Grid pattern emerging from complex dynamics of defects.

The formation process and growth dynamics of the grid pattern, a cellular convective pattern in the electroconvection of nematic liquid crystals, are investigated. The grid pattern appears via a disordered state called defect turbulence with the increasing of an applied voltage. The averaged defect density increases with the applied voltage and then the defects that have been in the continuous process of creation and annihilation are frozen as grid cells forming domain structures. The area fraction of the grid domains is adopted as the order parameter. The temporal growth of the area fraction for the step voltage was also measured. By applying the Kolmogorov-Avrami model to the results, it is suggested that the growth dynamics of the grid domain is not primarily governed by domain growth, but by the local transition of the rolls to the cellular flow via preliminary grid structures that transiently appear.

Emergence of different crystal morphologies using the coffee ring effect.

Macroscopic patterns in nature formed during crystal growth e.g. snow crystals have a significant influence on many material properties, such as macroscopic heat conduction, electrical conduction, and mechanical properties, even with the same microscopic crystal structure. Although crystal morphology has been extensively studied in bulk, the formation of patterns induced by re-crystallization during evaporation is still unclear. Here, we find a way to obtain concentric circles, a dendritic pattern, and a lattice pattern by pinning the edge of droplets using the coffee ring effect; only aggregates of crystallites are seen in the absence of pinning. Our systematic study shows that the macroscopic patterns depend both on initial concentration and evaporation rate. In addition, our qualitative analysis suggests that the local concentration of solute at the center of the pattern is related to the macroscopic patterns.

Active hole generation in a liquid droplet dissolving into a binary solvent.

In liquid-liquid dissolution, the critical point of phase separation is determined by the temperature. When the solvent consists of multi-components, in contrast, the mole fractions in the solvent also take on the role of control parameter. In this study an ionic liquid dissolves into a binary solvent composed of ethanol and water. It is found in this system that, near the critical point, a hole is spontaneously created in the droplet of the ionic liquid. The creation of the hole is initiated by a mutual interaction between the concentrations of the ionic liquid and the binary solvent via their affinity. A spatial inhomogeneity of the interfacial tension is induced through an amplification of fluctuation in the concentration due to an instability mechanism, and causes the Marangoni effect to create the hole. The hole moves inside the droplet and consequently leads to the motion of the droplet. The present system provides not only a new type of dissolution process but also a peculiar example of active matter realized in a liquid droplet.

Dynamical transition in a jammed state of a quasi-two-dimensional foam.

The states of foam are empirically classified into dry foam and wet foam by the volume fraction of the liquid. Recently, a transition between the dry foam state and the wet foam state has been found by characterizing the bubble shapes [Furuta et al., Sci. Rep. 6, 37506 (2016)2045-232210.1038/srep37506]. In the literature, it is indirectly ascertained that the transition from the dry to the wet form is related to the onset of the rearrangement of the bubbles, namely, the liquid fraction at which the bubbles become able to move to replace their positions. The bubble shape is a static property, and the rearrangement of the bubbles is a dynamic property. Thus, we investigate the relation between the bubble shape transition and the rearrangement event occurring in a collapsing process of the bubbles in a quasi-two-dimensional foam system. The current setup brings a good advantage to observe the above transitions, since the liquid fraction of the foam continuously changes in the system. It is revealed that the rearrangement of the bubbles takes place at the dry-wet transition point where the characteristics of the bubble shape change.

Close relationship between a dry-wet transition and a bubble rearrangement in two-dimensional foam.

Liquid foams are classified into a dry foam and a wet foam, empirically judging from the liquid fraction or the shape of the gas bubbles. It is known that physical properties such as elasticity and diffusion are different between the dry foam and the wet foam. Nevertheless, definitions of those states have been vague and the dry-wet transition of foams has not been clarified yet. Here we show that the dry-wet transition is closely related to rearrangement of the gas bubbles, by simultaneously analysing the shape change of the bubbles and that of the entire foam in two dimensional foam. In addition, we also find a new state in quite low liquid fraction, which is named "superdry foam". Whereas the shape change of the bubbles strongly depends on the change of the liquid fraction in the superdry foam, the shape of the bubbles does not change with changing the liquid fraction in the dry foam. Our results elucidate the relationship between the transitions and the macroscopic mechanical properties.

A new mechanism for dendritic pattern formation in dense systems.

Patterns are often formed when particles cluster: Since patterns reflect the connectivity of different types of material, the emergence of patterns affects the physical and chemical properties of systems and shares a close relationship to their macroscopic functions. A radial dendritic pattern (RDP) is observed in many systems such as snow crystals, polymer crystals and biological systems. Although most of these systems are considered as dense particle suspensions, the mechanism of RDP formation in dense particle systems is not yet understood. It should be noted that the diffusion limited aggregation model is not applicable to RDP formation in dense systems, but in dilute particle systems. Here, we propose a simple model that exhibits RDP formation in a dense particle system. The model potential for the inter-particle interaction is composed of two parts, a repulsive and an attractive force. The repulsive force is applied to all the particles all the time and the attractive force is exerted only among particles inside a circular domain, which expands at a certain speed as a wave front propagating from a preselected centre. It is found that an RDP is formed if the velocity of the wave front that triggers the attractive interaction is of the same order of magnitude as the time scale defined by the aggregation speed.

Dynamical transition of heat transport in a physical gel near the sol-gel transition.

