Fengmin Wu

Fractal analysis of effective thermal conductivity for three-phase (unsaturated) porous media

A fractal analysis of effective thermal conductivity for unsaturated fractal porous media is presented based on the thermal-electrical analogy and statistical self-similarity of porous media. Here, we derive a dimensionless expression of effective thermal conductivity without any empirical constant. The effects of the parameters of fractal porous media on the dimensionless effective thermal conductivity are discussed. From this study, it is shown that, when the thermal conductivity of solid phase and wet phase are greater than that of the gas phase (viz., ks∕kg>1, kw∕kg>1), the dimensionless effective thermal conductivity of unsaturated fractal porous media decreases with decreasing degree of saturation (Sw) and increasing fractal dimension for pore area (Df), fractal dimension for tortuosity (Dt), and porosity (ϕ); when the thermal conductivities of solid phase and wet phase are lower than that of the gas phase (viz., ks∕kg<1, kw∕kg<1), the trends were just opposite. Our model was validated by comparing ...

Electricity Resonance-Induced Fast Transport of Water through Nanochannels

We performed molecular dynamics simulations to study water permeation through a single-walled carbon nanotube with electrical interference. It was found that the water net flux across the nanochannel is greatly affected by the external electrical interference, with the maximal net flux occurred at an electrical interference frequency of 16670 GHz being about nine times as high as the net flux at the low or high frequency range of (<1000 GHz or >80,000 GHz). The above phenomena can be attributed to the breakage of hydrogen bonds as the electrical interference frequency approaches to the inherent resonant frequency of hydrogen bonds. The new mechanism of regulating water flux across nanochannels revealed in this study provides an insight into the water transportation through biological water channels and has tremendous potential in the design of high-flux nanofluidic systems.

Electromanipulating Water Flow in Nanochannels

In sharp contrast to the prevailing view that a stationary charge outside a nanochannel impedes water permeation across the nanochannel, molecular dynamics simulations show that a vibrational charge outside the nanochannel can promote water flux. In the vibrational charge system, a decrease in the distance between the charge and the nanochannel leads to an increase in the water net flux, which is contrary to that of the fixed-charge system. The increase in net water flux is the result of the vibrational charge-induced disruption of hydrogen bonds when the net water flux is strongly affected by the vibrational frequency of the charge. In particular, the net flux is reaches a maximum when the vibrational frequency matches the inherent frequency of hydrogen bond inside the nanochannel. This electromanipulating transport phenomenon provides an important new mechanism of water transport confined in nanochannels.

A vibration-charge-induced unidirectional transport of water molecules in confined nanochannels

We propose a novel nanoscale design for unidirectional transport of water molecules through a single-walled carbon nanotube (SWCNT). This is achieved by using a vibration charge and a composite SWCNT with asymmetrical surface energy. With the proposed system, we demonstrated, using molecular dynamics simulations, that a continuous unidirectional water flow can be driven by a vibration charge without osmotic pressure or a drop in hydrostatic pressure. It is shown that the net flux of continuous unidirectional water flow can be controlled by adjusting the parameters of periodic vibration charge, temperature, and the degree of heterogeneity in surface energy. The remarkable net flux was the combined effect of the kinetic energy provided by the vibration charge, and the water density gradient resulted from the heterogeneous surface energy of the SWCNT. The present nanoscale design can efficiently convert the energy of vibration charges to the transport of water molecules. It may find applications in liquid circulation without a pressure gradient, lab-on-a-chip technology, desalination of sea water, filtration of polluted water, etc.

