  An aqueous suspension of silica nanoparticles or nanofluid can alter the
wettability of surfaces, specifically by making them hydrophilic and
oil-repellent under water. Wettability alteration by nanofluids have important
technological applications, including for enhanced oil recovery and heat
transfer processes. A common way to characterize the wettability alteration is
by measuring the contact angles of an oil droplet with and without
nanoparticles. While easy to perform, contact angle measurements do not fully
capture the wettability changes to the surface. Here, we employed several
complementary techniques, such as cryo-scanning electron microscopy, confocal
fluorescence and reflection interference contrast microscopy and droplet probe
atomic force Microscopy (AFM), to visualize and quantify the wettability
alterations by fumed silica nanoparticles. We found that nanoparticles adsorbed
onto glass surfaces to form a porous layer with hierarchical micro- and
nano-structures. The porous layer is able to trap a thin water film, which
reduces contact between the oil droplet and the solid substrate. As a result,
even a small addition of nanoparticles (0.1 wt%) lowers the adhesion force for
a 20-$\mu$m-sized oil droplet by more than 400 times from 210$\pm$10 nN to
0.5$\pm$0.3 nN as measured using droplet probe AFM. Finally, we show that
silica nanofluids can improve oil recovery rates by 8% in a micromodel with
glass channels that resemble a physical rock network.
