Supercritical Casimir effect in carbon dioxide

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 CO2. 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 CO2 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.

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► First direct surface force measurement in supercritical CO2.
► Demonstration that supercritical ridge plays important role for confined sc. CO2.
► First report on ultra-long range (>100 nm) attractive forces in supercritical CO2.
► First experimental evidence for selective fluctuation depletion in confined sc. CO2.
► First measurement of solute structuring and apparent birefringence in confined solute/supercritical CO2.