Supercritical Casimir effect in carbon dioxide

► First direct surface force measurement in supercritical CO 2 . ► Demonstration that supercritical ridge plays important role for confined sc. CO 2 . ► First report on ultra-long range (>100 nm) attractive forces in supercritical CO 2 . ► First experimental evidence for selective fluctuation depletion in confined sc. CO 2 . ► First measurement of solute structuring and apparent birefringence in confined solute/supercritical CO 2 . We present the first measurement of surface forces across supercritical carbon dioxide. An important finding is that long-range attractive surface forces are detected not just near the critical point, but along the supercritical extension of the coexistence line, which is commonly discussed as the supercritical ridge. Using a precisely adjustable model slit pore, our experiment can measure the change of thermodynamic potential together with changes in refractive index (mass density) in the confined CO 2 . The model slit pore is realized between two atomically smooth mica surfaces in a specially designed high-pressure surface forces apparatus. Our data suggest that thermal fluctuations of higher density are selectively depleted, which results in observation of a time- and space-averaged density reduction in the confined film. Direct observation of this confinement effect has consequences for theoretical understanding as well as technological applications of CO 2 in porous materials. In the presence of small amounts of an acrylate solute (polyethyleneglycol-dimethacrylate), we found that multiple stable menisci of solute can be created between the surfaces and stretched out several micrometers. These liquid-liquid bridges are observed as an apparent birefringence effect. We demonstrate the critical character of the phenomena and discuss our findings in terms of a (super)critical Casimir effect.