北畑 裕之

The synchronized self-motion of two camphor boats on polygonal water chambers was investigated. The two boats synchronously moved depending on the number of corners in the polygon by changing the distance between the two boats through the corners. We regard the self-motion of a camphor boat as an oscillator; i.e., one cycle on the polygonal chamber corresponds to 2pi. Phase-locked synchronization at a phase difference of 2pi/3, which corresponds to the length of one side of the chamber, was observed with a triangular chamber. Two types of synchronized motion at phase differences of pi/2 and pi, which correspond to the length of one and two sides of the chamber, respectively, were observed with a square chamber. These characteristic features of synchronized self-motion were qualitatively reproduced by a numerical calculation that regarded the surface tension as the driving force and the number of corners in the chamber as a velocity-regulating mechanism. We believe that the present system may be a simple model of synchronization which depends on the geometry of the system.

Dynamical patterns of microscopic phase segregation under linear scanning of the focus of IR laser were investigated in a homogeneous oil/water mixture. When the scanning frequency is low, the generated oil droplet follows the motion of the laser focus. When the frequency is high, the generated oil droplet is located at a certain position. The mechanism of these characteristic trapping patterns is discussed in terms of the effective potential by the focused laser beam, which depends on the scan rate and the mode of scanning. (C) 2004 Elsevier B.V. All rights reserved.

生物は等温条件下で化学エネルギーをベクトル的な仕事に直接変換しつつ運動しており、そのメカニズムは未だ明らかではない。生体内でのエネルギーの損失は非常に少なく、その機構の解明は多くの恩恵をもたらすであろう。我々は、化学反応からベクトル的な仕事を取り出すために、Belousov-Zhabotinsky(BZ)反応が引き起こす対流現象を用いて物体を一方向へ輸送する系を作り、化学-機械エネルギー変換のメカニズムについて考察した。

Living organisms maintain their lives through the dissipation of chemical energy under isothermal conditions. They transduce chemical energy into macroscopic motion with high efficiency, but the mechanism is not fully understood yet. By considering experimental model systems together with experimental results reported for the molecular machinery, it will be possible to attain an essential understanding of mechanism on the chemo-mechanical energy transduction in living organisms. In the present paper, we introduce several simple experimental systems where spontaneous motion of a droplet is induced by chemical energy gradient or by photon flux.

The propagation of a chemical wave in a narrow, cone-shaped glass capillary was investigated. When a chemical wave propagates from the wider end to the narrower end, it slows, stops, and then disappears. A phenomenological model that considers the surface effect of the glass is proposed, and this model reproduces the experimental trends.

The synchronized self-motion of a camphor fragment and a mobile boundary on a water surface depending on the initial conditions and the asymmetry of the boundary was investigated. When a camphor fragment, which showed unidirectional rotation, was surrounded by a circular ring as a symmetric mobile boundary on a water surface, the circular ring also rotated but in the direction opposite to that of the camphor fragment. The velocity of the rotation of the mobile ring varied depending on the anisotropy of the shape of the ring. When a camphor fragment was placed on the inside of a circular ring floating on the water surface, the ring and camphor fragment exhibited translational motion without rotation. These characteristics of synchronized self-motion are discussed in relation to the surface tension as the driving force and convective flow due to the gradient of the surface-active camphor layer. (C) 2004 Elsevier Inc. All rights reserved.

The self-motion of a camphor disk on water was investigated both experimentally and theoretically. When the camphor disk moved along a one-dimensional water chamber, a convective flow was observed around the disk and the magnitude of convection decreased with an increase in the velocity of camphor motion. The camphor disk exhibited a characteristic motion that depended on the shape of the base of the chamber. The nature of the self-motion was reproduced by a numerical simulation that included the surface tension of the camphor layer as the driving force and Marangoni convection induced by the difference in surface tension around the camphor disk.

Recently, it has been found that a giant DNA molecule exhibits a repetitive change in conformation between an elongated coil and folded compact states under irradiation by a continuous IR laser. We report here a theoretical model to explain such spontaneous rhythmic motion. The generation of the rhythm can be attributed to the competition between optical attraction in the compact state and the instability of the compact state at the laser focus due to heating. This rhythmic conformational change is discussed in relation to the working mechanism of biological molecular motors. © 2004 The American Physical Society.

The generation of convective flow by a chemical wave was studied experimentally on Belousov-Zhabotinsky (BZ) reaction medium. A propagating chemical wave causes a transient increase in interfacial tension, which induces convection in the bulk phase. The observed flow profile was reproduced with a numerical simulation by coupled equation of a Navier-Stokes equation including interfacial tension with the chemical kinetics, named Oregonator. The present experimental system serves as a model toward the deeper understanding on the mechanism of direct coupling of chemical energy with mechanical motion.

We found that a focused laser can generate microscopic phase separation in an oil/water system. An oil droplet emerges and grows at the focus of the laser in a water-rich homogeneous medium. In contrast, in an oil-rich homogeneous phase, water droplets spring out in a successive manner from the focus of the laser, move away, and disappear in the surroundings, forming a flower-like pattern. The mechanism of this dynamic phase separation is discussed under the framework of the mean field theory. (C) 2003 American Institute of Physics.

The characteristic motion of a 1,10-phenanthroline grain on an FeSO4 aqueous phase was investigated as a simple example of an autonomous motor. The uniform motion of the grain changed to intermittent motion (periodic change between motion and rest) and its period and resting time increased depending on the concentration of FeSO4 (C-FeSO4). However, this intermittent motion reverted to uniform motion and the velocity increased with a further increase in C-FeSO4. The nature of this characteristic motion depending on C-FeSO4 is discussed in relation to the surface tension, which depends on surface concentration of surface active layer composed of 1,10-phenanthroline and a tris(1,10-phenanthroline) complex as the driving force.

生命のなぞに迫るためには,エネルギーや物質が流入・流出する散逸系であることを踏まえなければならない.これまで,散逸系の代表例として反応拡散系が取り上げられ,同心円やスパイラルなどの動的パターンの発生やチューリング (Turing) 不安定性など,実験,理論の両面から盛んに研究が行われてきた.ここでは,反応拡散系での基本的な仮定である「反応場が実空間上で静止している」という制約を取り除き,反応拡散対流系という枠組みを考える.これによって,本来,スカラー量の化学エネルギーからベクトル的な運動を直接取り出すことが可能となることを示す.

Rhythmic bursting on the order of seconds in a cluster of plastic beads under continuous irradiation of a focused neodymium-doped yttrium aluminum garnet (Nd:YAG) laser beam (1064 nm) is reported. The oscillatory instability is induced as a result of competition between trapping and scattering forces, where both forces are induced by the focused laser beam. Above a critical power of the laser beam, mode bifurcation from the stationary state into periodic bursting is observed. Our model employing ordinary differential equations reproduces the essential aspects of the experimental results. © 2002 The American Physical Society.