Imperfect bifurcation in the rotation of a propeller-shaped camphor rotor

Dynamics of Aggregation in Systems of Self-Propelled Rods

We highlight camphene-camphor-polypropylene plastic as a useful material for self-propelled objects that show aggregation while floating on a water surface. We consider self-propelled rods as an example of aggregation of objects characterized by non-trivial individual shapes with low-symmetry interactions between them. The motion of rods made of the camphene-camphor-polypropylene plastic is supported by dissipation of the surface-active molecules. The physical processes leading to aggregation and the mathematical model of the process are discussed. We analyze experimental data of aggregate formation dynamics and relate them to the system's properties. We speculate that the aggregate structure can be represented as a string of symbols, which opens the potential applicability of the phenomenon for information processing if objects floating on a water surface are regarded as reservoir computers.

Reduction-Induced Self-Propelled Oscillatory Motion of Perylenediimides on Water

The emergence of macroscopic self-propelled oscillatory motion based on molecular design has attracted continual attention in relation to autonomous systems in living organisms. Herein, a series of perylenediimides (PDIs) with various imide side chains was prepared to explore the impact of molecular design and alignment on the self-propelled motion at the air-water interface. When placed on an aqueous solution containing a reductant, a solid disk of neutral PDI was reduced to form the water-soluble, surface-active PDI dianion species, which induces a surface tension gradient in the vicinity of the disk for self-propelled motion. We found that centimeter-scale oscillatory motion could be elicited by controlling the supply rate of PDI dianion species through the reductant concentration and the structure of the imide side chains. Furthermore, we found that the onset and speed of the self-propelled motion could be changed by the crystallinity of PDI at the water surface. This design principle using π-conjugated molecules and their self-assemblies could advance self-propelled, non-equilibrium systems powered by chemical energy.

Mathematical modeling for the synchronization of two interacting active rotors

We investigate the synchronization of active rotors. A rotor is composed of a free-rotating arm with a particle that releases a surface-active chemical compound. It exhibits self-rotation due to the surface tension gradient originating from the concentration field of the surface-active compound released from the rotor. In a system with two active rotors, they should interact through the concentration field. Thus, the interaction between them does not depend only on the instantaneous positions, but also on the dynamics of the concentration field. By numerical simulations, we show that in-phase and antiphase synchronizations occur depending on the distance between the two rotors. The stability of the synchronization mode is analyzed based on phase reduction theorem through the calculation of the concentration field in the co-rotating frame with the active rotor. We also confirm that the numerical results meet the prediction by theoretical analyses.

A camphene-camphor-polymer composite material for the production of superhydrophobic absorbent microporous foams

In a recently published paper (doi.org/10.3390/molecules26113116) on self-propelled motion of objects on the water surface, we described a novel surface-active plastic material obtained by dissolution of camphor and polypropylene in camphene at 250 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ$$\end{document}∘C. The material has wax-like mechanical properties, can be easily formed to any moldable shape, and allows for longer and more stable self-propelled motion if compared with pure camphor or pure camphene or of a camphene-camphor wax. Here we use scanning electron microscopy to visualize and characterize the microporous structure of the solid polypropylene foam formed in the plastic for different polypropylene contents. The topology of foams remaining in the material after camphor and camphene molecules have been removed through evaporation or dissolution is similar to polypropylene foams obtained using thermally-induced phase separation. We show that the foams have a superhydrophobic surface but strongly absorb non-polar liquids, and suggest an array of potential scientific and industrial applications.

A Perfect Plastic Material for Studies on Self-Propelled Motion on the Water Surface

We describe a novel plastic material composed of camphene, camphor, and polypropylene that seems perfectly suited for studies on self-propelled objects on the water surface. Self-motion is one of the attributes of life, and chemically propelled objects show numerous similarities with animated motion. One of important questions is the relationship between the object shape and its motility. In our paper published in 2019, we presented a novel hybrid material, obtained from the solution of camphor in camphene, that allowed making objects of various shapes. This hybrid material has wax-like mechanical properties, but it has a very high tackiness. Here, we report that a small amount of polypropylene removed this undesirable feature. We investigated the properties of camphor–camphene–polypropylene plastic by performing the statistical analysis of a pill trajectory inside a Petri dish and compared them with those of camphor-camphene wax. The plastic showed the stable character of motion for over an hour-long experiment. The surface activity of objects made of plastic did not significantly depend on the weight ratios of the compounds. Such a significant increase in usefulness came from the polypropylene, which controlled the dissipation of camphor and camphene molecules.