The effect of diffusivity on the thermal transport of colloidal nanocrystal superlattices and nanofluids

Nanocrystals (NCs) integrated into a matrix can form an NC-based composite. The diffusivity of NCs significantly influences the thermal transport properties of these composites, yet its effects remain inadequately studied. This study investigates typical nanofluids and colloidal NC superlattices found in NC-based composites. By varying the diffusivity of NCs over eight orders of magnitude, we analyzed its impact using the Green-Kubo equilibrium molecular dynamics (GK-EMD) method. Our findings indicate that when the diffusivity exceeds 10 −5 Å 2 /ps, it artificially inflates the calculated thermal conductivity (TC) up to 60,000%, requiring a modification to the heat flux formulation to correct. Conversely, when diffusivity is below 10 −5 Å 2 /ps, it amplifies the fluctuations in the heat flux, leading to greater uncertainty in the results, which can be mitigated using the same heat flux modification. Based on the precise TC results for the two NC-based composites, we demonstrate that the vibrational mismatch between the two fluid layers closest to the NC surface, rather than between the NC surface and the fluid, governs the TC of the nanofluid. Although the diffusivity of NCs in the colloidal NC superlattice increases with temperature, it contributes less than 10% to the total TC at 400 K. Moreover, no evidence was found of collective vibrational modes contributing to thermal transport at low temperatures, contrary to a previous study [Materials Today Physics 22 (2022) 100601]. This discrepancy arises from the differences in heat flux calculation methods.