Masao Doi - Academia.edu (original) (raw)
Papers by Masao Doi
The Journal of Chemical Physics, 2022
Gas–liquid transition is generally a complex process, which involves nucleation of droplets and t... more Gas–liquid transition is generally a complex process, which involves nucleation of droplets and their growth by evaporation–condensation or collision–coalescence processes. Here, we focus on a microscopic system in which there is only one liquid droplet at most. In this case, we can develop an equilibrium theory for the formation of the droplet in the gas phase using the classical nucleation theory. We use the van der Waals fluid model with surface tension and calculate the size fluctuation of the droplet for various confinement conditions, NVT (in which the volume V of the system is fixed), NPT (in which the pressure P of the system is fixed), and NBT (in which the system is confined in a nano-bubble immersed in a host liquid, where both V and P can fluctuate). We show that in the NBT system, the size flexibility along with space confinement induces a wealth of properties that are not found in NVT and NPT. It exhibits richer phase behaviors: a stable droplet appears and coexists wi...
Physical Review E, 2021
Analogous to other porous solids, pinned nanobubbles serve as a zero-dimensional stable nanoscale... more Analogous to other porous solids, pinned nanobubbles serve as a zero-dimensional stable nanoscale chamber with controllable thermodynamic parameters, whereas they can respond to state change of guest molecule. Here we analyzed peculiarities of phase transitions in pinned nanobubbles, which were experimentally proved to be superstable. By combining molecular dynamics simulation and thermodynamic analysis, we reveal that guest molecules encapsulated inside a nanobubble exhibit distinct state behaviors: a state in vapor phase, a reversible two-state oscillation, and a stable nanodroplet@nanobubble state, depending on the number of guest molecules and the external pressure. The free-energy landscape shows how state metastability gradually develops with external stimuli and leads to the specific bistable state of two-state oscillation. The existence of strong coupling between nanobubble breathing and two-state oscillation is identified. Our simulation results demonstrate that the flexibility of pinned nanobubbles plays at least the same important role as space confinement in determining the states of guest molecules. Our findings indicate that pinned nanobubbles, serving as soft porous media that possess high stability and reversible transformability, show a wealth of properties that are not found in bulk solutions and in porous solids.
Masao Doi‡ Interdisciplinary Research Center, Beihang University, Beijing 100191, China. Abstract... more Masao Doi‡ Interdisciplinary Research Center, Beihang University, Beijing 100191, China. Abstract A new variational method is proposed to calculate the evolution of liquid film and liquid droplet moving on a solid substrate. An explicit time evolution equation is obtained for the contact angle of a liquid film that starts to move on a horizontal substrate. The equation predicts the dynamical transition at the receding side, and the ridge formation at the advancing side. The same method is applied for the evolution of a droplet that starts to move on an inclined solid surface, and again the characteristic shape change of the droplet is obtained. This demonstrates the potentiality of the present method to wider classes of problems of sliding liquids which involve contact line pinning, surface tension gradients and evaporation etc.
Gels, 2021
Some types of hydro-gels have almost the same equilibrium swelling volume in water and in ethylen... more Some types of hydro-gels have almost the same equilibrium swelling volume in water and in ethylene glycol (EG), a highly viscous liquid completely miscible with water. Experiments showed that when a gel fully swollen with EG is immersed into a large amount of water, it temporarily swells up and then relaxes to the equilibrium volume in water. The temporary swelling is explained by the friction force exerted on the gel network from the outward EG flux In this paper, we experimentally show that the temporary swelling is suppressed by adding linear PEG (polyethylene glycol) in the outer water. Although the suppression seems to be explained by the osmotic pressure (i.e., by the same mechanism as the conventional osmotic squeezing), our theoretical analysis reveals that the effect of PEG is much stronger than that expected from the equilibrium osmotic pressure, implying that the PEG chains are condensed on the gel surface.
Physics of Fluids, 2020
We calculate the shape and the velocity of a bubble rising in an infinitely large and closed Hele... more We calculate the shape and the velocity of a bubble rising in an infinitely large and closed Hele-Shaw cell using Park and Homsy's boundary condition which accounts for the change of the three dimensional structure in the perimeter zone. We first formulate the problem in the form of a variational problem, and discuss the shape change assuming that the bubble takes elliptic shape. We calculate the shape and the velocity of the bubble as a function of the bubble size, gap distance and the inclination angle of the cell. We show that the bubble is flattened as it rises. This result is in agreement with experiments for large Hele-Shaw cells.
