In a 3D printing method, a first layer of a build material is applied. A part layer is patterned by selectively applying a penetrating liquid functional material (PLFM) on at least a portion of the first layer. The PLFM includes (in amounts by weight based on total wt % of the PLFM): from about 5%-15% of a first metal oxide nanoparticle having a particle size ranging from about 0.5 nm up to 10 nm, from about 0.25%-10% of a second metal oxide nanoparticle having at least one dimension greater than 10 nm, from about 1%-10% of an electromagnetic radiation absorber, from about 5%-50% of an organic solvent, a surfactant, and a balance of water. The first layer having the PLFM applied thereon is exposed to electromagnetic radiation, whereby the portion of the first layer at least partially fuses to form the part layer.