arXiv subjects

Ko Okumura

Publications and source records attributed to Ko Okumura.

36 records · Page 2Linked to original sources

arXiv2019

Visco- and plasto-elastic fracture of nano-porous polymer sheets

We study the dependence of the fracture surface energy on the pulling velocity for nano-porous polypropylene (PP) sheets to find two components: the static and dynamic ones. We show that these terms can be interpreted respectively as plastoelastic and viscoelastic components, as has been shown for soft polyethylene (PE) foams in a previous work. Considering significant differences in the pore size, volume fraction and Young's modulus of the present PP and previous PE sheets, the present results suggest a universal physical mechanism for fracture of porous polymer sheets. The simple physical interpretation emerging from the mechanism could be useful for developing tough polymers. Equivalence of Griffith's energy balance in fracture mechanics to a stress criterion is also discussed and demonstrated using the present experimental data.

cond-mat.soft↗

arXiv2018

Self-similar dynamics of air film entrained by a solid disk in confined space: a simple prototype of topological transitions

In hydrodynamic topological transitions, one mass of fluid breaks into two or two merge into one. For example, in the honey-drop formation when honey dripping from a spoon, honey is extended to separate into two as the liquid neck bridging them thins down to micron scales. At the moment when topology changes due to the breakup, physical observables such as surface curvature locally diverges. Such singular dynamics have widely attracted physicists, revealing universality in their self-similar dynamics, which share much in common with critical phenomena in thermodynamics. Many experimental examples have been found, which include electric spout and vibration-induced jet eruption. However, only a few cases have been physically understood on the basis of equations that govern the singular dynamics and even in such a case the physical understanding is mathematically complicated inevitably involving delicate numerical calculations. Here, we study breakup of air film entrained by a solid disk into viscous liquid in a confined space, which leads to formation, thinning and breakup of the neck of air. As a result, we unexpectedly find that equations governing the neck dynamics can be solved analytically by virtue of two remarkable experimental features: only a single length scale linearly dependent on time remains near the singularity and universal scaling functions describing singular neck shape and velocity field are both analytic. The present solvable case would be essential for our better understanding of the singular dynamics and will help unveil the physics of unresolved examples intimately related to daily-life phenomena and diverse practical applications.

cond-mat.stat-mech↗

arXiv2017

Bursting dynamics of viscous film without circular symmetry: the effect of confinement

We experimentally investigate the bursting dynamics of confined liquid film suspended in air and find a viscous dynamics distinctly different from the non-confined counterpart, due to lack of circular symmetry in the shape of expanding hole: the novel confined-viscous bursting proceeds at a constant speed and a rim formed at the bursting tip does not grow. We find a confined-viscous to confined-inertial crossover, as well as a nonconfined-inertial to confined-inertial crossover, at which bursting speed does not change although the circular symmetry in the hole shape breaks dynamically.

physics.flu-dyn↗

arXiv2017

Viscous dynamics of drops and bubbles in Hele-Shaw cells: drainage, drag friction, coalescence, and bursting

In this review article, we discuss recent studies on drops and bubbles in Hele-Shaw cells, focusing on how scaling laws exhibit crossovers from the three-dimensional counterparts and focusing on topics in which viscosity plays an important role. By virtue of progresses in analytical theory and high-speed imaging, dynamics of drops and bubbles have actively been studied with the aid of scaling arguments. However, compared with three dimensional problems, studies on the corresponding problems in Hele-Shaw cells are still limited. This review demonstrates that the effect of confinement in the Hele-Shaw cell introduces new physics allowing different scaling regimes to appear. For this purpose, we discuss various examples that are potentially important for industrial applications handling drops and bubbles in confined spaces by showing agreement between experiments and scaling theories. As a result, this review provides a collection of problems in hydrodynamics that may be analytically solved or that may be worth studying numerically in the near future.

cond-mat.soft↗

arXiv2017

Stationary crack propagation in a two-dimensional visco-elastic network model

We investigate crack propagation in a simple two-dimensional visco-elastic model and find a scaling regime in the relation between the propagation velocity and energy release rate or fracture energy, together with lower and upper bounds of the scaling regime. On the basis of our result, the existence of the lower and upper bounds is expected to be universal or model-independent: the present simple simulation model provides generic insight into the physics of crack propagation, and the model will be a first step towards the development of a more refined coarse-grained model. Relatively abrupt changes of velocity are predicted near the lower and upper bounds for the scaling regime and the positions of the bounds could be good markers for the development of tough polymers, for which we provide simple views that could be useful as guiding principles for toughening polymer-based materials.

cond-mat.mtrl-sci↗

arXiv2017

Exactly solvable model for a velocity jump observed in crack propagation in viscoelastic solids

