Current Physical Chemistry - Volume 5, Issue 1, 2015

Numerous self-organized spatiotemporal patterns are seen in Nature, many of which are controlled via minute balances between the reactions and diffusion of the constituents in each system. The Liesegang phenomenon is one of the major mechanisms that yield static self-organized patterns in Nature, e.g., the beautiful patterns of agates. Since the discovery of the Liesegang phenomenon in chemistry, many researchers have attempted to identify the inherent mechanism to enable the engineering of desired patterns by chemical reactions. In this review, we briefly discuss the theoretical background of Liesegang phenomena, which can be used to quantitatively explain the experimental results in a number of Liesegang systems. Some recent developments in the controlled construction of Liesegang patterns are then reviewed.

A self-propelled object is a type of functional inanimate system used in the development of novel micro-robot designs. Many studies on the application of selfpropelled objects as drug delivery systems, cleaning systems for polluted water cleansing, and target DNA sorting systems have been reported. In order to realize additional functional systems, environmental responsivity is a desired feature of self-propelled objects. In this mini-review, we focus on self-propelled objects’ responses to environments with different chemical and light conditions, and classify the examined systems based on their sensitivity to various conditions.

Maze solving and finding the shortest path or all paths in a maze are interesting and challenging computational problems. These problems are important in robotics, urban transportation, and they provide model problems of network and graph theory. In the past decades, several unconventional computing methods have been proposed to solve a maze and to find its shortest path. In this review, we review the chemical-based methods proposed in the literature. In these methods, chemistry plays an important role in phenomena that drive maze solving.

The self-propelled motion of micrometer-sized oil droplets in nonequilibrium systems has attracted considerable attention as a primitive type of inanimate chemical machinery. We investigated the dynamics of n-heptyloxybenzaldehyde droplets in a cationic reactive surfactant solution with the objective of controlling the movement of self-propelled oil droplets. (1) We designed a novel molecular system that lengthened the locomotion time of oil droplets by using estercontaining cationic surfactants, (2) we evaluated the dynamics of oil droplets in a solution of a gemini cationic surfactant with various alkyl chain lengths in the linker to induce a locomotion mode change of the droplets and induce a conformational change in the flexible linked molecule, and (3) we examined the behavior of oil droplets in a microchannel in a solution containing a gemini cationic surfactant that has a carbonate linkage. These droplets exhibited two motions during the hydrolysis of the carbonate linkage, generating two surfactant species.

Viscous fingering (VF) is a phenomenon in which the interface between two fluids of having viscosities is fluid-dynamically unstable in porous media, with finger-like patterns forming as the less viscous fluid displacing the more viscous one. In this article, studies investigating the fundamental characteristics of the coupling between chemistry and hydrodynamics in reactive VF are reviewed. For this purpose, reactive VF has been divided into two types depending on whether or not the reaction changes the VF dynamics i.e., the active or passive cases of reactive VF, respectively. In this review, I first consider the passive and active cases, and then further discuss a third type of reactive VF as well. This third type of reactive is VF fully triggered by a chemical reaction in which VF does not take place in the absence of the chemical reaction. Finally, potential future topics of research are discussed.

We have studied stochastic motions of the microorganism Volvox, which displays phototactic behavior. Volvox colonies in a chamber were stimulated stepwise or sinusoidally by varying light intensities from both sides of the chamber, and their responses were systematically measured. We found that the colonies show a linear response to a relative light intensity difference, expressed as a ratio of the difference between the light intensities of the two sides to the sum of the intensities. An experimental formula obtained using our measurements describe the responses under various light conditions. We also found a linear relationship between fluctuation without the light stimulus and response to the stimulus; a group of colonies with a large fluctuation in velocity shows a larger response. This result indicates the existence of a relationship between fluctuations and responses in the motion of a living organism.

The research into the controllable micron-sized self-propelled particles provides basic principles for various applications such as micro robots, cell manipulation, drug delivery, etc. It is well known that we can adopt the fields such as concentration fields, temperature fields, and electric fields as the power source of self-propelled particles. Generally, the symmetry of the fields around particles is highly related to the shape of the particles. Utilizing this fact, recently, there are many attempts to realize various types of motion using rotationally asymmetric particles. By the control of the shape, the net torque and the net force exerted to the particles are finely controlled. This review article highlights the pioneering studies of the motion control of the self-propelled particles by the shape control. Not only the particles with fixed shapes but also the deformable self-propelled particles are introduced. The motions induced by the deformation are discussed with both the experimental studies and the theoretical studies. For the further development, we also introduce the recent development of the methods to produce rotationally asymmetric microparticles including our new easy-to-use method. We expect that these approaches will be useful for the wide range of applications in physical, life and biomedical science.

As a simple physico-chemical system that exhibits droplet motion induced by the pattern formation inside it, we investigate the motion of a droplet of Belousov- Zhabotinsky (BZ) reaction medium depending on the size and initial point of the chemical wave. We also observe the profile of Marangoni flow induced in the BZ droplet. In our previous paper, we reported the BZ-droplet motion and proposed a mechanism based on low-Reynolds-number hydrodynamics. Here, we discuss the validity of the suggested mechanism based on experimental results.

With numerous reports of creating propulsion in synthetic microsized particles, the next goal of this research field is to control the direction of propulsion so that it can be used to perform useful tasks. This mini-review summarizes the current state of this research including directing propulsion of microparticles using externally applied (e.g. electric) or naturally occurring fields (e.g. gravity), migration due to a chemical gradient (chemotaxis), and organization of particles’ position relative to other particles (clustering).