Moran Wang

Electrospun nanomaterials for ultrasensitive sensors

Increasing demands for ever more sensitive sensors for global environmental monitoring, food inspection and medical diagnostics have led to an upsurge of interests in nanostructured materials such as nanofibers and nanowebs. Electrospinning exhibits the unique ability to produce diverse forms of fibrous assemblies. The remarkable specific surface area and high porosity bring electrospun nanomaterials highly attractive to ultrasensitive sensors and increasing importance in other nanotechnological applications. In this review, we summarize recent progress in developments of the electrospun nanomaterials with applications in some predominant sensing approaches such as acoustic wave, resistive, photoelectric, optical, amperometric, and so on, illustrate with examples how they work, and discuss their intrinsic fundamentals and optimization designs. We are expecting the review to pave the way for developing more sensitive and selective nanosensors.

Electrochemical charge of silica surfaces at high ionic strength in narrow channels

We present a theoretical framework to calculate the electrochemical charge on silica surfaces in contact with high-ionic-strength solutions in narrow channels. Analytical results indicate that the contribution of the adsorbed metal cations to the total surface charge is not negligible when the salinity is larger than 1 mM. The electrical triple-layer model is proved much better than other models for high ionic strength. The charge regulation caused by the double-layer overlap in narrow channels will reduce the surface charge density but increase the zeta potential on silica surfaces.

Non-Fourier heat conductions in nanomaterials

We study the non-Fourier heat conductions in nanomaterials based on the thermomass theory. For the transient heat conduction in a one-dimensional nanomaterial with a low-temperature step at both ends, the temperature response predicted by the present model is consistent with those by the existing theoretical models for small temperature steps. However, if the step is large, the unphysical temperature distribution under zero predicted by the other models, when two low-temperature cooling waves meet, does not appear in the predictions by the present model. The steady-state non-Fourier heat conduction equation derived by the present model has been applied to predict the effective thermal conductivities of nanomaterials. The temperature and size dependences of effective thermal conductivities of nanofilms, nanotubes, and nanowires from the present predictions agree well with the available data from experiments in the literature and our molecular dynamics simulation results, which again proves the validity of ...

Roughness and cavitations effects on electro-osmotic flows in rough microchannels using the lattice Poisson–Boltzmann methods

This paper investigates the effects of roughness and cavitations in microchannels on the electro-osmotic flow behaviors using the Lattice Poisson–Boltzmann methods which combined one lattice evolution method for solving the non-linear Poisson–Boltzmann equation for electric potential distribution with the other lattice evolution method for solving the Navier–Stokes equations for fluid flow. The boundary conditions are correctly treated for consistency between the both. The results show that for the electro-osmotic flows in homogeneously charged rough channels, the flow rate does not vary with the roughness height or the interval space monotonically. The flow rate varies slightly with the roughness height or even increases a little when the roughness is very small, and then decreases when the roughness height is larger than 5% channel width. The flow rate decreases first and then increase with the roughness interval space. An interval space at twice roughness width makes the flow rate minimum. For the heterogeneously charged rough channel, the flow rate increases with the roughness surface potential at a super-linear rate. For the electro-osmotic flows in microchannels with cavitations, the flow rate change little with the cavitations depth when the depth value is very low and decreases sharply when the depth is greater than 3% channel width. The flow rate trends to be a constant when the cavitations are very deep. The flow rate decreases with the cavitations width but increases with the cavitations interval.

Three-dimensional effect on the effective thermal conductivity of porous media

A three-dimensional mesoscopic method is developed for predicting the effective thermal conductivity of multiphase random porous media. The energy transport equations are solved by a lattice Boltzmann method for multiphase conjugate heat transfer through a porous structure whose morphology is characterized by a random generation-growth algorithm. Our numerical results show that the cell number in the third dimension influences the resulting effective thermal conductivity of three-dimensional porous media. The predicted effective thermal conductivity varies with the cell number in the third dimension following an exponential relationship, and it requires in the examples at least 10 cells along the third dimension before the predictions stabilize. Comparisons with the experimental data show that the effective thermal conductivities measured by the hot-probe and hot-wire techniques agree well with the predicted results by the two-dimensional model, whereas those measured by the transient comparative method agree more with the three-dimensional predictions.

