If a drop is placed on a smooth, horizontal surface, it is generally not in the equilibrium state. Hence, it spreads until an equilibrium contact radius is reached (partial wetting). While taking into account capillary, gravitational, and viscous contributions, the drop radius as a function of time can be expressed as
For the complete wetting situation, the drop radius at any time during the spreading process is given by
where
:*γ is surface tension of the fluid
:*V is drop volume
:*η is viscosity of the fluid
:*ρ is density of the fluid
:*g is gravitational constant
:*λ is shape factor, 37.1m
:*t is experimental delay time
:*r is drop radius in equilibrium