Thanks@Hossein Soleimani. Please provide full text access to your research article.

For O/W method you need to dissolve the polymer/drug in organic solvent and then add it to excess volume of Water/aqueous phase under high speed stirring or homogenization (with typically a surfactant added in the water phase). This will lead to precipitation of the nanoparticles. On centrifugation, the precipitate should settle to the bottom of the centrifuge tube which can be washed and resuspended.

Thanks @Arun Iyer. But precipate is formed in the form of an interfacial layer not at the bottom of the centrifuge tube. Are the nanoparticles present just above interfacial layer or at the bottom of the centrifuge tube?