Abstract：
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 has 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 nanostructures. The porous layer can 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 μm sized oil droplet by more than 400 times from 210 ± 10 to 0.5 ± 0.3 nN as measured by 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.
摘要：
二氧化硅纳米颗粒或纳米流体的水性悬浮液可以改变表面的润湿性，特别是通过使它们在水下亲水和拒油。纳米流体的润湿性改变具有重要的技术应用,包括强化采油和传热过程。表征润湿性改变的常见方式是通过测量具有和不具有纳米颗粒的油滴的接触角。虽然易于执行，接触角测量不能完全捕获到表面的润湿性变化。这里,我们采用了几种互补的技术，如低温扫描电子显微镜，共聚焦荧光和反射干涉对比显微镜，和液滴探针原子力显微镜（AFM），可视化和量化气相二氧化硅纳米颗粒的润湿性变化。我们发现纳米颗粒吸附到玻璃表面上，形成具有分层微观和纳米结构的多孔层。多孔层可以捕获薄的水膜，这减少了油滴和固体基质之间的接触。因此,即使少量添加纳米颗粒（0.1wt％），也会使20μm大小的油滴的附着力从210±10降低到0.5±0.3nN，降低400倍以上。最后,我们表明，二氧化硅纳米流体可以提高石油采收率8%的微模型与玻璃通道，类似于物理岩石网络。