We experimentally study heat transport in a gelatin solution near a reversible sol-gel transition point where viscosity strongly depends on temperature. We visualize the temperature field and velocity field using thermochromic liquid crystals and polystyrene latex particles, respectively. During the initial stages of heating, we find that heat transport undergoes a dynamical transition from conductive to convective. Subsequently, during later stages, we observe that the transport dynamics are much more complex than conventional thermal convections. At the sample's surface we observe the formation of stagnant domains, which lack fluid flow. Their formation is not due to the effects of local cooling. We determine that it is the dynamics of these stagnant domains that induce convective-conductive-convective transitions.

Lagrangian chaos and particle diffusion in electroconvection of planar nematic liquid crystals.

Two types of spatiotemporal chaos in the electroconvection of nematic liquid crystals, such as defect turbulence and spatiotemporal intermittency, have been statistically investigated according to the Lagrangian picture. Here fluctuations are traced using the motion of a single particle driven by chaotic convection. In the defect turbulence (fluctuating normal rolls), a particle is mainly trapped in a roll but sometimes jumps to a neighboring roll. Its activation energy is then obtained from the jumping (hopping) rate. This research clarifies that diffusion in the defect turbulence regime in electroconvection can be regarded as a kind of hopping process. The spatiotemporal intermittency appears as a coexistent state of ordered grid domains and turbulent domains. The motion of a single particle shows weak and strong diffusion, respectively, in the ordered and turbulent domains. The diffusion characteristics intermittently change from one to another with certain durations as the domains change. This research has found that the distribution function of the duration that a particle remains in an ordered area has a power-law decay for which the index is different from that obtained by the Eulerian measurement.

Collective behavior of Dictyostelium discoideum monitored by impedance analysis.

Dictyostelium discoideum cells respond to periodic signals of extracellular cAMP by collective changes of cell-cell and cell-substrate contacts. This was confirmed by dielectric analysis employing electric cell-substrate impedance sensing (ECIS) and impedance measurements involving cell-filled micro channels in conjunction with optical microscopy providing a comprehensive picture of chemotaxis under conditions of starvation.

Chemotaxis of Dictyostelium discoideum: collective oscillation of cellular contacts.

Chemotactic responses of Dictyostelium discoideum cells to periodic self-generated signals of extracellular cAMP comprise a large number of intricate morphological changes on different length scales. Here, we scrutinized chemotaxis of single Dictyostelium discoideum cells under conditions of starvation using a variety of optical, electrical and acoustic methods. Amebas were seeded on gold electrodes displaying impedance oscillations that were simultaneously analyzed by optical video microscopy to relate synchronous changes in cell density, morphology, and distance from the surface to the transient impedance signal. We found that starved amebas periodically reduce their overall distance from the surface producing a larger impedance and higher total fluorescence intensity in total internal reflection fluorescence microscopy. Therefore, we propose that the dominant sources of the observed impedance oscillations observed on electric cell-substrate impedance sensing electrodes are periodic changes of the overall cell-substrate distance of a cell. These synchronous changes of the cell-electrode distance were also observed in the oscillating signal of acoustic resonators covered with amebas. We also found that periodic cell-cell aggregation into transient clusters correlates with changes in the cell-substrate distance and might also contribute to the impedance signal. It turned out that cell-cell contacts as well as cell-substrate contacts form synchronously during chemotaxis of Dictyostelium discoideum cells.

Controlling chaos for spatiotemporal intermittency.

This paper reports the control of spatiotemporal intermittency in an electroconvective system in a nematic liquid crystal. In the spatiotemporal intermittency, an ordered structure [the defect lattice (DL)] coexists with turbulence. Control of the spatiotemporal intermittency, in which the turbulent state changes to a DL, is achieved by a few percent amplitude modulation of the applied ac voltage. The optimal control frequency is equal to the intrinsic oscillation frequency of the DL. The transition from the turbulent state to a DL occurs not by nucleation of the DL domain, but by penetration of the DL domain into the turbulent one. Control of the spatiotemporal intermittency is achieved through a resonance of the DL oscillation with respect to the modulation frequency that leads to spatial entrainment.

Order-disorder phase transition in a chaotic system.

For soft-mode turbulence, which is essentially the spatiotemporal chaos caused by the nonlinear interaction between convective modes and Goldstone modes in electroconvection of homeotropic nematics, a type of order-disorder phase transition was revealed, in which a new order parameter was introduced as pattern ordering. We calculated the spatial correlation function and the anisotropy of the convective patterns as a 2D XY system because the convective wave vector could freely rotate in the homeotropic system. We found the hidden order in the chaotic patterns observed beyond the Lifshitz frequency f(L), and a transition from a disordered to a hidden ordered state occurred at the f(L) with the increase of the frequency of the applied voltages.

Formation of a defect lattice in electroconvection of nematics.

The formation of a defect lattice--i.e., a periodic orientational structure of numerous defect pairs--is experimentally investigated in the electroconvection of nematics. Specific twist structures of the director as a background field play an important role in the formation of a defect lattice. The observed formation sequence is as follows. With increasing applied voltage, normal rolls change into abnormal rolls due to the twisting deformation of the director. This process leads to belt-shaped domains along the abnormal rolls, in which the twist angle of the director alternates between positive and negative angles. The period of the defect lattice perpendicular to the rolls is equal to that of the domain structure in the abnormal rolls. Further increasing the applied voltage induces a secondary short-wavelength mode by the skewed varicose instability, which in turn induces defects. The periodicity of the defect lattice parallel to the rolls is due to the beating mode of the normal rolls and the secondary mode.