Figure 1

Experimental scheme. (a) Crossed cylinder configuration. Droplets of fluid were placed between crossed cylinders of mica and force was applied in the normal direction, causing the formation of a circular area of flattened contact. (b) Fluorescence correlation spectroscopy (FCS) was performed after compressing the films with a mean normal pressure of $2–3\mathrm{M}\mathrm{P}\mathrm{a}$ to $2.5\mathrm{n}\mathrm{m}$ (1,2-propane diol) and 3.0 nm (OMCTS). The sequence of smaller circles illustrates that the focus of the laser beam (diameter $\approx 0.35\mu \mathrm{m}$) was scanned across the contact (radius $\approx 10\mu \mathrm{m}$), enabling spatially resolved measurements.Reuse & Permissions

Figure 2

Fluorescence autocorrelation functions, normalized to unity, are plotted against logarithmic time for rhodamine 123 in 1,2-propane diol (a) and coumarin 153 in OMCTS (b). The focus was at distances $a$ from the center of the contact and the ratio $x\equiv a/r$ was considered, where $r$ is the contact radius. In (a), $x\approx 0.95$ (circles), 0.82 (down triangles), 0.7 (squares), and 0.6 (up triangles). In (b), $x=0.95$ (circles), 0.8 (up triangles), and 0.7 (squares). Lines through the autocorrelation curves are the least-squares fit to a single Fickian diffusion process. The magnitude of the fluorescence autocorrelation function at short times implies that, on average, one sole dye resided within the focus. The averaging time was 45 min. In the inset of (b), fluorescence counts (counts per second) are plotted against time for the same data that led to the slowest autocorrelation curve for the OMCTS system, demonstrating photostability (less than 3% decrease of counts in 2400 sec).Reuse & Permissions

Figure 3

Quantification of the effective diffusion coefficients ($D_{\mathrm{e}\mathrm{f}\mathrm{f}}$) inferred from the raw data in the previous figure. (a) The ratio, $D_{\mathrm{e}\mathrm{f}\mathrm{f}}/D_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}$, is plotted against focus position, $a/r$. (b) The ratio, $D_{\mathrm{e}\mathrm{f}\mathrm{f}}/D_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}$, is plotted against relative pressure squeezing the surfaces together, $P_r=P/P_{\mathrm{m}\mathrm{a}\mathrm{x}}$ ($P_{\mathrm{m}\mathrm{a}\mathrm{x}}\approx 4\mathrm{M}\mathrm{P}\mathrm{a}$), assuming Hertzian pressure distribution. Open circles (main figures) denote rhodamine 123 in 1,2-propane diol ($D_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}\approx 8\mu {\mathrm{m}}^2{\mathrm{s}\mathrm{e}\mathrm{c}}^{-1}$). Filled circles (insets) denote coumarin 153 in OMCTS ($D_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}\approx 190\mu {\mathrm{m}}^2{\mathrm{s}\mathrm{e}\mathrm{c}}^{-1}$). Because photobleaching was prohibitively large, it was not possible to make measurements closer to the center than shown. Because mechanical pressure is believed to be overestimated near the edge of the contact (see discussion in Ref. 10), the apparent slowing down only above a critical pressure is not believed to be significant.Reuse & Permissions