Optimizing the design of nanostructures for improved thermal conduction within confined spaces

Maintaining constant temperature is of particular importance to the normal operation of electronic devices. Aiming at the question, this paper proposes an optimum design of nanostructures made of high thermal conductive nanomaterials to provide outstanding heat dissipation from the confined interior (possibly nanosized) to the micro-spaces of electronic devices. The design incorporates a carbon nanocone for conducting heat from the interior to the exterior of a miniature electronic device, with the optimum diameter, D0, of the nanocone satisfying the relationship: D02 (x) ∝ x1/2 where x is the position along the length direction of the carbon nanocone. Branched structure made of single-walled carbon nanotubes (CNTs) are shown to be particularly suitable for the purpose. It was found that the total thermal resistance of a branched structure reaches a minimum when the diameter ratio, β* satisfies the relationship: β* = γ-0.25bN-1/k*, where γ is ratio of length, b = 0.3 to approximately 0.4 on the single-walled CNTs, b = 0.6 to approximately 0.8 on the multiwalled CNTs, k* = 2 and N is the bifurcation number (N = 2, 3, 4 ...). The findings of this research provide a blueprint in designing miniaturized electronic devices with outstanding heat dissipation.PACS numbers: 44.10.+i, 44.05.+e, 66.70.-f, 61.48.De

Toward the hydrophobic state transition by the appropriate vibration of substrate

On the basis of molecular-dynamics simulations, we demonstrate the transition from hydrophobic to super-hydrophobic or hydrophilic to hydrophobic by substrate vibration. The wetting degree can be modulated by the substrate vibration period and substrate vibration amplitude. A phase diagram emerging out of the vibration amplitude threshold from the simulations and can offer a quantitative guideline toward a feasible and robust physical approach to applications. Furthermore, we show that the wettability of a graphene surface can be controlled by tuning its surface energy via an external field. Our work demonstrates that vibration is a promising method to apply in practice.

Mesostructural origin of stress-induced magnetic anisotropy in Fe-based nanocrystalline ribbons

The cross-sectional mesostructure of a stress-annealed Fe-based (Fe73.5Cu1Nb3Si13.5B9) ribbon (SAR) has been investigated by atomic force microscopy. The magnetic anisotropy field was measured via the curves of the giant magnetoimpedence effect in SAR. The stress-induced magnetic anisotropy field (ΔHk) showed a linear correlation with the transverse congregating vector Kv of the agglomerated grains. This indicated that the ΔHk originated from the directional congregation of the agglomerated grains formed in SAR. A model for this directional congregation has been established. The previous diverse viewpoints on the mechanism of stress-induced magnetic anisotropy have been unified in this model.

A controllable collapsed/circular nanoactuator based on carbon nanotube

We performed molecular dynamics simulations to investigate the transformation between collapsed and circular cross-sectioned single walled carbon nanotube (SWCNT). It is shown that, by tuning the surface energy of SWCNT via an external field, the shape of a SWCNT can be transformed from the collapsed form to circular cross-sectioned form or vice versa, demonstrating promising applications as actuators and motors in nanomechanical systems. Phase diagrams of the surface energy threshold with varying diameters of the SWCNT and environmental temperature were computed, providing quantitative guidelines for the design of such a nanoactuator.

Migration-collision scheme of Lattice Boltzmann method for heat conduction problems involving solidification

The phase-change problem is solved by the migration-collision scheme of lattice Boltzmann method. After formula derivation, we find that this method can give a rigorous numerical value for the phase-change temperature, which is of crucial importance. One-dimensional solidification in half-space and two-dimensional solidification in a corner are simulated. The phase change temperature and the liquid-solid interface are both obtained, and the results conform to the analytical solution.

Optimizing Permeability in Fractal Tree-like Branched Networks

The overall permeability of composites with self-similar fractal tree-like networks is studied. Under the constraint of total volume, we derived a dimensionless expression of effective permeability and discussed in detail the relationship between the dimensionless effective permeability and the geometrical parameters of the tree-like network including diameter ratio, length ratio, branching number and fractal dimension. From the study, it was shown that, the dimensionless effective permeability of the tree-like network decreases with the increase of bifurcation number (N), branching length ratio ( ^ ) , branching levels (m) or fractal dimensions of channel length (D) when other parameters are kept constant. It was also found that, the dimensionless effective permeability of the tree-like networks reaches maximum when the diameter ratio Ρ satisfies ß , where ^ = 3 , Ν is the bifurcation number N=2, 3, 4, This optimal diameter ratio for maximum effective permeability of the fractal tree-like networks obeys Murrys law, but does not obey the WBE model of plants.