Physical Review Letters, 2017
Evaporating droplets are known to show complex motion that has conventionally been explained by t... more Evaporating droplets are known to show complex motion that has conventionally been explained by the Marangoni effect (flow induced by the gradient of surface tension). Here, we show that the droplet motion can be induced even in the absence of the Marangoni effect due to the gradient of evaporation rate. We derive an equation for the velocity of a droplet subject to non-uniform
ACS Macro Letters, 2021
Plasticized poly(vinyl chloride) (PVC) is a jelly-like soft dielectric material that attracted su... more Plasticized poly(vinyl chloride) (PVC) is a jelly-like soft dielectric material that attracted substantial interest recently as a new type of electro-active polymers. Under electric fields of several hundred volt/mm, PVC gels undergo large deformations. These gels can be used as artificial muscles and other soft robotic devices, with striking deformation behavior that is quite different from conventional dielectric elastomers. Here, we present a simple model for the electro-activity of PVC gels and show a nonlinear effect of free ions on its dielectric behaviors. It is found that their particular deformation behavior is due to an electro-wetting effect and to a change in their interfacial tension. In addition, we derive analytical expressions for the surface tension as well as for the apparent dielectric constant of the gel. The theory indicates that the size of the mobile free ions has a crucial role in determining the electro-induced deformation, opening up the way to novel and innovative designs of electro-active gel actuators.
Physics of Fluids, 2020
This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Dynamics of moving contact ... more This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Dynamics of moving contact line on a transversely patterned inclined surface
Macromolecules
When a gel absorbs a solvent from the surrounding, a stress field is created in the gel, and this... more When a gel absorbs a solvent from the surrounding, a stress field is created in the gel, and this causes complex dynamics of the swelling behavior. Here, we study this effect for a diskshaped gel by rigorously solving the diffusio-mechanical coupling equation. We show that (a) while the macroscopic thickness and the radius of the gel increase monotonically in time, the gel is compressed near the midplane, and that (b) while the swelling time depends on the shear modulus G of the gel, its dependence is weak, and the time is mainly determined by the friction constant of the gel network and the osmotic bulk modulus of the gel. We also show that these characteristic features are reproduced accurately by a simple variational calculation for the gel deformation. An analytical expression is given for the swelling time.
Journal of Fluid Mechanics
NIPPON GOMU KYOKAISHI
Rubber industries utilize many kinds of soft matter, polymer, colloids, surfactants etc. They are... more Rubber industries utilize many kinds of soft matter, polymer, colloids, surfactants etc. They are complex materials, but they all obey the law of physics, and their complex behaviors can be understood in terms of physics. In this series of lectures, I will try to explain the principles of physics which governs the complex behavior of these materials. In this first chapter, I will discuss the elasticity of rubbers: where does the unique elasticity of rubber come from, and how to understand the strange behaviors of rubber balloons.
Physical Review Fluids
When a polymer solution is uniaxially stretched and held fixed at both ends, the solution quickly... more When a polymer solution is uniaxially stretched and held fixed at both ends, the solution quickly separates into droplets connected by strings and takes the beads-on-string structure. The string then becomes thinner by capillary forces. Here we develop a theoretical framework on viscoelastic fluids based on Onsager principle, and apply it to the dynamics of viscoelastic filaments. We show that the beads-on-string structure is a thermodynamic quasi-equilibrium state, and derive an equation for the coexistence condition in the pseudo-equilibrium state. Using the condition, we solve the evolution equation analytically and show that the string radius and the tensile stress vary exponentially as predicted by the classical theory of Entov and Hinch [J. Non-Newtonian Fluid Mech. 72, 31 (1997)], but the prefactor for the tensile stress is different from their theory and agrees with the numerical solutions of Clasen et al. [J. Fluid Mech. 556, 283 (2006)].
Soft Matter
The dynamics of liquid imbibition in the bulk and fingers are analyzed accounting for the couplin... more The dynamics of liquid imbibition in the bulk and fingers are analyzed accounting for the coupling of the two parts.
Journal of the Chemical Society, Faraday Transactions 2
Journal of Polymer Science: Polymer Physics Edition
The stress relaxation of concentrated polymer solutions after double-step strain is analyzed by t... more The stress relaxation of concentrated polymer solutions after double-step strain is analyzed by the model of Doi and Edwards (DE model). The theory explains the experiments of Osaki et al., who showed that the Bernstein-Kearsley-Zapas (BKZ) constitutive equation works reasonably well when the successive deformations are applied in the same direction, hut that it fails when the direction of the second deformation is reversed. This experimental result has heen considered contrary to the DE model, since the previous theory of Doi and Edwards provides a BKZ-type constitutive equation. In the present paper the discrepancy is reconciled by showing that (i) the source of the discrepancy is a mathematical approximation used in the previous DE theory, and that (ii) if this approximation is removed, the agreement between the theory and experiment becomes fairly good.
