Microscopic analysis of the influence of nanoparticle shape and solid-liquid interfacial layer density on the thermal conductivity of nanofluids: A molecular dynamics study on Cu-H2O nanofluids

Abstract

Nanofluids are known for their excellent heat transfer properties, making the investigation of mechanisms to enhance thermal conductivity highly significant. Although previous studies have examined how thermal conductivity varies with temperature, nanoparticle shape, and volume fraction, detailed microscopic analyses of how nanoparticle shape and the interfacial layer between nanoparticles and the base fluid influence thermal conductivity remain limited. In this study, we focus on Cu-H₂O nanofluids, employing molecular dynamics combined with the non-equilibrium molecular dynamics (NEMD) method to simulate the thermal conductivity under various conditions, including temperature, nanoparticle shape (with different surface area-to-volume ratios, S/V), and volume fraction. We observed that nanoparticles with higher surface area-to-volume (S/V) ratios significantly enhanced the thermal conductivity of nanofluids. For instance, as the S/V ratio increased from 0.31746 to 0.805, the thermal conductivity enhancement rate rose from 11 % to 30 %. Visualization and number density analyses revealed that the aggregation of water molecules around nanoparticles decreased with lower S/V ratios, as indicated by the reduction in peak number density from 0.0749333 to 0.0745075. This indicates that nanoparticle shape influences nanofluid thermal conductivity by altering the density of the solid-liquid interfacial layer. RDF analysis showed that in water-based nanofluids, the Cu-O peak position shifted closer to the nanoparticle surface as the volume fraction increased (e.g., for spherical nanoparticles, the peak shifted from 3.125 Å at 0.5 % to 2.925 Å at 2.5 %), revealing the microscopic mechanism by which increased volume fractions enhance thermal conductivity. Further analysis demonstrated that the S/V ratio of nanoparticles significantly affects the interfacial layer density, as reflected in the Cu-O g(r) peak height, which followed the order: sheet-like (1.21293) > cubic (0.8675) > spherical (0.83449) > cylindrical (0.67553). This sequence corresponds to the S/V ratio and aligns with the observed trend in thermal conductivity. Finally, MSD analysis of dispersion performance indicated that the diffusion coefficients of nanoparticles were ordered as cylindrical (3.833), spherical (4.000), cubic (4.167), and sheet-like (4.525), consistent with their respective S/V ratios. These findings confirm that nanoparticle motion intensity, reflected by diffusion coefficients, is closely related to S/V ratios, further supporting the microscopic mechanism by which S/V ratios influence nanofluid thermal conductivity.This study provides a deeper understanding of the relationship between nanoparticle shape, interfacial density, and thermal conductivity, offering insights into the microscopic mechanisms and guiding future molecular dynamics studies of nanofluids.