  Experimental measurements of interactions in ionic liquids and concentrated
electrolytes over the past decade or so have revealed simultaneous monotonic
and oscillatory decay modes. These observations have been hard to interpret
using classical theories, which typically allow for just one electrostatic
decay mode in electrolytes. Meanwhile, substantial progress in the theoretical
description of dielectric response and ion correlations in electrolytes has
illuminated the deep connection between density and charge correlations and the
multiplicity of decay modes characterising a liquid electrolyte. The challenge
in front of us is to build connections between the theoretical expressions for
pair correlation functions and the directly measured free energy of interaction
between macroscopic surfaces in experiments. Towards this aim, we here present
measurements and analysis of the interactions between macroscopic bodies across
a fluid mixture of two ionic liquids of widely diverging ionic size. The
measured oscillatory interaction forces in the liquid mixtures are
significantly more complex than for either of the pure ionic liquids, but can
be fitted to a superposition of two oscillatory and one monotonic mode with
parameters matching those of the pure liquids. We discuss this empirical
finding, which hints at a kind of wave mechanics for interactions in liquid
matter.