Journal of Polymer Science: Polymer Symposia
The Journal of Chemical Physics, 2022
Gas–liquid transition is generally a complex process, which involves nucleation of droplets and t... more Gas–liquid transition is generally a complex process, which involves nucleation of droplets and their growth by evaporation–condensation or collision–coalescence processes. Here, we focus on a microscopic system in which there is only one liquid droplet at most. In this case, we can develop an equilibrium theory for the formation of the droplet in the gas phase using the classical nucleation theory. We use the van der Waals fluid model with surface tension and calculate the size fluctuation of the droplet for various confinement conditions, NVT (in which the volume V of the system is fixed), NPT (in which the pressure P of the system is fixed), and NBT (in which the system is confined in a nano-bubble immersed in a host liquid, where both V and P can fluctuate). We show that in the NBT system, the size flexibility along with space confinement induces a wealth of properties that are not found in NVT and NPT. It exhibits richer phase behaviors: a stable droplet appears and coexists wi...
Physical Review E, 2021
Analogous to other porous solids, pinned nanobubbles serve as a zero-dimensional stable nanoscale... more Analogous to other porous solids, pinned nanobubbles serve as a zero-dimensional stable nanoscale chamber with controllable thermodynamic parameters, whereas they can respond to state change of guest molecule. Here we analyzed peculiarities of phase transitions in pinned nanobubbles, which were experimentally proved to be superstable. By combining molecular dynamics simulation and thermodynamic analysis, we reveal that guest molecules encapsulated inside a nanobubble exhibit distinct state behaviors: a state in vapor phase, a reversible two-state oscillation, and a stable nanodroplet@nanobubble state, depending on the number of guest molecules and the external pressure. The free-energy landscape shows how state metastability gradually develops with external stimuli and leads to the specific bistable state of two-state oscillation. The existence of strong coupling between nanobubble breathing and two-state oscillation is identified. Our simulation results demonstrate that the flexibility of pinned nanobubbles plays at least the same important role as space confinement in determining the states of guest molecules. Our findings indicate that pinned nanobubbles, serving as soft porous media that possess high stability and reversible transformability, show a wealth of properties that are not found in bulk solutions and in porous solids.
Masao Doi‡ Interdisciplinary Research Center, Beihang University, Beijing 100191, China. Abstract... more Masao Doi‡ Interdisciplinary Research Center, Beihang University, Beijing 100191, China. Abstract A new variational method is proposed to calculate the evolution of liquid film and liquid droplet moving on a solid substrate. An explicit time evolution equation is obtained for the contact angle of a liquid film that starts to move on a horizontal substrate. The equation predicts the dynamical transition at the receding side, and the ridge formation at the advancing side. The same method is applied for the evolution of a droplet that starts to move on an inclined solid surface, and again the characteristic shape change of the droplet is obtained. This demonstrates the potentiality of the present method to wider classes of problems of sliding liquids which involve contact line pinning, surface tension gradients and evaporation etc.
Gels, 2021
Some types of hydro-gels have almost the same equilibrium swelling volume in water and in ethylen... more Some types of hydro-gels have almost the same equilibrium swelling volume in water and in ethylene glycol (EG), a highly viscous liquid completely miscible with water. Experiments showed that when a gel fully swollen with EG is immersed into a large amount of water, it temporarily swells up and then relaxes to the equilibrium volume in water. The temporary swelling is explained by the friction force exerted on the gel network from the outward EG flux In this paper, we experimentally show that the temporary swelling is suppressed by adding linear PEG (polyethylene glycol) in the outer water. Although the suppression seems to be explained by the osmotic pressure (i.e., by the same mechanism as the conventional osmotic squeezing), our theoretical analysis reveals that the effect of PEG is much stronger than that expected from the equilibrium osmotic pressure, implying that the PEG chains are condensed on the gel surface.
Physics of Fluids, 2020
We calculate the shape and the velocity of a bubble rising in an infinitely large and closed Hele... more We calculate the shape and the velocity of a bubble rising in an infinitely large and closed Hele-Shaw cell using Park and Homsy's boundary condition which accounts for the change of the three dimensional structure in the perimeter zone. We first formulate the problem in the form of a variational problem, and discuss the shape change assuming that the bubble takes elliptic shape. We calculate the shape and the velocity of the bubble as a function of the bubble size, gap distance and the inclination angle of the cell. We show that the bubble is flattened as it rises. This result is in agreement with experiments for large Hele-Shaw cells.
