278, (b-2) 283, and (b-3) 288 K (A = 0.14 nm2 molecule−1; time interval, 0.4 s). The individual snapshots (b-1, b-2, and b-3) were trimmed in the same space. θ in (b-1), (b-2), and (b-3) was 0.64π, 0.38π, and 0.42π rad, respectively. Movies of (b-1) and (b-2) are provided as Supporting Information.

at A = 0.14 nm2 molecule−1. Because the trimmed snapshots (b1, b-2, and b-3 in Figure 2) are individually of the same space and size, a camphor disk exhibits one-dimensional reciprocating motion. Here, we introduced R(θ) as a polar coordinate to represent the reciprocating motion (see definition in Figure 2a). The amplitude of the reciprocating motion increased with an increase in the temperature between 278 and 288 K (Figure 2b). Reciprocating motion started at t ∼ 65, 60, and 40 s at 278, 283, and 288 K, respectively. After this reciprocating motion at 278, 283, and 288 K was maintained for 2, 3, and 4 min, respectively, it changed to irregular motion. The direction of the reciprocating motion was independent of the direction of compression. The temperature of the aqueous phase was measured with an infrared camera when a camphor disk moved. The spatiotemporal difference in the temperature was maintained within 1 K, as shown in Figure S1 of Supporting Information. Figure 3 shows the (a) area, (b) speed, (c) amplitude of L, and (d) period of camphor motion depending on the temperature of the aqueous phase at A = 0.14 nm2 molecule−1.

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RESULTS The surface pressure−area isotherms for C18ANA monolayers at different temperatures of the aqueous phase are shown in Figure 1. Above 281 K, the local minimum and local maximum of π were maintained at Amin (∼0.2 nm2 molecule−1) and Amax (∼0.3 nm2 molecule−1), respectively, and π at around Amax was maintained at ∼23 mN m−1. With a decrease in temperature, π increased around Amin, and as a result the local minimum disappeared. Figure 2 shows snapshots of a camphor disk for different temperatures of the aqueous phase as observed from above. In this experiment, a camphor disk was placed on the C18ANA molecular layer at the elapsed time t = 0 after A reached 0.14 nm2 molecule−1 by compression. We selected A = 0.14 nm2 molecule−1 because reciprocating motion was stably generated 14889

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Figure 5. (a) N−H and (b) CO stretching regions of the IR spectra for C18ANA at different temperatures. A graph of the wavenumber at the absorbance peak versus temperature for N−H stretching is shown in the inset in (a).

Figure 3. (a) Area, (b) speed, (c) amplitude of L at θmax (filled circles), and (d) period of camphor motion depending on the temperature of the aqueous phase. θmax (rad) was 0.64π at 278 K, 0.32π at 281 K, 0.38π at 283 K, 0.69π at 286 K, and 0.42π at 288 K. Empty circles in (c) denote the amplitude at θmin (rad) = θmax + π/2.

for N−H stretching (∼3340 cm−1) shifted to a lower wavenumber and the absorbance at 1684 cm−1, which is the lower wavenumber in the absorption band for CO stretching, increased slightly. To observe the status of the C18ANA monolayer under the compression, BAM images were monitored under the compression, as shown in Figure 6. All of the BAM images

No motion, reciprocating motion, and irregular motion were observed at 276, 278−288, and 293−308 K, respectively, and only reciprocating motion was analyzed in (c) and (d). Here, θmax (rad) is the angle θ at the maximum amplitude of L, and θmin (rad) = θmax + π/2. Self-motion was analyzed for 50 s for every examination, and the average values and their error bars were obtained for 3 examinations. With an increase in the temperature of the aqueous phase, the area of camphor motion increased. The speed of camphor motion increased below 283 K with an increase in the temperature but was almost constant above 283 K. Both the amplitude and period of reciprocating motion for θmax increased with an increase in the temperature. Figure 4 shows the relationship between the local minimum value of the surface pressure (πmin) and the surface pressure at

