  We measure the colloidal interaction between two silica microspheres in
aqueous solution in the distance range from $0.2\,\mu$m to $0.5\,\mu$m with the
help of optical tweezers. When employing a sample with a low salt
concentration, the resulting interaction is dominated by the repulsive
double-layer interaction which is fully characterized. The double-layer
interaction is suppressed when adding $0.22\,$M of salt to our sample, thus
leading to a purely attractive Casimir signal. When analyzing the experimental
data for the potential energy and force, we find good agreement with
theoretical results based on the scattering approach. At the distance range
probed experimentally, the interaction arises mainly from the unscreened
transverse magnetic contribution in the zero-frequency limit, with nonzero
Matsubara frequencies providing a negligible contribution. In contrast, such
unscreened contribution is not included by the standard theoretical model of
the Casimir interaction in electrolyte solutions, in which the zero-frequency
term is treated separately as an electrostatic fluctuational effect. As a
consequence, the resulting attraction is too weak in this standard model, by
approximately one order of magnitude, to explain the experimental data.
Overall, our experimental results shed light on the nature of the thermal
zero-frequency contribution and indicate that the Casimir attraction across
polar liquids has a longer range than previously predicted.