Physical Review Letters, 2017
Evaporating droplets are known to show complex motion that has conventionally been explained by t... more Evaporating droplets are known to show complex motion that has conventionally been explained by the Marangoni effect (flow induced by the gradient of surface tension). Here, we show that the droplet motion can be induced even in the absence of the Marangoni effect due to the gradient of evaporation rate. We derive an equation for the velocity of a droplet subject to non-uniform
ACS Macro Letters, 2021
Plasticized poly(vinyl chloride) (PVC) is a jelly-like soft dielectric material that attracted su... more Plasticized poly(vinyl chloride) (PVC) is a jelly-like soft dielectric material that attracted substantial interest recently as a new type of electro-active polymers. Under electric fields of several hundred volt/mm, PVC gels undergo large deformations. These gels can be used as artificial muscles and other soft robotic devices, with striking deformation behavior that is quite different from conventional dielectric elastomers. Here, we present a simple model for the electro-activity of PVC gels and show a nonlinear effect of free ions on its dielectric behaviors. It is found that their particular deformation behavior is due to an electro-wetting effect and to a change in their interfacial tension. In addition, we derive analytical expressions for the surface tension as well as for the apparent dielectric constant of the gel. The theory indicates that the size of the mobile free ions has a crucial role in determining the electro-induced deformation, opening up the way to novel and innovative designs of electro-active gel actuators.
Physics of Fluids, 2020
This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Dynamics of moving contact ... more This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Dynamics of moving contact line on a transversely patterned inclined surface
Macromolecules
When a gel absorbs a solvent from the surrounding, a stress field is created in the gel, and this... more When a gel absorbs a solvent from the surrounding, a stress field is created in the gel, and this causes complex dynamics of the swelling behavior. Here, we study this effect for a diskshaped gel by rigorously solving the diffusio-mechanical coupling equation. We show that (a) while the macroscopic thickness and the radius of the gel increase monotonically in time, the gel is compressed near the midplane, and that (b) while the swelling time depends on the shear modulus G of the gel, its dependence is weak, and the time is mainly determined by the friction constant of the gel network and the osmotic bulk modulus of the gel. We also show that these characteristic features are reproduced accurately by a simple variational calculation for the gel deformation. An analytical expression is given for the swelling time.
Journal of Fluid Mechanics
NIPPON GOMU KYOKAISHI
Rubber industries utilize many kinds of soft matter, polymer, colloids, surfactants etc. They are... more Rubber industries utilize many kinds of soft matter, polymer, colloids, surfactants etc. They are complex materials, but they all obey the law of physics, and their complex behaviors can be understood in terms of physics. In this series of lectures, I will try to explain the principles of physics which governs the complex behavior of these materials. In this first chapter, I will discuss the elasticity of rubbers: where does the unique elasticity of rubber come from, and how to understand the strange behaviors of rubber balloons.
Physical Review Fluids
When a polymer solution is uniaxially stretched and held fixed at both ends, the solution quickly... more When a polymer solution is uniaxially stretched and held fixed at both ends, the solution quickly separates into droplets connected by strings and takes the beads-on-string structure. The string then becomes thinner by capillary forces. Here we develop a theoretical framework on viscoelastic fluids based on Onsager principle, and apply it to the dynamics of viscoelastic filaments. We show that the beads-on-string structure is a thermodynamic quasi-equilibrium state, and derive an equation for the coexistence condition in the pseudo-equilibrium state. Using the condition, we solve the evolution equation analytically and show that the string radius and the tensile stress vary exponentially as predicted by the classical theory of Entov and Hinch [J. Non-Newtonian Fluid Mech. 72, 31 (1997)], but the prefactor for the tensile stress is different from their theory and agrees with the numerical solutions of Clasen et al. [J. Fluid Mech. 556, 283 (2006)].
Soft Matter
The dynamics of liquid imbibition in the bulk and fingers are analyzed accounting for the couplin... more The dynamics of liquid imbibition in the bulk and fingers are analyzed accounting for the coupling of the two parts.
Journal of the Chemical Society, Faraday Transactions 2
Journal of Polymer Science: Polymer Physics Edition
The stress relaxation of concentrated polymer solutions after double-step strain is analyzed by t... more The stress relaxation of concentrated polymer solutions after double-step strain is analyzed by the model of Doi and Edwards (DE model). The theory explains the experiments of Osaki et al., who showed that the Bernstein-Kearsley-Zapas (BKZ) constitutive equation works reasonably well when the successive deformations are applied in the same direction, hut that it fails when the direction of the second deformation is reversed. This experimental result has heen considered contrary to the DE model, since the previous theory of Doi and Edwards provides a BKZ-type constitutive equation. In the present paper the discrepancy is reconciled by showing that (i) the source of the discrepancy is a mathematical approximation used in the previous DE theory, and that (ii) if this approximation is removed, the agreement between the theory and experiment becomes fairly good.
Journal of Polymer Science: Polymer Symposia