Akio Nakahara

Position control of desiccation cracks by memory effect and Faraday waves.

Pattern formation of desiccation cracks on a layer of a calcium carbonate paste is studied experimentally. This paste is known to exhibit a memory effect, which means that a short-time application of horizontal vibration to the fresh paste predetermines the direction of the cracks that are formed after the paste is dried. While the position of the cracks (as opposed to their direction) is still stochastic in the case of horizontal vibration, the present work reports that their positioning is also controllable, at least to some extent, by applying vertical vibration to the paste and imprinting the pattern of Faraday waves, thus breaking the translational symmetry of the system. The experiments show that the cracks tend to appear in the node zones of the Faraday waves: in the case of stripe-patterned Faraday waves, the cracks are formed twice more frequently in the node zones than in the anti-node zones, presumably due to the localized horizontal motion. As a result of this preference of the cracks to the node zones, the memory of the square lattice pattern of Faraday waves makes the cracks run in the oblique direction differing by 45 degrees from the intuitive lattice direction of the Faraday waves.Graphical abstract

Experimental Investigation on the Validity of Population Dynamics Approach to Bacterial Colony Formation

We have investigated the dynamics of a two-dimensional spreading of a bacterial population in a surface environment. After point inoculation of flagellated bacteria ( Bacillus subtilis ) on nutrient-rich semi-solid medium, the bacterial population grew up by multiplication and translocation of cells, and developed a homogeneous round colony. By comparing experimental results with those of numerical simulations of the model equation, we found that this homogeneous population growth of bacteria is an actual manifestation of growth dynamics described by the Fishers equation.

Desiccation cracks and their patterns: Formation and modelling in science and nature

The ideal team of authors, combining experimental and theoretical backgrounds, and with experience in both physical and earth sciences, discuss how the study of cracks can lead to the design of crack-resistant materials, as well as how cracks can be grown to generate patterned surfaces at the nanoand micro-scales. Important research and recent developments on tailoring desiccation cracks by different methods are covered, supported by straightforward, yet deep theoretical models.

Transition in the pattern of cracks resulting from memory effects in paste.

Experiments involving vibrating pastes before drying were performed for the purposes of controlling the crack patterns that appear in the drying process. These experiments revealed a transition in the direction of lamellar cracks from perpendicular to parallel when compared with the direction of the initial external vibration as the solid volume fraction of the paste is decreased. This result suggests a transition in the memory in paste, which is visually represented as morphological changes in the crack pattern. Given that the memory in paste represents the flow pattern induced by the initial external vibration, it should then be possible to control and design various crack patterns, such as cellular, lamellar, radial, ring, spiral, and others.

Imprinting Memory into Paste and Its Visualization as Crack Patterns in Drying Process

In the drying process of a paste, we can imprint a memory into the paste that determines how it will be broken in the future. That is, if we vibrate the paste before it is dried, it remembers the direction of the initial external vibration, and the morphology of the resultant crack patterns is determined solely by the memory of this direction. The morphological phase diagram of crack patterns and the rheological measurement of the paste show that this memory effect is induced by the plasticity of the paste.

Morphological Diversity in the Crystal Growth of Potassium Dichromate in Gelatin Gel.

Potassium dichromate (K 2 Cr 2 O 7 ) grown in gelatin has been found to exhibit five morphological phases, i.e., right- or left-handed spiral, spherulite, square polelike, platelike and DLA-like crystals when varying the initial concentrations of K 2 Cr 2 O 7 and gelatin. The results are summarized in a morphological phase diagram. Although we reported in our previous letter that spiral crystals obtained were all right-handed, only a few left-handed spiral crystals have also been observed in a specific subregion of the spiral growth region in the phase diagram. Square polelike crystals have shown tendency to grow and fuse together side by side in a region adjacent to the spiral growth in the phase diagram. This strongly suggests that spirals are composed of small square polelike crystals fused together. It is conjectured that right-handed triple helices of collagen molecules constituting gelatin-gel perturb the growth process to form right-handed spirals obtained mainly in the present experiments.

Self-Organized Critical Density Waves of Granular Material Flowing through a Pipe

Fourier analysis has been applied to density waves of granular material (sand) which flows through a pipe. We let sand flow from a hopper through a vertical glass pipe due to gravity. The flow appeared intermittent, reminiscent of a chain of traffic jams on a crowded highway, over the length of the pipe except for the uppermost part where the sand flowed smoothly and homogeneously from the hopper. The FFT power spectra were found to change from a rather structureless form to a stable power-law form of 1/ f α with α≃1.5 as we varied the measuring position from the hopper down to the pipe end. This result shows the occurrence of self-organized critical density waves in granular material flow through a pipe.

Morphological Diversity of the Colony Produced by Bacteria Proteus mirabilis

Morphological changes of colonies have been investigated for a bacterial strain of Proteus mirabilis , which is a famous species for producing concentric-ring-like colonies. It was found that colony patterns can be classified into three types, i.e., cyclic spreading, diffusion-limited growth (DLA-like) and three-dimensional growth (inside the agar medium) patterns. Cyclic spreading patterns can further be classified into three subgroups, i.e., concentric-ring, homogeneous and spatiotemporal patterns. These subgroups were classified by examining the development of colony structure after colonies spread all over petri-dishes. Comparison of the results with those of another bacterial species Bacillus subtilis is also discussed.

Self-organized critical density waves of granular particles flowing through a pipe

Abstract Density waves of granular material which flows through a pipe have been investigated. Granules (mainly sand) were drained from a hopper through a vertical glass pipe. The flow looked intermittent over the length of the pipe except for the uppermost part where the granules streamed smoothly and homogeneously from the hopper aperture. Transmission light intensity across the pipe was measured as time-series signals. The FFT power spectra were found to change from white-noise-like form to a stable and robust power-law form of 1/ f α with α ≊1.5 as the measuring position was varied from just below the hopper to the pipe end. This indicates the occurrence of self-organized critical density waves in the granular flow through a pipe. It is pointed out that the back-flow of air is responsible for the occurrence of the density waves.

Spatiotemporal Patterns Produced by Bacteria

Spatiotemporal patterns formed by a bacterial colony of Proteus mirabilis on an agar plate were observed. About half or one hour after the colony spread over the entire surface of the agar medium in a petridish, various patterns including target and spiral patterns appeared. They are very similar to those seen in other dissipative systems, such as chemical oscillations and electrohydrodynamic convective systems. Microscopic observations revealed that the collective motion of bacterial cells is responsible for the formation of these spatiotemporal patterns.