Computer Simulations On The Transfer Processes In Nano/Micrometer Scale

Micro-chemical engineering attracts extensive interest for its widelyapplications, and becomes a new research field in chemistry and chemicalengineering. It presents an ability in the precisely control of physical andchemical processes confined in the nano/miro-channels. Understanding of thestructures and properties of fluids and interfaces is a fundamental topic for thedesign and development of micro-chemical engineering devices. The limit ofexperimental methods in characterizing the structures and properties ofliquid-liquid interfaces makes it necessary to carry out molecular simulationsto investigate the transfer processes across the liquid-liquid interface.The molecular dynamic method and the lattice Boltzmann method havebeen used in this dissertation to study the transfer processes, including theboundary conditions, heat transfer and mass transfer across liquid-liquid interfaces, the mass transfer confined in nanochannel, and the generation ofconcentration gradients in mcirochannel networks. The main conclusions aresummarized as follows.1. Molecular dynamics simulations have been carried out on the boundaryconditions, heat transfer and mass transfer at the nanoscale interface betweentwo immiscible liquids.(1) The flow boundary conditions for the interface between twoimmiscible liquids under the condition of low shear rates, in thepresence or absence of surfactants have been studied. Three boundaryconditions at immiscible liquid-liquid interfaces, including slip, no-slip,and locking boundary conditions, have been observed, depending onthe interfacial surfactant concentration. The slip, no-slip and lockingboundary conditions yield the positive, zero and negative slip lengths,respectively. The dependence of boundary slip on shear rate at differentinterfacial surfactant concentrations has also been investigated.(2) The heat transfer at the liquid-liquid interface of two immiscibleliquids in the presence or absence of surfactants has been studied. Achange of heat flux in the system does not affect the structure of theinterface, so that the heat resistance remains unchanged. The presenceof surfactants induces a higher density profile at the interface, whichaffects the process of the heat transfer. The heat resistance at theinterface first decreases quickly to a minimum value with the increase of surfactant concentration, and then mainly keeps unchanged. Theboundary slip occurs at the interface between two immiscible liquidwhen the liquids flow, which results in a fast increase of the heatresistance. As the increase of velocity, the heat resistance increases to amaximum and then decli.(3) The mass transfer of solutes across the liquid-liquid interface of twoimmiscible liquids in the presence or absence of surfactants has beeninvestigated. The evolution process with time for solute concentrationand the mean force potential of solute at the interface demonstrate aresistance for solute transfer across the liquid-liquid interface. Thesimulation results illustrate that the solubility of solute in the solventaffects the rate of mass transfer. The rate of mass transfer depends onthe concentration of surfactants at the interface. With the increase ofsurfactant concentration, the transfer rate increases first and thendecreases, which is opposite to that in the microscopic experiment.2. Molecular dynamics simulations on the process of the water flow throughcarbon nanotubes under the condition of external pressure have beenconducted. The appearance of a gap for the density distribution of solvent inthe region near the inlet demonstrates that there exists a resistance at the inletof nanotube. With the increase of external pressure, four states of watermolecules appear: the state of water out of the carbon nanotube, the state ofwater entering and subfill the nanotube, the state of water fulfilling the nanotube and the state of water flow through the nanotube. Compared with thesystem of a single carbon nanotube, it is easier for water molecules to flowthrough carbon nanotubes in the system with two carbon nanotubes. Theinvestigation on the effect of the LJ potential shows that the system with alonger LJ potential cutoff radius is easier to achieve the state of water flowingthrough the nanotube. All these results illustrate that water molecules indifferent nanotubes affect each others. Finally, the rotation of water moleculein the carbon nanotube has also been investigated.3. The generation of concentration gradients in a microfluidic networks hasbeen simulated by using the Lattice Boltzmann (LB) method. A model ofmicrofluidic networks was developed, and the results obtained by this modelare in agreement with experimental data. The simulation results indicated thatthe relative positions of the branching plates in different levels exertsignificant effects on the shape of concentration gradients. Extensivesimulations have been performed to study the dependence of the shape ofconcentration gradient on the velocity of the flow u, diffusion coefficients ofsolutes D, and the length of the microchannel L. A dimensionlessparameter,L/uis the hydraulic diameter of the mainH2, in which microchannel, is proposed in this work. It is found that for geometricallysimilar microfluidic networks, the parameter alone determines the shape of thegenerated concentration gradient.