Modeling electrokinetic flows in microchannels using coupled lattice Boltzmann methods

We present a numerical framework to solve the dynamic model for electrokinetic flows in microchannels using coupled lattice Boltzmann methods. The governing equation for each transport process is solved by a lattice Boltzmann model and the entire process is simulated through an iteration procedure. After validation, the present method is used to study the applicability of the Poisson-Boltzmann model for electrokinetic flows in microchannels. Our results show that for homogeneously charged long channels, the Poisson-Boltzmann model is applicable for a wide range of electric double layer thickness. For the electric potential distribution, the Poisson-Boltzmann model can provide good predictions until the electric double layers fully overlap, meaning that the thickness of the double layer equals the channel width. For the electroosmotic velocity, the Poisson-Boltzmann model is valid even when the thickness of the double layer is 10 times of the channel width. For heterogeneously charged microchannels, a higher zeta potential and an enhanced velocity field may cause the Poisson-Boltzmann model to fail to provide accurate predictions. The ionic diffusion coefficients have little effect on the steady flows for either homogeneously or heterogeneously charged channels. However the ionic valence of solvent has remarkable influences on both the electric potential distribution and the flow velocity even in homogeneously charged microchannels. Both theoretical analyses and numerical results indicate that the valence and the concentration of the counter-ions dominate the Debye length, the electrical potential distribution, and the ions transport. The present results may improve the understanding of the electrokinetic transport characteristics in microchannels.

Electrokinetic Transport in Microchannels with Random Roughness

We present a numerical framework to model the electrokinetic transport in microchannels with random roughness. The three-dimensional microstructure of the rough channel is generated by a random generation-growth method with three statistical parameters to control the number density, the total volume fraction, and the anisotropy characteristics of roughness elements. The governing equations for the electrokinetic transport are solved by a high-efficiency lattice Poisson-Boltzmann method in complex geometries. The effects from the geometric characteristics of roughness on the electrokinetic transport in microchannels are therefore modeled and analyzed. For a given total roughness volume fraction, a higher number density leads to a lower fluctuation because of the random factors. The electroosmotic flow rate increases with the roughness number density nearly logarithmically for a given volume fraction of roughness but decreases with the volume fraction for a given roughness number density. When both the volume fraction and the number density of roughness are given, the electroosmotic flow rate is enhanced by the increase of the characteristic length along the external electric field direction but is reduced by that in the direction across the channel. For a given microstructure of the rough microchannel, the electroosmotic flow rate decreases with the Debye length. It is found that the shape resistance of roughness is responsible for the flow rate reduction in the rough channel compared to the smooth channel even for very thin double layers, and hence plays an important role in microchannel electroosmotic flows.

LATTICE BOLTZMANN SIMULATIONS OF MIXING ENHANCEMENT BY THE ELECTRO-OSMOTIC FLOW IN MICROCHANNELS

The Lattice Boltzmann methods are used to study the mixing enhancements by the electro-osmotic flow in microchannel. Three sets of lattice evolution methods are performed for the fluid flow, for the electrical potential distribution, and for the concentration propagation. The simulation results show that the electro-osmotic flow induces y-directional velocity which enhances the mixing in microchannels. The mixing enhancement is related with the surface zeta potential arrangement and the external electric field strength.

Elastic property of multiphase composites with random microstructures

We propose a computational method with no ad hoc empirical parameters to determine the elastic properties of multiphase composites of complex geometries by numerically solving the stress-strain relationships in heterogeneous materials. First the random microstructure of the multiphase composites is reproduced in our model by the random generation-growth method. Then a high-efficiency lattice Boltzmann method is employed to solve the governing equation on the multiphase microstructures. After validated against a few standard solutions for simple geometries, the present method is used to predict the effective elastic properties of real multiphase composites. The comparisons between the predictions and the existing experimental data have shown that the effects of pores/voids in composites are not negligible despite their seemingly tiny amounts. Ignorance of such effects will lead to over-predictions of the effective elastic properties compared with the experimental measurements. When the pores are taken into account and treated as a separate phase, the predicted Youngs modulus, shear modulus and Poissons ratio agree well with the available experimental data. The present method provides an alternative tool for analysis, design and optimization of multiphase composite materials.

Irreversibility of Heat Conduction in Complex Multiphase Systems and Its Application to the Effective Thermal Conductivity of Porous Media

The irreversibility of heat conduction in porous media, its relation to effective thermal conductivities (ETCs), and the optimization of thermal conduction process are investigated in this work based on the concept of entransy dissipation. Two more new concepts of reference entransy dissipation and nondimensional entransy dissipation are first introduced. Then it is showed that the nondimensional entransy dissipation rate (NER) can be employed as an objective function to evaluate the efficiency of a thermal transfer process in a porous material. By using this criterion and a newly developed structure growth algorithms, different porous structures were generated and the corresponding values of both ETC and NER were derived to illustrate the usefulness and power of using NER to assess the thermal performance of the materials. The results show that the effective thermal conductivity not only influences the heat transfer ability of porous media, but also reflects the irreversibility of heat conduction in porous media, which is a dissipation coefficient for heat transfer. Meanwhile, decreasing the structural particle size will increase the contact points, i.e. more heat bridges, decrease the temperature gradient nearby the contact points, and hence significantly increase the effective thermal coefficient of porous media. Essentially, decreasing the particle size will result in a more uniform distribution of both temperature gradient and local entransy dissipation rate along the heat flow direction, and consequently lead to a larger effective thermal conductivity.