Needs to impart appropriate elasticity and high toughness to viscoelastic polymer materials are ubiquitous in industries such as concerning automobiles and medical devices. One of the major problems to overcome for toughening is catastrophic failure linked to a velocity jump, i.e., a sharp transition in the velocity of crack propagation occurred in a narrow range of the applied load. However, its physical origin has remained an enigma despite previous studies over 35 years. Here, we propose an exactly solvable model that exhibits the velocity jump incorporating linear viscoelasticity with a cutoff length for a continuum description. With the exact solution, we elucidate the physical origin of the velocity jump: it emerges from a dynamic glass transition in the vicinity of the propagating crack tip. We further quantify the velocity jump together with slow- and fast-velocity regimes of crack propagation, which would stimulate the development of tough polymer materials.

cond-mat.soft↗

arXiv2016

Meandering instability of air flow in a granular bed: self-similarity and fluid-solid duality

Meandering instability is familiar to everyone through river meandering or small rivulets of rain flowing down a windshield. However, its physical understanding is still premature, although it could inspire researchers in various fields, such as nonlinear science, fluid mechanics and geophysics, to resolve their long-standing problems. Here, we perform a small-scale experiment in which air flow is created in a thin granular bed to successfully find a meandering regime, together with other remarkable fluidized regimes, such as a turbulent regime. We discover that phase diagrams of the flow regimes for different types of grains can be universally presented as functions of the flow rate and the granular-bed thickness when the two quantities are properly renormalized. We further reveal that the meandering shapes are self-similar as was shown for meandering rivers. The experimental findings are explained by theory, with elucidating the physics. The theory is based on force balance, a minimum-dissipation principle, and a linear-instability analysis of a continuum equation that takes into account the fluid-solid duality, i.e., the existence of fluidized and solidified regions of grains along the meandering path. The present results provide fruitful links to related issues in various fields, including fluidized bed reactors in industry.

cond-mat.soft↗

arXiv2016

Scaling crossover in thin-film drag dynamics of fluid drops in the Hele-Shaw cell

We study both experimentally and theoretically the descending motion due to gravity of a fluid drop surrounded by another immiscible fluid in a confined space between two parallel plates, i.e., in the Hele-Shaw cell. As a result, we show a new scaling regime of a nonlinear drag friction in viscous liquid that replaces the well-known Stokes' drag friction through a clear collapse of experimental data thanks to the scaling law. In the novel regime, the dissipation in the liquid thin film formed between the drop and cell walls governs the dynamics. The crossover of this scaling regime to another scaling regime in which the dissipation inside the droplet is dominant is clearly demonstrated and a phase diagram separating these scaling regimes is presented.

physics.flu-dyn↗

arXiv2016

Initial rigid response and softening transition of highly stretchable kirigami sheet materials

We study, experimentally and theoretically, the mechanical response of sheet materials on which line cracks or cuts are arranged in a simple pattern. Such sheet materials, often called kirigami (the Japanese words, kiri and gami, stand for cut and paper, respectively), demonstrate a unique mechanical response promising for various engineering applications such as stretchable batteries: kirigami sheets possess a mechanical regime in which sheets are highly stretchable and very soft compared with the original sheets without line cracks, by virtue of out-of-plane deformation. However, this regime starts after a transition from an initial stiff regime governed by in-plane deformation. In other words, the softness of the kirigami structure emerges as a result of a transition from the two-dimensional to three-dimensional deformation, i.e., from stretching to bending. We clarify the physical origins of the transition and mechanical regimes, which are revealed to be governed by simple scaling laws. The results could be useful for controlling and designing the mechanical response of sheet materials including cell sheets for medical regeneration and relevant to the development of materials with tunable stiffness and mechanical force sensors.

cond-mat.soft↗

arXiv2016

Stationary bubble formation and cavity collapse in wedge-shaped hoppers

The hourglass is one of the apparatuses familiar to everyone, but reveals intriguing behaviors peculiar to granular materials, and many issues are remained to be explored. In this study, we examined the dynamics of falling sand in a special form of hourglass, i.e., a wedge-shaped hopper, when a suspended granular layer is stabilized to a certain degree. As a result, we found remarkably different dynamic regimes of bubbling and cavity. In the bubbling regime, bubbles of nearly equal size are created in the sand at a regular time interval. In the cavity regime, a cavity grows as sand beads fall before a sudden collapse of the cavity. Bubbling found here is quite visible to a level never discussed in the physics literature and the cavity regime is a novel phase, which is neither continuous, intermittent nor completely blocked phase. We elucidate the physical conditions necessary for the bubbling and cavity regimes and develop simple theories for the regimes to successfully explain the observed phenomena by considering the stability of a suspended granular layer and clogging of granular flow at the outlet of the hopper. The bubbling and cavity regimes could be useful for mixing a fluid with granular materials.

cond-mat.soft↗

arXiv2003

Bouncing gel balls: impact of soft gels onto rigid surface

After thrown onto a solid substrate, very soft spherical gels bounce repeatedly. Separate rheological measurements suggest that these balls can be treated as nearly elastic. The Hertz contact deformation expected in the static (elastic) limit was observed only at very small impact velocities. For larger velocities, the gel ball deformed into flattened forms like a pancake. We measured the size of the gel balls at the maximal deformation and the contact time as a function of velocities for the samples different in the original spherical radius and the Young modulus. The experimental results revealed a number of scaling relations. To interpret these relations, we developed scaling arguments to propose a physical picture.

