Structure Of Nanoparticle Aggregates Research Articles

Factors that influence the aggregation structure of nanoparticles in nanofluids: A molecular dynamics simulation

Nanofluids have received considerable attention from researchers due to their excellent heat transfer properties compared with traditional heat transfer media. The aggregation structure of nanoparticle agglomerates is one of the important mechanisms for the enhanced thermal conductivity of nanofluids. In this study, molecular dynamics was used to study the factors that influenced the aggregation structure of nanoparticle agglomerates in Cu–Ar nanofluids. The aggregation behavior of two and four nanoparticle agglomerates at different particle sizes and initial distances was simulated. The results show that when the initial distance of nanoparticle agglomerates is 4 Å, the increase of particle size does not change the aggregation structure to reaction-limited aggregation. When the initial distance is 3 Å, the nanoparticle agglomerates form a reaction-limited aggregation structure after the particle size increases beyond a threshold value (two nanoparticle agglomerates with the same particle size: 10 Å; two nanoparticle agglomerates with different particle sizes: 9 Å; four nanoparticle agglomerates with the same particle size: 5 Å). This indicates that there is a threshold value for the initial distance of nanoparticle agglomerates. When the initial distance is larger than the threshold value, the nanoparticle agglomerates cannot break the repulsive barrier and form reaction-limited aggregates, no matter how the particle size increases. When the initial distance is less than the threshold value, the particle size becomes a key factor in determining the aggregation structure.

Investigation on the aggregation structure of nanoparticle on the thermal conductivity of nanofluids by molecular dynamic simulations

In this paper, equilibrium molecular dynamic simulations were used to calculate the thermal conductivity of Cu-Ar nanofluid with various initial aggregation structures at different volume fractions. The microscopic mechanism of the effect of nanoparticle agglomerate aggregation mode on the thermal conductivity of nanofluids was analyzed. Results showed that the particle size is the key factor to determine the aggregation structure of nanoparticles. The enhancement efficiency of diffusion-limited aggregation on the thermal conductivity of nanofluids was found to be greater than that of reaction-limited aggregation, and the effect of different aggregation structures on the diffusion coefficient of nanofluidic systems may be one of the potential mechanisms affecting the thermal conductivity of nanofluids.

Nanoparticle Aggregation in Ionic Solutions and Its Effect on Nanoparticle Translocation Across the Cell Membrane

Nanoparticle (NP) aggregation can not only change the unique properties of NPs but also affect NP transport and membrane penetration behavior in biological systems. Coarse-grained (CG) molecular dynamics (MD) simulations were performed in this work to investigate the aggregation behavior of NPs with different properties in ionic solutions under different temperature conditions. Four types of NPs and NP aggregates were modeled to analyze the effects of NP aggregation on NP translocation across the cell membrane at different temperatures. Hydrophilic modification and surface charge modification inhibited NP aggregation, whereas stronger hydrophobicity and higher temperature resulted in a higher degree of NP aggregation and a denser structure of NP aggregates. The final aggregation percentage of hydrophobic NPs in the NaCl solution at 37 °C is 87.5%, while that of hydrophilic NPs is 0%, and the time required for hydrophobic NPs to reach 85% aggregation percentage at 42 °C is 6 ns, while it is 9.2 ns at 25 °C. The counterions in the solution weakened the effect of surface charge modification, thereby realizing good dispersity. High temperature could promote the NP membrane penetration for the same NP, while it also could enhance the NP aggregation which would increase the difficulty in NP translocation across cell membrane, especially for the hydrophobic NPs. Therefore, suitable surface modification of NPs and temperature control should be comprehensively considered in promoting NP membrane penetration in biomedical applications.

Microwave extinction characteristics of nanoparticle aggregates

Structure of nanoparticle aggregates plays an important role in microwave extinction capacity. The diffusion-limited aggregation model (DLA) for fractal growth is utilized to explore the possible structures of nanoparticle aggregates by computer simulation. Based on the discrete dipole approximation (DDA) method, the microwave extinction performance by different nano-carborundum aggregates is numerically analyzed. The effects of the particle quantity, original diameter, fractal structure, as well as orientation on microwave extinction are investigated, and also the extinction characteristics of aggregates are compared with the spherical nanoparticle in the same volume. Numerical results give out that proper aggregation of nanoparticle is beneficial to microwave extinction capacity, and the microwave extinction cross section by aggregated granules is better than that of the spherical solid one in the same volume.

Stability of nanofluids in quiescent and shear flow fields

An experimental study was conducted to investigate the structural stability of ethylene glycol-based titanium dioxide nanoparticle suspensions (nanofluids) prepared by two-step method. The effects of particle concentration, fluid temperature, shear rate and shear duration were examined. Particle size and thermal conductivity measurements in quiescent state indicated the existence of aggregates and that they were stable in temperatures up to 60°C. Shear stability tests suggested that the structure of nanoparticle aggregates was stable in a shear interval of 500-3000 s-1 measured over a temperature range of 20-60°C. These findings show directions to resolve controversies surrounding the underlying mechanisms of thermal conduction and convective heat transfer of nanofluids.

Theoretical Framework for Nanoparticle Reactivity as a Function of Aggregation State

Theory is developed that relates the reactivity of nanoparticles to the structure of aggregates they may form in suspensions. This theory is applied to consider the case of reactive oxygen species (ROS) generation by photosensitization of C(60) fullerenes. Variations in aggregate structure and size appear to account for an apparent paradox in ROS generation as calculated using values for the photochemical kinetics of fullerene (C(60)) and its hydroxylated derivative, fullerol (C(60)(OH)(22-24)) and assuming that structure varies between compact and fractal objects. A region of aggregation-suppressed ROS production is identified where interactions between the particles in compact aggregates dominate the singlet oxygen production. Intrinsic kinetic properties dominate when aggregates are small and/or are characterized by low fractal dimensions. Pseudoglobal sensitivity analysis of model input variables verifies that fractal dimension, and by extension aggregation state, is the most sensitive model parameter when kinetics are well-known. This theoretical framework qualitatively predicts ROS production by fullerol suspensions 2 orders of magnitude higher compared with aggregates of largely undifferentiated C(60) despite nearly an order of magnitude higher quantum yield for the undifferentiated C(60) based on measurements for single molecules. Similar to C(60), other primary nanoparticles will exist as aggregates in many environmental and laboratory suspensions. This work provides a theoretical basis for understanding how the structure of nanoparticle aggregates may affect their reactivity.