Figure 6. (a) BAM images for a C18ANA monolayer at A = (a-1) 0.31, (a-2) 0.24, (a-3) 0.22, (a-4) 0.17, and (a-5) 0.12 nm2 molecule−1 and (b) simultaneous measurement of π−A isotherm for a C18ANA monolayer at 298 K. The compression rate of A was 0.031 nm2 molecule−1 min−1. Figure 4. Relationship between the local minimum value of the surface pressure (πmin) and the surface pressure at A = 0.14 nm2 molecule−1 (πc) for different temperatures of the aqueous phase. Empty and filled circles correspond to reciprocating and irregular motion, respectively. Typical trajectories of reciprocating and irregular motion for 5 min are shown (blue line, top view). The rest and fluctuation of motion at the initial state were included in the trajectory of reciprocating motion. A thick outline around the trajectory corresponds to the water chamber.

exhibit coexisting disordered (dark contrast) and ordered (bright contrast) phases, i.e., the dark and bright contrasts correspond to the pure water surface and the C18ANA molecular layer, respectively. The spatial homogeneity of the bright image increased with a decrease in the area A (from 0.31 to 0.24 nm2 molecule−1). When A = 0.22 nm2 molecule−1, which corresponded to a decrease in the surface pressure from the local maximum, brighter domains were locally distributed. At A = 0.17 nm2 molecule−1, which corresponded to the local minimum of the surface pressure, a dark region was partly observed. A homogeneous bright image was observed again at A < 0.14 nm2 molecule−1. Figure 7 shows the reciprocating motion of a camphor disk along a line that was artificially drawn on the C18ANA molecular layer. In this experiment, the line was drawn with a

A = 0.14 nm2 molecule−1 (πc) for different temperatures of the aqueous phase. Reciprocating and irregular motion were distinguished based on πmin and πc, i.e., reciprocating motion and irregular motion were observed at higher and lower values of πmin and πc, respectively. Figure 5 shows the IR spectra for C18ANA at different temperatures to elucidate the interaction between C18ANA molecules. With a decrease in temperature, the absorption peak 14890

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C18ANA molecules. This irreversible condensation increases the surface area without C18ANA; therefore, π is decreased with compression. The surface pressure for the condensed C18ANA monolayer is increased by further compression; therefore, π−A isotherms indicate a characteristic curve. The decrease in the local minimum value with an increase in the temperature (Figure 1) suggests that the distribution of the C18ANA monolayer at a higher temperature is more heterogeneous than that at a lower temperature. A suggested mechanism for the reciprocating motion of a camphor disk is schematically shown in Figure 9a. As for the

Figure 7. (a) Trajectory of reciprocating motion of a camphor disk (red line) on a molecular layer of C18ANA after a line (or a character “−”) was drawn with a camphor pen (blue dotted line), (b) timevariation of reciprocating motion on the y-axis in panel a, and (c) timevariation of reciprocating motion on x-axis of panel a. Snapshots of drawing with a camphor pen (time interval, 1.2 s) and reciprocating motion of a camphor disk (time interval, 1/3 s) are shown beside panel c individually. The outline in panel a corresponds to the water chamber.

camphor disk, which was held with a pair of tweezers by hand, on the C18ANA molecular layer at A = 0.14 nm2 molecule−1 and 288 K (Figure 7c). Another camphor disk was then placed on the line. The camphor disk indicated reciprocating motion along the artificial line. Figure 8 shows the reciprocating motion

Figure 9. Schematic illustration of the suggested mechanism of (a) reciprocating motion on a C18ANA monolayer (side view) and (b) temperature-dependence at a lower temperature (left panel) and a higher temperature (right panel) (top view). The gray region in panel b denotes the existence of a C18ANA molecular layer.

initial condition at A = 0.14 nm2 molecule−1, the camphor disk cannot move because π for camphor is lower than that for C18ANA. The settled disk is destabilized by the accumulation of the camphor molecules dissolved from the disk. Therefore, unidirectional motion occurs because of the symmetry breaking of camphor distribution.25 The surface density of the C18ANA monolayer is increased because of the compression by camphor motion. The condensed C18ANA monolayer with a higher surface pressure can suppress the camphor motion any more (state I in Figure 9a). The low density of C18ANA molecules on the trajectory of camphor motion is maintained because of the irreversible condensation. The camphor disk reverses because of the sublimation of camphor on the trajectory and inversion of the gradient in the surface pressure around the disk.26 Therefore, the camphor disk draws the same trajectory (state II in Figure 9a). Because the C18ANA monolayer on the opposite side is similarly condensed by camphor motion, one-dimensional reciprocating motion is repeated on the same trajectory. Thus, the irreversibly condensed C18ANA monolayer maintains the amplitude of reciprocating motion. As for the temperature-dependence of camphor motion, Figures 2, 3a, and 4 suggest that the surface area of motion is related to πc and πmin. The higher πc can suppress the selfmotion of a camphor disk because the trajectory of camphor motion can be immediately occupied by the C18ANA molecular layer. Therefore, the surface area of motion increases with a decrease in πc (Figures 2, 3a, and 4). In contrast, the lower πmin

