Tuning Density Profiles and Mobility of Inhomogeneous Fluids

Density profiles are the most common measure of inhomogeneous structure in confined fluids, but their connection to transport coefficients is poorly understood. We explore via simulation how tuning particle-wall interactions to flatten or enhance the particle layering of a model confined fluid impacts its self-diffusivity, viscosity, and entropy. Interestingly, interactions that eliminate particle layering significantly reduce confined fluid mobility, whereas those that enhance layering can have the opposite effect. Excess entropy helps to understand and predict these trends.

Figure 1

(a) Natural and flat density profiles $\rho (z)$, and (b) natural and structured density profiles for a confined WCA fluid with average density ${\rho }_{\mathrm{avg}}=0.6$ and $H=4$, as discussed in the text. (c) The associated particle-boundary interactions ${\varphi }_{\mathrm{ext}}(z)$.

Figure 2

Effect of boundary interaction (shape of density profile) on (a) excess entropy per particle $\Delta s^{\mathrm{ex}}(\lambda )=s^{\mathrm{ex}}(\lambda )-s^{\mathrm{ex}}(0)$ and entropy per particle $\Delta s(\lambda )=s(\lambda )-s(0)$, (b) self-diffusivity $D$, and (c) viscosity $\eta$ for the confined WCA fluid with ${\rho }_{\mathrm{avg}}=0.6$ and $H=4$. The center line corresponds to the fluid with the natural density profile of Figs. 1a, 1b. From center to left, the density profile is systematically flattened: ${\varphi }_0(z)={\lambda }_f{\varphi }_{0,f}(z)$, where ${\lambda }_f=1$ yields the flat profile shown in Fig. 1a. From center to right, the density profile is structured: ${\varphi }_0(z)={\lambda }_s{\varphi }_{0,s}(z)$, where ${\lambda }_s=1$ produces the structured profile shown in Fig. 1b. Symbols are simulation data, and curves are guides to the eye.

Figure 3

(a) Excess entropy per particle $s^{\mathrm{ex}}$, (b) self-diffusivity $D$, and (c) viscosity $\eta$, of the confined WCA fluid versus average density ${\rho }_{\mathrm{avg}}$ at $H=4$. Symbols are simulation data, and curves are guides to the eye.