I want to know that why density of drilling fluid which is a mixture of water, barrite, bentonite and carboxymethyl cellulose increases when i am adding silica nanoparticles. My ain of adding these nanoparticles is to improve the viscosity.
If your question is why density increases with the addition of nanoparticle then i have to recommend this quora post.
If the attractive forces between the constituent particles of the solid with the water molecules are high enough, the total amount of intermolecular space is reduced and the total volume of the solution can be smaller that the combined volume of its components, leading to an extra increase in density....
Surface of silica nanoparticles have siloxane($Si-O-Si$) and silanol ($Si-OH$) groups. Because of the presence of these functional groups silica nanoparticles tend to form hydrogen bonds between themselves and/or with other polar molecules such as water. Its these interactions which reduces the intermoleculr space which further reduces the volume thus increase in density.
If the silica nanoparticles interect with themselves and form a linked three dimensional networks then it will also increase the viscosity of the solution.