Scaling behavior of thin films on chemically heterogeneous walls

We study the adsorption of a fluid in the grand canonical ensemble occurring at a planar heterogeneous wall which is decorated with a chemical stripe of width $L$. We suppose that the material of the stripe strongly preferentially adsorbs the liquid in contrast to the outer material which is only partially wet. This competition leads to the nucleation of a droplet of liquid on the stripe, the height $h_m$ and shape of which (at bulk two-phase coexistence) has been predicted previously using mesoscopic interfacial Hamiltonian theory. We test these predictions using a microscopic Fundamental Measure Density Functional Theory which incorporates short-ranged fluid-fluid and fully long-ranged wall-fluid interactions. Our model functional accurately describes packing effects not captured by the interfacial Hamiltonian but still we show that there is excellent agreement with the predictions $h_m\approx L^{1/2}$ and for the scaled circular shape of the drop even for $L$ as small as 50 molecular diameters. For smaller stripes the droplet height is considerably lower than that predicted by the mesoscopic interfacial theory. Phase transitions for droplet configurations occurring on substrates with multiple stripes are also discussed.

Figure 1

Schematic illustration of the shape of a drop, of height $h_m$, above a composite planar wall containing stripe of material, of finite width $L$ but infinite depth and length, which is completely wet. The outer parts of the substrate only show microscopic adsorption corresponding to partial wetting (or indeed drying).

Figure 2

Equilibrium density profiles as obtained from DFT at a temperature $T=0.95T_c$ and bulk two-phase equilibrium for stripe widths (from top to bottom): $L=20\sigma$, $L=40\sigma$, and $L=80\sigma$.

Figure 3

Dependence of the drop height $h_m$ on the stripe width $L$. Symbols denote the DFT results with $h_m$ determined from the equilibrium density profile using the criterion $\rho (0,h_m)=({\rho }_v+{\rho }_l)/2)$. The straight line fit to the data for $L\ge 40\sigma$ has slope 0.5079.

Figure 4

DFT results for the droplet shape for different stripe widths $L$ in rescaled dimensionless units $\tilde{x}=x/L$ and $\tilde{h}\left(x\right)=h\left(x\right)/h_m$. The dashed red line is the prediction of the effective Hamiltonian model.