Biomedical subjects

Shin-ichi Sasa

Publications and source records attributed to Shin-ichi Sasa.

8 recordsLinked to original sources

PubMed2006

Systematic derivation of coarse-grained fluctuating hydrodynamic equations for many Brownian particles under nonequilibrium conditions.

We study the statistical properties of many Brownian particles under the influence of both a spatially homogeneous driving force and a periodic potential with period l in a two-dimensional space. In particular, we focus on two asymptotic cases lint< >l , where lint represents the interaction length between two particles. We derive fluctuating hydrodynamic equations describing the evolution of a coarse-grained density field defined on scales much larger than l for both cases. Using the obtained equations, we calculate the equal-time correlation functions of the density field to the lowest order of the interaction strength. We find that the system exhibits long-range correlation of the type r-d (d=2) for the case lint>>l , while no such behavior is observed for the case lint<<l .

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PubMed2006

Energy dissipation and violation of the fluctuation-response relation in nonequilibrium Langevin systems.

The fluctuation-response relation is a fundamental relation that is applicable to systems near equilibrium. On the other hand, when a system is driven far from equilibrium, this relation is violated in general because the detailed-balance condition is not satisfied in nonequilibrium systems. Even in this case, it has been found that for a class of Langevin equations, there exists an equality between the extent of violation of the fluctuation-response relation in the nonequilibrium steady state and the rate of energy dissipation from the system into the environment [T. Harada and S.-i. Sasa, Phys. Rev. Lett. 95, 130602 (2005)]. Since this equality involves only experimentally measurable quantities, it serves as a proposition to determine experimentally whether the system can be described by a Langevin equation. Furthermore, the contribution of each degree of freedom to the rate of energy dissipation can be determined based on this equality. In this paper, we present a comprehensive description on this equality, and provide a detailed derivation for various types of models including many-body systems, Brownian motor models, time-dependent systems, and systems with multiple heat reservoirs.

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PubMed2005

Equality connecting energy dissipation with a violation of the fluctuation-response relation.

In systems driven away from equilibrium, the velocity correlation function and the linear-response function to a small perturbation force do not satisfy the fluctuation-response relation (FRR) due to the lack of detailed balance in contrast to equilibrium systems. In this Letter, an equality between an extent of the FRR violation and the rate of energy dissipation is proved for Langevin systems under nonequilibrium conditions. This equality enables us to calculate the rate of energy dissipation by quantifying the extent of the FRR violation, which can be measured experimentally.

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PubMed2005

Statistics of unstable periodic orbits of a chaotic dynamical system with a large number of degrees of freedom.

For a simple model of chaotic dynamical systems with a large number of degrees of freedom, we find that there is an ensemble of unstable periodic orbits (UPOs) with the special property that the expectation values of macroscopic quantities can be calculated using only one UPO sampled from the ensemble. Evidence to support this conclusion is obtained by generating the ensemble by Monte Carlo calculation for a statistical mechanical model described by a space-time Hamiltonian that is expressed in terms of Floquet exponents of UPOs. This result allows us to interpret the recent interesting discovery that statistical properties of turbulence can be obtained from only one UPO [G. Kawahara and S. Kida, J. Fluid Mech. 449, 291 (2001); S. Kato and M. Yamada, Phys. Rev. E 68, 025302(R) (2003)].

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PubMed2005

Anomalous time correlation in two-dimensional driven diffusive systems.

We study the time-correlation function of a density field in two-dimensional driven diffusive systems within the framework of fluctuating hydrodynamics. It is found that the time correlation exhibits power-law behavior in an intermediate time regime in the case that the fluctuation-dissipation relation is violated and that the power-law exponent depends on the extent of this violation. We obtain this result by employing a renormalization group method to treat a logarithmic divergence in time.

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PubMed2004

Effective temperature in nonequilibrium steady states of Langevin systems with a tilted periodic potential.

We theoretically study Langevin systems with a tilted periodic potential. It is known that the ratio Theta of the diffusion constant D to the differential mobility mu(d) is not equal to the temperature of the environment (multiplied by the Boltzmann constant), except in the linear response regime, where the fluctuation dissipation theorem holds. In order to elucidate the physical meaning of Theta far from equilibrium, we analyze a modulated system with a slowly varying potential. We derive a large scale description of the probability density for the modulated system by use of a perturbation method. The expressions we obtain show that Theta plays the role of the temperature in the large scale description of the system and that Theta can be determined directly in experiments, without measurements of the diffusion constant and the differential mobility. Hence the relation D= mu(d) Theta among the independent measurable quantities D, mu(d), and Theta can be interpreted as an extension of the Einstein relation.

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PubMed2003

Thermodynamic relations in a driven lattice gas: numerical experiments.

We explore thermodynamic relations in nonequilibrium steady states with numerical experiments on a driven lattice gas. After operationally defining the pressure and chemical potential in the driven lattice gas, we confirm numerically the validity of the integrability condition (the Maxwell relation) for the two quantities whose values differ from those for an equilibrium system. This implies that a free-energy function can be constructed for the nonequilibrium steady state that we consider. We also investigate a fluctuation relation associated with this free-energy function. Our result suggests that the compressibility can be expressed in terms of density fluctuations even in nonequilibrium steady states.

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PubMed2003

Anomalous pressure in fluctuating shear flow.

We investigate how the pressure in fluctuating shear flow depends on the shear rate S and on the system size L by studying fluctuating hydrodynamics under shear conditions. We derive anomalous forms of the pressure for two limiting values of the dimensionless parameter lambda=SL(2)/nu, where nu is the kinematic viscosity. In the case lambda << 1, the pressure is not an intensive quantity because of the influence of the long-range spatial correlations of momentum fluctuations. In the other limit lambda >> 1, the long-range correlations are suppressed at large distances, and the pressure is intensive. In this case, however, there is the interesting effect that the nonequilibrium correction to the pressure is proportional to S(3/2), which was previously obtained with the projection operator method [K. Kawasaki and J. D. Gunton, Phys. Rev. A 8, 2048 (1973)].

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