arXiv subjects

Naoko Nakagawa

Publications and source records attributed to Naoko Nakagawa.

At least 19 recordsLinked to original sources

arXiv2026

Pressure-drop localization and momentum insulation in liquid-gas coexistence Poiseuille flow

We study Poiseuille flow of a one-component fluid between adiabatic plates in liquid-gas coexistence with a stationary interface. Using only bulk transport laws and particle and energy conservation, we identify small dimensionless parameters $A^\mathrm{L}$ and $A^\mathrm{G}$ governed by squared microscopic-to-macroscopic length ratios. In weak driving with macroscopic bulk regions and a regular interfacial temperature response, the pressure drop is concentrated across the interface, strongly suppressing the particle current. For equal-temperature reservoirs, the residual current produces interfacial cooling. Experimental feasibility is also discussed.

physics.flu-dyn↗

arXiv2026

Gibbs Factorials Become Kinetic in History-Dependent Reactions

Gibbs factorials are usually regarded as equilibrium counting factors. We show that they can also appear directly in a measurable kinetic observable when products formed at different stages are statistically distinguished in the history ensemble. In a model designed to isolate the essential ingredients, transient AB$_2$ complexes are stabilized as C molecules either in a single operation or through a two-stage procedure. The mean-waiting-time ratio is governed by a kinetic advantage factor $\mathcal{A}_C$ defined from equilibrium probabilities. A fluctuation-theorem argument identifies the dominant contribution $n_{\mathrm C}!/[n_{\mathrm m}!(n_{\mathrm C}-n_{\mathrm m})!]$, where $n_{\mathrm m}$ is the intermediate product number and $n_{\mathrm C}$ is the final target. This Gibbs factorial arises because products formed before and after the intermediate operation are statistically distinguished in the history ensemble, although the final molecules are macroscopically identical. Molecular dynamics simulations confirm the predicted combinatorial scaling of the mean-waiting-time ratio.

cond-mat.stat-mech↗

arXiv2026

Global thermodynamics for heat-conducting fluids under weak gravity

We study liquid-gas coexistence under gravity and heat conduction from the viewpoint of global thermodynamics. We construct a variational free-energy function for the fixed-global-temperature description and decompose it into two parts. The first has the same configurational form as the equilibrium weak-gravity free energy with gravity replaced by the effective gravity, and it determines the first-order configurational transition between the two separated liquid-gas arrangements. The second is a residual excess-latent-heat contribution that vanishes without heat conduction. Although it does not decide which separated liquid-gas arrangement is thermodynamically favored, this residual part is needed to derive the fundamental relation in the laboratory variables and to recover thermodynamic observables such as the spatially averaged pressure. The same residual contribution reshapes the barrier geometry, ridge/valley structure, and interfacial anomalies of the fixed-global-temperature free-energy landscape. Numerical examples based on the van der Waals model illustrate the resulting landscape structure, and estimates of experimental scales suggest a setup for detecting the effective-gravity inversion.

cond-mat.stat-mech↗

arXiv2025

Thermodynamic Variational Principle Unifying Gravity and Heat Flow

Predicting the stable phase configuration in a liquid-gas system becomes a fundamental challenge when the stratification favored by gravity conflicts with arrangements induced by heat flow, particularly because standard equilibrium thermodynamics is insufficient in such non-equilibrium steady states. We propose a variational principle based on an extended thermodynamics, called global thermodynamics, to address this state selection problem. Our key finding is that gravity and heat flow effects are unified into a single parameter, ``effective gravity'' ($g_\mathrm{eff}$), within this framework. Crucially, the sign of $g_\mathrm{eff}$ determines the stable configuration: liquid is at the bottom if $g_\mathrm{eff} > 0$, and floats above the gas if $g_\mathrm{eff} < 0$. This provides a quantitative tool for the configuration prediction under competing drives.

cond-mat.stat-mech↗

arXiv2025

Global thermodynamics for isothermal fluids under gravity

We develop a formulation of global thermodynamics for equilibrium systems under the influence of gravity. The free energy for simple fluids is extended to include a dependence on $(T, V, N, mgL)$, where $L$ represents the vertical system length in the direction of gravity. A central idea in this formulation is to uniquely fix the reference point of the gravitational potential, ensuring a consistent thermodynamic framework. Using this framework, we derive the probability density of thermodynamic quantities, which allows us to define a variational function for determining equilibrium liquid-gas coexistence under gravity. The resulting free energy landscape, derived from the variational function, reveals the local stability of liquid-gas configurations. Specifically, the liquid phase resides at the lower portion of the system due to gravity, while the inverted configuration (with liquid on top) is also locally stable in this landscape. Furthermore, we characterize the transition between these liquid-gas configurations as a first-order phase transition using the thermodynamic free energy of $(T,V,N,mgL)$. Finally, we validate the predictions of global thermodynamics through molecular dynamics simulations, demonstrating the applicability and accuracy of the proposed framework.

