Viscosity of liquids and suspensions

Hi Everyone !

We are so used to the term viscosity that we never try to imagine the phenomena at molecular  level. At molecular level in a liquid at rest continuous rearrangement of molecules takes place within a confined volume due to closed packing,this random motion of molecules is the cause of viscosity ,thus at a molecular level viscosity is a function of the attractive forces of the molecules of the liquid.The fluids which obey linear relationship between stress (momentum flux) and velocity gradient are Newtonian fluids but as the rearrangements of molecules increases with existence of velocity gradient and applied shear the relationship no longer remains linear and Non-Newtonian liquids cones into picture.

 

 Now lets discuss about the viscosity of suspensions, they consists of solid and liquid phase and the viscosity mainly depends on inter particle forces which plays a crucial role specially in case of concentrated suspensions. Einstein was the first one to propose the relation for viscosity of dilute suspensions assuming that the solid phase is in the form of rigid spherical particles. This was done by solving equations for flow around a single sphere ,avoiding inter particle interaction thus making the suspension as single  phase entity.He gave the effective viscosity as a function of volume fraction of solid spheres. Thus dilute suspensions can be treated as Newtonian-fluid but for concentrated suspensions the viscosity depends on velocity gradient which makes them Non-Newtonian in nature.

A lot of research prevails in the field of determinig exact viscosity for concentrated suspension and Stoesian Dynamics is one of the tool to study such rheological properties but for Stokes flow regime (Re << 1).