Figure 8. Trajectory of the reciprocating motion of a camphor disk on a C18ANA monolayer (red line) after the characters “S”, “C”, and “I” were drawn with a camphor pen (blue dotted line).

of camphor disks on the characters “S”, “C”, and “I” which were drawn on a C18ANA molecular layer. In this experiment, the characters were individually drawn with a camphor disk, which was held with a pair of tweezers, by hand on the C18ANA molecular layer at A = 0.14 nm2 molecule−1 and 288 K. Another camphor disk was then placed on each character. These camphor disks individually exhibited reciprocating motion following these characters.

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DISCUSSION On the basis of the experimental results and related studies,21,25,26,31,32 we discuss the temperature-dependent transient reciprocating motion of a camphor disk on a C18ANA molecular layer. When the area A becomes less than that at the local maximum (Amax ∼ 0.3 nm2 molecule−1) under the compression, irreversible condensation between C18ANA molecules is generated by the molecular interaction between 14891

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can enhance the freedom of camphor motion; therefore, reciprocating motion changes to irregular motion with a decrease in πmin. The fact that πc and πmin decrease with an increase in the temperature may be due to the weakening of the interaction between the condensed C18ANA molecules. The decrease in the wavenumber for N−H (Figure 5a) and the increase in the relative intensity of absorbance at the lower shoulder (1684 cm−1) for CO (Figure 5b) with a decrease in temperature implies that the hydrogen bonding between CO in a C18ANA molecule and N−H in another C18ANA molecule is slightly strengthened. Thus, a C18ANA monolayer is further condensed through the increase in the hydrogen bonding of the monolayer when the temperature of C18ANA is low. Therefore, the surface area where the camphor disk moves is restricted at the lower temperature because the C18ANA monolayer with strong interaction is homogeneously distributed and it is difficult for the camphor disk to extend the surface area of motion (left side of Figure 9b). In contrast, the camphor disk can extend the surface area of motion by crowding out the C18ANA molecular layer because of the weak interaction at a higher temperature (right side of Figure 9b). Figure 4 suggests that both the higher surface pressure (πc) at the measurement area, A (= 0.14 nm2 molecule−1), and the higher local minimum of the surface pressure (πmin) play an important role in the induction of reciprocating motion. On the other hand, the lower πc and lower πmin cannot produce reciprocating motion and instead induce irregular motion due to the lower surface pressure around the camphor disk. The BAM images in Figure 6 support the suggested status of the C18ANA molecular layer in Figure 9. Figures 7 and 8 suggest that the trajectory of reciprocating motion can be determined by artificially scratching the C18ANA monolayer, and it is difficult for the C18ANA monolayer to fill the scratched area without C18ANA because of the strong interaction between the C18ANA molecules at a lower temperature. It is known that Marangoni flow occurs in the camphor system,5,17,18 and the relationship between speed of motion and Marangoni flow was reported in separate papers.33,34 We consider the effect of Marangoni flow on the reciprocating motion in future work.

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +81-82-424-7409. E-mail: [email protected]. jp. Present Address ⊥

Y.K.: Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research (25410094) and the Sekisui Chemical Grant Program for Research on Manufacturing Based on Innovations Inspired by Nature to S.N.

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CONCLUSION We found that a camphor disk exhibits transient reciprocating motion on a C18ANA monolayer depending on the temperature of the aqueous phase. The C18ANA monolayer induces the nonlinear phenomena, such as reciprocating motion, i.e., oscillation, and mode-bifurcation among no motion, reciprocating motion, and irregular motion. The π−A curve and reciprocating motion depending on the temperature were considered regarding the interaction between C 18 ANA molecules at the air−aqueous interface and the irreversible condensation. Our results suggest that the nature of a selfpropelled motor can be designed based on chemical information, such as chemical structure and concentration.