cond-mat.stat-mech↗

arXiv2024

Non-equilibrium phase coexistence in boundary-driven diffusive systems

Liquid-gas phase coexistence in a boundary-driven diffusive system is studied by analyzing fluctuating hydrodynamics of a density field defined on a one-dimensional lattice with a space interval $Λ$. When an interface width $\ell$ is much larger than $Λ$, the discrete model becomes the standard fluctuating hydrodynamics, where the phase coexistence condition is given by the local equilibrium thermodynamics. In contrast, when $\ell < Λ$, the most probable density profile is determined by a new variational principle, where the chemical potential at the interface is found to deviate from the equilibrium coexistence chemical potential. This means that metastable states at equilibrium stably appear near the interface as the influence of the particle current. The variational function derived in the theoretical analysis is also found to be equivalent to the variational function formulated in an extended framework of thermodynamics called global thermodynamics. Finally, the validity of the theoretical result is confirmed by numerical simulations.

cond-mat.stat-mech↗

arXiv2024

Heat-induced liquid hovering in liquid-gas coexistence under gravity

We study a liquid-gas coexistence system in a container under gravity with heat flow in the direction opposite to gravity. By molecular dynamics simulation, we find that the liquid buoys up and continues to float steadily. The height at which the liquid floats is determined by a dimensionless parameter related to the ratio of the temperature gradient to gravity. We confirm that supercooled gas remains stable above the liquid. We provide a phenomenological argument for explaining the phenomenon from a simple thermodynamic assumption.

cond-mat.stat-mech↗

arXiv2023

Control of Metastable States by Heat Flux in the Hamiltonian Potts Model

The local equilibrium thermodynamics is a basic assumption of macroscopic descriptions of the out of equilibrium dynamics for Hamiltonian systems. We numerically analyze the Hamiltonian Potts model in two dimensions to study the violation of the assumption for phase coexistence in heat conduction. We observe that the temperature of the interface between ordered and disordered states deviates from the equilibrium transition temperature, indicating that metastable states at equilibrium are stabilized by the influence of a heat flux. We also find that the deviation is described by the formula proposed in an extended framework of the thermodynamics.

cond-mat.stat-mech↗

arXiv2022

Unique extension of the maximum entropy principle to phase coexistence in heat conduction

The maximum entropy principle determines the values of thermodynamic variables in thermally isolated equilibrium systems. This paper extends the principle to a variational principle that applies to liquid-gas coexistence in heat conduction. We show the uniqueness of the extension under the assumption that the variational principle and the fundamental thermodynamic relation are simultaneously extended in the linear response regime with the total energy fixed. Using the extended variational principle, we calculate the thermodynamic quantities in this steady state and find that the temperature of the liquid-gas interface deviates from the equilibrium transition temperature, which should be verified in experiments.

cond-mat.stat-mech↗

arXiv2022

Quasi-static decomposition and the Gibbs factorial in small thermodynamic system

For small thermodynamic systems in contact with a heat bath, we determine the free energy by imposing the following two conditions. First, the quasi-static work in any configuration change is equal to the free energy difference. Second, the temperature dependence of the free energy satisfies the Gibbs-Helmholtz relation. We find that these prerequisites uniquely lead to the free energy of a classical system consisting of $N$-interacting identical particles, up to an additive constant proportional to $N$. The free energy thus determined contains the Gibbs factorial $N!$ in addition to the phase space integration of the Gibbs-Boltzmann factor. The key step in the derivation is to construct a quasi-static decomposition of small thermodynamic systems.

cond-mat.stat-mech↗

arXiv2022

Work relation for determining the mixing free energy of small-scale mixtures

In thermodynamically characterizing a mixture comprising a finite number of molecules, we consider two kinds of protocol for producing a mixture from a pure substance. The first is a single alchemical operation, whereas the second is a series of processes with feedback control in information thermodynamics and conventional mixing with semipermeable membranes. A comparison of the two numerically determined free-energy changes provides a combinatorial factor that indicates the indistinguishability of the molecules and an alternative Jarzynski equality. The comparison also uncovers a work relation for determining the mixing free energy without using semipermeable membranes. We demonstrate a numerical calculation of applying the work relation to a mixture of argon and krypton. The mixing free energy clearly shows the characteristics of liquid--vapor transition.

cond-mat.stat-mech↗

arXiv2020

Effective Langevin equations leading to large deviation function of time-averaged velocity for a nonequilibrium Rayleigh piston

We study fluctuating dynamics of a freely movable piston that separates an infinite cylinder into two regions filled with ideal gas particles at the same pressure but different temperatures. To investigate statistical properties of the time-averaged velocity of the piston in the long-time limit, we perturbatively calculate the large deviation function of the time-averaged velocity. Then, we derive an infinite number of effective Langevin equations yielding the same large deviation function as in the original model. Finally, we provide two possibilities for uniquely determining the form of the effective model.

