| It is well known that critical Casimir forces occur in binary liquid mixtures in the vicinity of a critical point. Here we investigate the influence of additional salt ions on the particle-wall interaction which is determined with total internal reflection microscopy. We observe the onset of strong attractive forces even 6 K below the critical temperature. In addition, the sign of these salt induced forces remains attractive independent of the boundary conditions (symmetric or antisymmetric). Because both findings are in contrast to earlier measurements without salt ions,currently it is not clear whether these observations can be entirely attributed to critical Casimir forces or whether e.g. the coupling between electrostatic interactions and critical fluctuations has to be considered. |
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