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ASSOCIATED CONTENT

S Supporting Information *

Infrared image of water surface when a camphor disk was floated on it (Figure S1) and movies of camphor motion as presented in Figure 2. This material is available free of charge via the Internet at http://pubs.acs.org. 14892

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(18) Luo, C.; Qiao, L.; Li, H. Dramatic Squat and Trim Phenomena of mm-Scaled SU-8 Boats Induced by Marangoni Effect. Microfluid. Nanofluid. 2010, 9, 573−577. (19) Tomlinson, C. On the Motions of Camphor on the Surface of Water. Proc. R. Soc. London 1860, 11, 575−577. (20) Rayleigh, L. Measurements of the Amount of Oil Necessary in Order to Check the Motions of Camphor upon Water. Proc. R. Soc. London 1890, 47, 364−367. (21) Nakata, S.; Iguchi, Y.; Ose, S.; Kuboyama, M.; Ishii, T.; Yoshikawa, K. Self-Rotation of a Camphor Scraping on Water: New Insight into the Old Problem. Langmuir 1997, 13, 4454−4458. (22) Nagayama, M.; Yadome, M.; Murakami, M.; Kato, N.; Kirisaka, J.; Nakata, S. Bifurcation of Self-Motion Depending on the Reaction Order. Phys. Chem. Chem. Phys. 2009, 11, 1085−1090. (23) Iida, K.; Suematsu, N. J.; Miyahara, Y.; Kitahata, H.; Nagayama, M.; Nakata, S. Experimental and Theoretical Studies on the SelfMotion of a Phenanthroline Disk Coupled with Complex Formation. Phys. Chem. Chem. Phys. 2010, 12, 1557−1563. (24) Suematsu, N. J.; Miyahara, Y.; Matsuda, Y.; Nakata, S. SelfMotion of a Benzoquinone Disk Coupled with a Redox Reaction. J. Phys. Chem. C 2010, 114, 13340−13343. (25) Nakata, S.; Murakami, M. Self-Motion of a Camphor Disk on an Aqueous Phase Depending on the Alkyl Chain Length of Sulfate Surfactants. Langmuir 2010, 26, 2414−2417. (26) Hayashima, Y.; Nagayama, M.; Nakata, S. A Camphor Grain Oscillates while Breaking Symmetry. J. Phys. Chem. B 2001, 105, 5353−5357. (27) Suematsu, N. J.; Ikura, Y.; Nagayama, M.; Kitahata, H.; Kawagishi, N.; Murakami, M.; Nakata, S. Mode-Switching of the SelfMotion of a Camphor Boat Depending on the Diffusion Distance of Camphor Molecules. J. Phys. Chem. C 2010, 114, 9876−9882. (28) Kohira, M. I.; Hayashima, Y.; Nagayama, M.; Nakata, S. Synchronized self-motion of two camphor boats. Langmuir 2001, 17, 7124−7129. (29) Ikura, Y. S.; Heisler, E.; Awazu, A.; Nishimori, H.; Nakata, S. Collective motion of symmetric camphor papers in an annular water channel. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2013, 88, 012911. (30) Miyamoto, Y.; Kaifu, K.; Koyano, T.; Saito, M.; Kato, M. Second Harmonic Generation from Mixed Langmuir-Blodgett Films of NAcyl-p-Nitroaniline and Its Homologous Amphiphile. Thin Solid Films 1992, 210/211, 178−181. (31) Nakata, S.; Miyaji, T.; Sato, T.; Hoshikawa, M.; Ikura, Y. S.; Izumi, S. Reciprocating Motion of a Self-Propelled Object on a Molecular Layer. ChemPhysChem 2012, 13, 4129−4133. (32) Nakata, S.; Miyaji, T.; Ueda, T.; Sato, T.; Ikura, Y. S.; Izumi, S.; Nagayama, M. Reciprocating Motion of a Self-Propelled Object on a Molecular Layer with a Local Minimum and a Local Maximum Isotherm. J. Phys. Chem. C 2013, 117, 6346−6352. (33) Kitahata, H.; Hiromatsu, S.; Doi, Y.; Nakata, S.; Islam, M. R. Self-Motion of a Camphor Disk Coupled with Convection. Phys. Chem. Chem. Phys. 2004, 6, 2409−2414. (34) Ikura, Y. S.; Tenno, R.; Kitahata, H.; Suematsu, N. J.; Nakata, S. Suppression and Regeneration of Camphor-Driven Marangoni Flow with the Addition of Sodium Dodecyl Sulfate. J. Phys. Chem. B 2012, 116, 992−996.

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