cond-mat↗

arXiv2003

Phase transitions of nematic rubbers

Single crystal nematic elastomers undergo a transition from a strongly ordered phase N to an "isotropic" phase I. We show that: (a) samples produced under tension by the Finkelmann procedure are intrinsically anisotropic and should show a small (temperature dependent) birefringence in the high temperature I phase. (b) for the I->Ntransition via cooling there is a spinodal limit but for the N->I transition via heating there is no soft mode at the standard spinodal temperature. (c) the N->I transition is reminiscent of a martensitic transformation: nucleation of the I phase should occur in the form of platelets, making a well defined angle with the director.

cond-mat.soft↗

arXiv2002

Physical picture for fractures in stratified materials: viscoelastic effects in large cracks

We present an intuitive physical picture for fractures in nacre-type stratified materials via scaling arguments: strain distributions around a fracture are rather different depending on directions and size of fractures. We thus observe that viscoelastic effects are important for parallel fractures. These effect are taken into account via the simplest viscoelastic model for weakly cross-linked polymer. Within a certain limit, we find a trumpet crack shape similar to that in certain polymers and make predictions for the fracture energy of certain stratified materials in this mode.

cond-mat.soft↗

arXiv2002

Viscoelastic fractures in stratified composite materials: "lenticular trumpet"

We consider fractures in a stratified composite material with solid layers separated by thin slices of extremely soft matter. Viscoelastic effects associated with the soft layers are taken into account via the simplest model for weakly cross-linked polymers. We find that certain small cracks running along layers take a new "trumpet" shape quite different from previously known shapes.

cond-mat.soft↗

arXiv2002

Energy-level diagrams and their contribution to fifth-order Raman and second-order infrared responses: Distinction between relaxation mechanisms by two-dimensional spectroscopy

We develop a Feynman rule for energy-level diagrams emphasizing their connections to the double-sided Feynman diagrams and physical processes in the Liouville space. Thereby we completely identify such diagrams and processes contributing to the two-dimensional response function in the Brownian oscillator model. We classify such diagrams or processes in quartet and numerically present signals separately from each quartet of diagrams or Liouville-space processes. We find that the signal from each quartet is distinctly different from the others; we can identify each peaks in frequency domain with a certain quartet. This offers the basis for analyzing and assigning actual two-dimensional peaks and suggests the possibility of Liouville-space-path selective spectroscopy. As an application we demonstrate an example in which two familiar homogeneous mechanisms of relaxation are distinguished by existence or non-existence of certain peaks on the two-dimensional map; appearance or disappearance of certain peak is sensitive to the coupling mechanism. We also point out some confusion in the literature with regard to inclusion of relaxation effects.

physics.chem-ph↗

arXiv2002

Why is nacre strong? II: remaining mechanical weakness for cracks propagating along the sheets

In our previous paper (Eur. Phys. J. E 4, 121 (2001)) we proposed a coarse-grained elastic energy for nacre, or stratified structure of hard and soft layers. We then analyzed a crack running perpendicular to the layers and suggested one possible reason for the enhanced toughness of this substance. In the present paper, we consider a crack running parallel to the layers. We propose a new term added to the previous elastic energy, which is associated with the bending of layers. We show that there are two regimes for the parallel-fracture solution of this elastic energy; near the fracture tip the deformation field is governed by a parabolic differential equation while the field away from the tip follows the usual elliptic equation. Analytical results show that the fracture tip is lenticular, as suggested in a paper on a smectic liquid crystal (P. G. de Gennes, Europhys. Lett. 13 (8), 709 (1990)). On the contrary, away from the tip, the stress and deformation distribution recover the usual singular behaviors (sqrt(x) and 1/sqrt(x), respectively, where x is the distance from the tip). This indicates there is no enhancement in toughness in the case of parallel fracture.

cond-mat.soft↗

arXiv1993

The supergroup U(M/N) with regard to electronic Hamiltonians

We study the U(M/N) supergroup keeping in mind its connection with electronic Hamiltonians. It is explicitly shown that the generators of the supergroup U(N/N) can be expressed by Clifford operators or Fermi operators. A multi-band supersymmetric electronic model is suggested.

cond-mat↗

arXiv1993

Various condensed matter Hamiltonians in terms of U(2/2) operators and their symmetry structures

We rewrite various lattice Hamiltonian in condensed matter physics in terms of U(2/2) operators that we introduce. In this representation the symmetry structure of the models becomes clear. Especially, the Heisenberg, the supersymmetric t-J and a newly proposed high-$T_c$ superconducting Hamiltonian reduce to the same form $H=$$-t\sum_{ }\sum_{ac}X^{ac}_j X^{ca}_k (-1)^{F(c)}$. This representation also gives us a systematic way of searching for the symmetries of the system.

cond-mat↗