Abstract
An optical technique for tracking single particles has been used to evaluate the particle diameter at which diffusion transitions from molecular behaviour described by the fractional Stokes–Einstein relationship to particle behaviour described by the classical Stokes–Einstein relationship. The results confirm a prior prediction from molecular dynamic simulations that there is a particle size at which transition occurs and show it is inversely dependent on concentration and viscosity but independent of particle density. For concentrations in the range 5 × 10−3 to 5 × 10−6 mg ml−1 and viscosities from 0.8 to 150 mPa s, the transition was found to occur in the diameter range 150–300 nm.
Highlights
It is well known that the motion of molecules and particles in a fluid is different
The values for the diffusion coefficient, DT are shown in figure 1 based on equation (2.2) using measurements for the full range of particle sizes that were tracked, i.e. from 10 to 500 nm diameters, together with the corresponding prediction using the classical Stokes–Einstein relationship in equation
It can be seen that the transition to classical Stokes–Einstein behaviour occurs for particles with a diameter of about 300 nm at a viscosity of 0.8 mPa s, but with a diameter of about 150 nm at a viscosity of 150 mPa s, when the prediction lies on the edge of the error bar for the measurements
Summary
It is well known that the motion of molecules and particles in a fluid is different. The issue investigated in this study was the conditions at which diffusion behaviour transitions from one type of behaviour to the other; in other words, how large a cluster of molecules, in the form of a solute particle, is required to induce Stokes–Einstein-type motion in a simple fluid?. Others have considered the change in diffusive behaviour with particle size in dense fluids, for example Rudyak et al [3], and Ould-Kaddour & Levesque [4], and at extreme temperatures [5,6]. Molecular dynamics simulations have been used to establish that, in a simple fluid, 2017 The Authors.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
