Dongxing Song

Modeling of anisotropic flow and thermodynamic properties of magnetic nanofluids induced by external magnetic field with varied imposing directions

Magnetic field can enhance both thermal conductivity and Lorentz force resistance of the magnetic nanofluids (MNFs), in which the former is favored while the latter often leads to pressure drop of the flow. It is assumed that there would exist a balance between the magnetic field induced thermal conductivity and Lorentz force if one can appropriately adjust the angle of the imposing magnetic field with respect to the direction of the flow. In the present study, the effects of direction of magnetic field ( α) on anisotropic thermodynamic properties of magnetic nanofluids in channel were studied. The effects of direction of magnetic field on thermal conductivity, Nusselt number, global total entropy generation, and other parameters, such as velocity, temperature, and concentration, have been discussed in detail. A greater α can lead to a larger thermal conductivity normal to the walls of channel and a more uniform temperature field. However, the velocity of magnetic nanofluid tends to decrease. There is a t...

The effects of nanoparticle aggregation on the convection heat transfer investigated by a combined NDDM and DPM method

ABSTRACT In this study, convective heat transfer of Al2O3-water nanofluids (NFs) of various concentrations under laminar flow in a horizontal circular tube with and without considering particle aggregation was numerically investigated by a combined nanoparticle diameter distribution model (NDDM) with the Discrete Phase Model (DPM). Heat transfer coefficient (h), temperature distribution in axial and radial direction, and pressure drop (ΔP) were studied. It turns out that beside a slight pressure drop, heat transfer near the wall will significantly deteriorate when particle aggregation occurs. With an increase in Re numbers, the effects of particle aggregation on the heat transfer performance of the fluid will decrease.

Insight into the localized surface plasmon resonance property of core-satellite nanostructures: Theoretical prediction and experimental validation

Regulation of the localized surface plasmon resonance (LSPR) of nanoparticles by changing the dielectric constant of the surrounding medium has been exploited in many practical applications. In this study, using Ag-nanodot-decorated SiO2 nanoparticles (Ag-decorated SiO2NPs) with different solvents, we investigated the potential of using such core-satellite nanostructures as a liquid sensor for the determination of melamine. The dielectric constant effect of the surrounding medium on the LSPR property was given particular attention. It was found that colloids with water as solvent display a LSPR shift of 14nm, and this value was 18nm for ethanol. For colloids with methanol and glycol as solvents, the peak shifts are negligible. Finite-difference time-domain (FDTD) simulations were used to assign the LSPR peaks of Ag-decorated SiO2NPs and to monitor the effect of the substrate and solvent on the LSPR properties. In the calculations, the wavelength positions of the LSPR peaks for Ag-decorated SiO2NPs in various solvents were successfully predicted in the order methanol<water<ethanol<glycol, as also verified by experiments. The separation distance of Ag nanodots and their relative positions on the SiO2 substrate with respect to the incident light were also found to be crucial to the characteristic LSPR peak positions. The LSPR peak undergoes a shift in the presence of different concentrations of melamine. We proposed a multi-mode absorption model to describe the effect of melamine absorption on the LSPR peak shifts of Ag-decorated SiO2NPs. Based on this model, we were able to quantitatively explain the LSPR peak shift of Ag-decorated SiO2NPs in the presence of various concentrations of melamine. Our work is expected to be valuable for theoretical guidance in design of new materials and devices based on LSPR effects.

Sedimentation of particles and aggregates in colloids considering both streaming and seepage

Sedimentation of colloids is a common phenomenon in various industrial processes. Aggregation of nanoparticles is expected to occur during the processes. However, previous studies often ignore the important features of aggregates, e.g. porosity and possible seepage, leading to a mathematical description of the sedimentation processes of low reliability. In this study, we successfully elaborated the partial differential equation of the dynamic concentration distribution of regimented colloids based on the Stokes approximation and diffusion along the negative gradient of concentration. The permeability of aggregates was obtained by the finite volume method and the ratios of the velocities of flowing around (u f) to seepage through (u s) aggregates over various primary particle sizes and aggregation structures were obtained based on the Darcy equations. After validation of the model, the effects of size and density of the particles and aggregates on the concentration profiles were investigated. Our results indicate that both an increase in size and density of particles and aggregates can accelerate the sedimentation process, and lead to more thorough sedimentation. We mathematically explain why suspensions with high particle concentration are more unstable. What is more, replacing gravity with other volume forces, e.g. centrifugal force and magnetic forces, our model is expected to be applicable to centrifugation or magnetic sedimentation processes.

Controlled formation of intense hot spots in Pd@Ag core-shell nanooctapods for efficient photothermal conversion

Plasmonic Ag nanostructures have been of great interest for such applications in cancer therapy and catalysis, etc. However, the relatively week Ag-Ag interaction and spontaneous atom diffusion make it very difficult to generate concaved or branched structures in Ag nanocrystals with sizes less than 100u2009nm, which has been considered very favorable for plasmonic effects. Herein, by employing a cubic Pd seed and a specific reducing agent to restrict the surface diffusion of Ag atoms, Pd@Ag core-shell nanooctapod structures where Ag atoms can be selectively deposited onto the corner sites of the Pd cubes were obtained. Such selective decoration enables us to precisely control the locations for the hot spot formation during light irradiation. We find that the branched nanooctapod structure shows strong absorption in the visible-light region and generates intense hot spots around the octapod arms of Ag. As such, the photothermal conversion efficiency could be significantly improved by more than 50% with a coll...