19 research outputs found
Wetting hysteresis induces effective unidirectional water transport through a fluctuating nanochannel
We propose a water pump that actively transports water molecules through
nanochannels. Spatially asymmetric thermal fluctuations imposed on the channel
radius cause unidirectional water flow without osmotic pressure, which can be
attributed to hysteresis in the cyclic transition between the wetting/drying
states. We show that the water transport depends on fluctuations, such as
white, Brownian, and pink noises. Because of the high-frequency components in
white noise, fast switching of open and close states inhibits channel wetting.
Conversely, pink and Brownian noises generate high-pass filtered net flow.
Brownian fluctuation leads to a faster water transport rate, whereas pink noise
has a higher capability to overcome osmotic pressure in the opposite direction.
A trade-off relationship exists between the resonant frequency of the
fluctuation and the flow amplification. The proposed pump can be considered as
an analogy for the reversed Carnot cycle, which is the upper limit on the
energy conversion efficiency
Ionic conductivities of non-equivalent molten salts by molecular dynamics simulations
この論文は国立情報学研究所の電子図書館事業により電子化されました。研究会報告これまで我々は等価溶融塩のイオン伝導度について検討してきたが、本研究は理論面での非等価溶融塩への拡張である。イオン伝導度は速度相関関数および二体ポテンシャルで表され、またLangevin方程式を用いてイオン伝導度の比σ^+/σ^-が|z^+|m^-/|z^-|m^+に等しいことが示された。これは等価溶融塩の場合の逆質量比の関係の非等価溶融塩への拡張となっている。また、溶融CaCl_2及びAlF_3についての理論の計算結果とシミュレーションの良い一致を得た
Large-scale molecular-dynamics simulation of nanoscale hydrophobic interaction and nanobubble formation
We performed large-scale molecular-dynamics simulation of nanoscale hydrophobic interaction manifested by the formation of nanobubble between nanometer-sized hydrophobic clusters at constrained equilibrium. Particular attention is placed on the tendency of formation and stability of nanobubbles in between model nanoassemblies which are composed of hydrophobic clusters (or patches) embedded in a hydrophilic substrate. On the basis of physical behavior of nanobubble formation, we observed a change from short-range molecular hydrophobic interaction to midrange nanoscopic interaction when the length scale of hydrophobe approaches to about 1 nm. We investigated the behavior of nanobubble formation with several different patterns of nonpolar-site distribution on the nanoassemblies but always keeping a constant ratio of nonpolar to polar monomer sites. Dynamical properties of confined water molecules in between nanoassemblies are also calculated
Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface
Water droplets on rugged hydrophobic surfaces typically exhibit one of the following two states: (i) the Wenzel state [Wenzel RN (1936) Ind Eng Chem 28:988–994] in which water droplets are in full contact with the rugged surface (referred as the wetted contact) or (ii) the Cassie state [Cassie, ABD, Baxter S (1944) Trans Faraday Soc 40:546–551] in which water droplets are in contact with peaks of the rugged surface as well as the “air pockets” trapped between surface grooves (the composite contact). Here, we show large-scale molecular dynamics simulation of transition between Wenzel state and Cassie state of water droplets on a periodic nanopillared hydrophobic surface. Physical conditions that can strongly affect the transition include the height of nanopillars, the spacing between pillars, the intrinsic contact angle, and the impinging velocity of water nanodroplet (“raining” simulation). There exists a critical pillar height beyond which water droplets on the pillared surface can be either in the Wenzel state or in the Cassie state, depending on their initial location. The free-energy barrier separating the Wenzel and Cassie state was computed on the basis of a statistical-mechanics method and kinetic raining simulation. The barrier ranges from a few tenths of kBT0 (where kB is the Boltzmann constant, and T0 is the ambient temperature) for a rugged surface at the critical pillar height to ≈8 kBT0 for the surface with pillar height greater than the length scale of water droplets. For a highly rugged surface, the barrier from the Wenzel-to-Cassie state is much higher than from Cassie-to-Wenzel state. Hence, once a droplet is trapped deeply inside the grooves, it would be much harder to relocate on top of high pillars
Molecular Dynamics Simulation of Water Nanodroplet Bounce Back from Flat and Nanopillared Surface
Molecular
dynamics simulations of impinging nanodroplets were performed
to study the bounce-back condition for flat and nanopillared surfaces.
We found that the bounce-back condition can be closely related to
the degree of droplet deformation upon collision with the solid surface.
When the droplets have little or small deformation, the bounce-back
condition solely depends on the hydrophobicity of the surface. On
the other hand, when the droplet deformation is large, the impinging
velocity dependence of the bounce-back condition becomes stronger
due to the increase of the liquid–vapor interfacial area of
colliding droplet, which is proportional to the liquid–vapor
surface energy. The impinging droplet simulations with nanopillared
hydrophobic surfaces were also performed. The contribution of droplet
deformation in this case is relatively small because the surface hydrophobicity
is enhanced due to the existence of pillars. Finally, we find that
the maximum spreading diameter of the impinging droplets exhibits
a consistent trend, in terms of the Weber number dependence, as the
experimental measurements with macrodroplets
Understanding Molecular Motor Walking along a Microtubule: A Themosensitive Asymmetric Brownian Motor Driven by Bubble Formation
The “asymmetric Brownian ratchet
model”, a variation of Feynman’s ratchet and pawl system,
is invoked to understand the kinesin walking behavior along a microtubule.
The model system, consisting of a motor and a rail, can exhibit two
distinct binding states, namely, the random Brownian state and the
asymmetric potential state. When the system is transformed back and
forth between the two states, the motor can be driven to “walk”
in one direction. Previously, we suggested a fundamental mechanism,
that is, bubble formation in a nanosized channel surrounded by hydrophobic
atoms, to explain the transition between the two states. In this study,
we propose a more realistic and viable switching method in our computer
simulation of molecular motor walking. Specifically, we propose a
thermosensitive polymer model with which the transition between the
two states can be controlled by temperature pulses. Based on this
new motor system, the stepping size and stepping time of the motor
can be recorded. Remarkably, the “walking” behavior
observed in the newly proposed model resembles that of the realistic
motor protein. The bubble formation based motor not only can be highly
efficient but also offers new insights into the physical mechanism
of realistic biomolecule motors