Fluctuation Forces and Wetting Layers in Colloid-Polymer Mixtures

We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting.

Figure 1

Phase diagram of the colloid-polymer system with size ratio $q=R_g/a=1.04$ represented as a function of the colloid volume fraction $\varphi$ and the polymer concentration $c$ relative to the overlap concentration $c^{*}$. The solid line is the experimental phase boundary between the stable one-fluid and the two-fluid regions and the star marks the approximate location of the critical point. The state points studied are indicated as S1 ($\varphi =0.120$, $c/c^{*}=0.408$), S2 (0.092, 0.314) and S3 (0.082, 0.278). Inset: Averaged roughness $w_0$ (circles) and surface tension $\gamma$ (triangles) of the free interface as a function of colloid volume fraction. Lines are guides to the eye.

Figure 2

Example of a confocal image stack taken at the cell wall and showing the free interface and the vertical wetting layer. Top left: frame taken at the layer’s interface. Top right: frame taken in the bulk of the sample. Bottom left: side view of the stack showing the liquid meniscus and the wetting layer at the cell wall. Bottom right: example of the derivative of the frame-averaged fluorescence intensity (inset) as a function of depth $z$; the solid line is a fit that allows to identify (i) the position of the wall and the interface (vertical dotted lines) (ii) the width at the cell wall ${\sigma }_z$ and the width at the interface $\sigma$.

Figure 3

Measured layer thickness $\ell$ (a) and interface width $w$ (b) as a function of height $h$ from the bulk-gas–liquid interface for sample S1 (triangles), S2 (circles) and S3 (squares). The solid line in (a) is Eq. (7). (c) Interface width as a function of layer thickness. The solid line is $w^2=0.29\mu \mathrm{m}\ell$ and the dashed line is $w^2=0.23\mu \mathrm{m}\ell$, describing the theoretical prediction of Eq. (3).