Yumihiko S. Ikura

Suppression and Regeneration of Camphor-Driven Marangoni Flow with the Addition of Sodium Dodecyl Sulfate

We investigated the Marangoni flow around a camphor disk on water with the addition of sodium dodecyl sulfate (SDS). The flow velocity decreased with an increase in the concentration of SDS in the aqueous phase, and flow was hardly observed around the critical micelle concentration (cmc), because SDS reduced the driving force of Marangoni flow. However, the flow velocity increased with a further increase in the concentration of SDS. Thus, the Marangoni flow is maximally inhibited around the cmc of SDS. In this paper, we concluded that the regeneration of Marangoni flow originates from an increase in the dissolution rate of camphor into the SDS aqueous solution.

Oscillation of a polymer gel entrained with a periodic force.

The oscillation of a polymer gel induced by the Belousov-Zhabotinsky (BZ) reaction was investigated under an external force composed of a square wave. The oscillation of the BZ reaction entrained to the periodic force and the features of this entrainment changed depending on the period and duty cycle of the square wave. The experimental results suggest that the change in the volume of the gel also gave feedback to the BZ reaction. The mechanism of entrainment is discussed in relation to the compression of the gel and the reaction-diffusion system in the BZ reaction.

Oscillation and Synchronization in the Combustion of Candles

We investigate a simple experimental system using candles; stable combustion is seen when a single candle burns, while oscillatory combustion is seen when three candles burn together. If we consider a set of three candles as a component oscillator, two oscillators, that is, two sets of three candles, can couple with each other, resulting in both in-phase and antiphase synchronization depending on the distance between the two sets. The mathematical model indicates that the oscillatory combustion in a set of three candles is induced by a lack of oxygen around the burning point. Furthermore, we suggest that thermal radiation may be an essential factor of the synchronization.

Mode Change in the Self-Motion of a Benzoquinone Disk Coupled with a NADPH System

The self-motion of a benzoquinone (BQ) disk on NADPH was investigated as the coupling of an autonomous motor and an enzyme reaction. In the absence of the enzyme reaction, features of motion changed depending on the concentration of NADPH, that is, continuous motion→ intermittent oscillatory motion→ no motion. When the reverse reaction from NADP(+) to NADPH was introduced into the system with the addition of an enzyme reaction, continuous motion changed to intermittent oscillatory motion with small amplitude. The mechanism of this mode change is discussed in relation to the surface tension as a driving force and the time course of UV spectra as a window to the progress of the reaction. Characteristic features of the mode change were qualitatively reproduced by a numerical calculation.

Periodic Oscillatory Motion of a Self‐Propelled Motor Driven by Decomposition of H2O2 by Catalase

A self-propelled motor driven by the enzymatic reaction of catalase adsorbed onto a filter paper floating on an aqueous solution of H2 O2 was used to study nonlinear behavior in the motors motion. An increase in the concentration of H2 O2 resulted in a change from no motion to irregular oscillatory motion, periodic oscillatory motion, and continuous motion. The mechanisms underlying oscillation and mode bifurcation are discussed based on experimental results on O2 bubble formation and growth on the underside of the motor.

Reciprocating Motion of a Self‐Propelled Object on a Molecular Layer

The mode change of a simple autonomous motor depending on the nature of a monolayer on water is investigated. A camphor disk is floated on a molecular layer of N-stearoyl-p-nitroaniline (C(18)ANA), which gives a surface-pressure (π)-area per molecule (A) isotherm with a local maximum and a local minimum. The nature of the camphor motion changes depending on A, and in particular, reciprocating motion is observed at a lower A while cutting out its own trajectory of motion. The characteristic motion of a camphor disk depending on A is discussed in relation to the π-A isotherm of C(18)ANA and the influence of the molecular interaction between molecules on the driving force of motion.

Chemical wave propagation preserved on an inhibitory field in the ruthenium-catalyzed Belousov-Zhabotinsky reaction.

The propagation of a chemical wave on an inhibitory field, which was wedged between two excitable fields, was investigated for the photosensitive Belousov-Zhabotinsky (BZ) reaction. With an increase in the width of the inhibitory field between the excitable fields (W), the chemical wave divided into two waves at W = Wα. The divided chemical waves then coalesced at W = Wβ with a decrease in W. Wα was larger than Wβ, i.e., hysteresis on the width of the inhibitory field was observed between the division and coalescence of the chemical wave. The experimental results were qualitatively reproduced by a numerical calculation based on a three-variable Oregonator model modified for the photosensitive BZ reaction. These results suggest that the chemical wave may be preserved on the inhibitory field due to an activator supplied from a chemical wave on the excitable field.