cond-mat.stat-mech↗

arXiv2020

Stochastic order parameter dynamics for phase coexistence in heat conduction

We propose a stochastic order parameter equation for describing phase coexistence in steady heat conduction near equilibrium. By analyzing the stochastic dynamics with a non-equilibrium adiabatic boundary condition, where total energy is conserved over time, we derive a variational principle that determines thermodynamic properties in non-equilibrium steady states. The resulting variational principle indicates that the temperature of the interface between the ordered region and the disordered region becomes greater (less) than the equilibrium transition temperature in the linear response regime when the thermal conductivity in the ordered region is less (greater) than that in the disordered region. This means that a super-heated ordered (super-cooled disordered) state appears near the interface, which was predicted by an extended framework of thermodynamics proposed in [N. Nakagawa and S.-i. Sasa, Liquid-gas transitions in steady heat conduction, Phys. Rev. Lett. {\bf 119}, 260602, (2017).]

cond-mat.stat-mech↗

arXiv2019

Global Thermodynamics for Heat Conduction Systems

We propose the concept of global temperature for spatially non-uniform heat conduction systems. With this novel quantity, we present an extended framework of thermodynamics for the whole system such that the fundamental relation of thermodynamics holds, which we call "global thermodynamics" for heat conduction systems. Associated with this global thermodynamics, we formulate a variational principle for determining thermodynamic properties of the liquid-gas phase coexistence in heat conduction, which corresponds to the natural extension of the Maxwell construction for equilibrium systems. We quantitatively predict that the temperature of the liquid-gas interface deviates from the equilibrium transition temperature. This result indicates that a super-cooled gas stably appears near the interface.

cond-mat.stat-mech↗

arXiv2019

Multiplicative Langevin Equation to Reproduce Long-time Properties of Nonequilibrium Brownian Motion

We statistically examine long time sequences of Brownian motion for a nonequilibrium version of the Rayleigh piston model and confirm that the third cumulant of a long-time displacement for the nonequilibrium Brownian motion linearly increases with the observation time interval. We identify a multiplicative Langevin equation that can reproduce the cumulants of the long-time displacement up to at least the third order, as well as its mean, variance and skewness. The identified Langevin equation involves a velocity-dependent friction coefficient that breaks the time-reversibility and may act as a generator of the directionality. Our method to find the Langevin equation is not specific to the Rayleigh piston model but may be applied to a general time sequence in various fields.

cond-mat.stat-mech↗

arXiv2018

Unattainability of Carnot efficiency in thermal motors: Coarse-graining and entropy production of Feynman-Smoluchowski ratchet

We revisit and analyze the thermodynamic efficiency of the Feynman-Smoluchowski (FS) ratchet, a classical thought experiment describing an autonomous heat-work converter. Starting from the full kinetics of the FS ratchet and deriving the exact forms of the hidden dissipations resulting from coarse-graining, we restate the historical controversy over its thermodynamic efficiency. The existence of hidden entropy productions implies that the standard framework of stochastic thermodynamics applied to the coarse-grained descriptions fails in capturing the dissipative feature of the system. In response to this problem, we explore an extended framework of stochastic thermodynamics to reconstruct the hidden entropy production from the coarse-grained dynamics. The approach serves as a key example of how we can systematically address the problem of thermodynamic efficiency in a multi-variable fluctuating non-equilibrium system.

cond-mat.stat-mech↗

arXiv2017

Liquid-gas transitions in steady heat conduction

We study liquid-gas transitions of heat conduction systems in contact with two heat baths under constant pressure in the linear response regime. On the basis of local equilibrium thermodynamics, we propose an equality with a global temperature, which determines the volume near the equilibrium liquid-gas transition. We find that the formation of the liquid-gas interface is accompanied by a discontinuous change in the volume when increasing the mean temperature of the baths. A super-cooled gas near the interface is observed as a stable steady state.

cond-mat.stat-mech↗

arXiv2016

Numerical examination of steady-state thermodynamics from the entropy connected to the excess heat

We numerically determine the entropy for heat-conducting states, which is connected to the so-called excess heat considered as a basic quantity for steady-state thermodynamics in nonequilibrium. We adopt an efficient method to estimate the entropy from the bare heat current and find that the obtained entropy agrees with the familiar local equilibrium hypothesis well. Our method possesses a wider applicability than local equilibrium and opens a possibility to compare thermodynamic properties of complex systems with those in the local equilibrium. We further investigate the entropy for heat-conducting states and find that it exhibits both extensive and additive properties; however, the two properties do not degenerate each other differently from those at equilibrium. The separation of the extensivity and additivity makes it difficult to apply powerful thermodynamic methods.

cond-mat.stat-mech↗