Single-walled carbon nanotubes (SWCNTs) are reported to spontaneously align in a rotational pattern by drying a liquid droplet of toluene containing polyfluorene as a dispersant. By situating a droplet of an SWCNT solution around a glass bead, spiral patterns are generated. The parallel alignment of SWCNTs along one stripe of such a pattern is confirmed using scanning electron microscopy and polarized optical microscopy. The orientation order increases toward the outer edge of a stripe. The stripe width in the pattern is proportional to the solute concentration, and the width and position of the stripes follow geometric sequences. The growth of the rotational pattern is also observed in real time. The process of spiral pattern formation is visualized, indicating the role of the annihilation of counter-traveling accompanied by continuous depinning. The geometric sequences for the stripe width and position are explained by the near-constant traveling speed and solute enrichment at the droplet periphery.
A sustainable droplet motion that is driven by pH oscillation was obtained. The pH oscillation is only of a single pulse in a batch reactor. However, it shows continuous oscillation around the moving droplet, as the motion itself controls the diffusion flux in an asymmetric manner. Various types of motions that are spontaneous in nature may be obtained by a single-pulse oscillation coupled with mass transport.
Herein, the oscillation of an oil droplet on the surface of water is studied. The droplet contains an anionic surfactant that can react with the cations present in water. The oscillation starts after a random motion, and the oscillation pattern apparently depends on the cation species in the water phase. However, a common pattern is included. The cation species only affects the amplitude and frequency and sometimes perturbs the regular pattern owing to the instability at the oil/water interface. This common pattern is explained by a simple model that incorporates the surfactant transport from the droplet to the surrounding water surface. The dependency of the amplitude and frequency on cation species is expressed quantitatively by a single parameter, the product of the amplitude and square of frequency. This parameter depends on the cationic species and can be understood in terms of the spreading coefficient. The simple model successfully explains this dependency.
Biological functions are maintained by various types of molecular motors driven at several pico-newtons, where the driving force is obtained from a chemical potential difference within the microscale. Here, we show in detail artificial vesicles that generate mechanical work from a local pH gradient. This study demonstrates that they can be regarded as a molecular assembly machine. We have previously reported that the vesicles are composed of oleate and oleic acid and exhibit rhythmic shape changes. This cyclic motion involves both rotation of the entire vesicle and its inside-out inversion, which constitute relaxation and excitation processes, respectively, that sustain the cycle. These motions were observed under a quasi-steady state pH gradient, and the driving force of rotation was determined to be of the order of 10−2–10−1 pN, which is consistent with the membrane elasticity driving the deformation (vesicle inversion).
The synchronization of chemically driven oscillators plays a crucial role in various biological motions. An artificial model system of these chemo-mechanical oscillators is proposed in this study. The oscillator is composed of three liquid layers (oil/water/oil), which exhibit a back and forth motion in a glass tube. This motion is caused by the chemical reaction between a water-soluble surfactant and oil-soluble anions. The frequency is unique for an individual experimental setup because it depends on the surface state of the glass sensitively. When the glass tube with the liquid oscillator is placed on a plate with mechanical vibration, the frequencies of the oscillator and mechanical vibration become similar within a certain frequency range of mechanical vibration. When two or more glass tubes are placed in a boat floating on a water surface, all frequencies agree with each other by the joggling motion of the boat. The entrainment into the external vibration and mutual synchronization on the boat are explained by a simple mathematical model. The proposed chemo-mechanical oscillator may be used as a primitive model system for studying the interplay of macroscopic motion and molecular scale processes that control chemically driven motion.
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