  Recent experiments and a series of subsequent theoretical studies suggest the
occurrence of universal underscreening in highly concentrated electrolyte
solutions. We performed a set of systematic Atomic Force Spectroscopy
measurements for aqueous salt solutions in a concentration range from 1 mM to 5
M using chloride salts of various alkali metals as well as mixed concentrated
salt solutions (involving both mono- and divalent cations and anions), that
mimic concentrated brines typically encountered in geological formations.
Experiments were carried out using flat substrates and submicrometer-sized
colloidal probes made of smooth oxidized silicon immersed in salt solutions at
pH values of 6 and 9 and temperatures of 25 {\deg}C and 45 {\deg}C. While
strong repulsive forces were observed for the smallest tip-sample separations,
none of the conditions explored displayed any indication of anomalous long
range electrostatic forces as reported for macroscopic mica surfaces. Instead,
forces are universally dominated by attractive van der Waals interactions at
tip-sample separations of $\approx$2nm and beyond for salt concentrations of 1
M and higher. Complementary calculations based on classical density functional
theory for the primitive model support these experimental observations and
display a consistent decrease in screening length with increasing ion
concentration.
