Casimir force in dense confined electrolytes
Abstract
Understanding the force between charged surfaces immersed in an electrolyte solution is a classic problem in soft matter and liquidstate theory. Recent experiments showed that the force decays exponentially but the characteristic decay length in a concentrated electrolyte is significantly larger than what liquidstate theories predict based on analysing correlation functions in the bulk electrolyte. Inspired by the classical Casimir effect, we consider an additional mechanism for force generation, namely the confinement of density fluctuations in the electrolyte by the walls. We show analytically within the random phase approximation, which assumes the ions to be point charges, that this fluctuationinduced force is attractive and also decays exponentially, albeit with a decay length that is half of the bulk correlation length. These predictions change dramatically when excluded volume effects are accounted for within the mean spherical approximation. At high ion concentrations the Casimir force is found to be exponentially damped oscillatory as a function of the distance between the confining surfaces. Our analysis does not resolve the riddle of the anomalously long screening length observed in experiments, but suggests that the Casimir force due to mode restriction in density fluctuations could be an hitherto underappreciated source of surfacesurface interaction.
 Publication:

Molecular Physics
 Pub Date:
 November 2018
 DOI:
 10.1080/00268976.2018.1478137
 arXiv:
 arXiv:1803.00071
 Bibcode:
 2018MolPh.116.3147L
 Keywords:

 Casimir force;
 density fluctuations;
 concentrated electrolytes;
 correlation length;
 Condensed Matter  Soft Condensed Matter;
 Condensed Matter  Statistical Mechanics;
 Physics  Chemical Physics
 EPrint:
 doi:10.1080/00268976.2018.1478137