(208f) Contact Line Instability in Receding Dynamic Contact Angles

Correlation of the dynamic contact angles as a function of the capillary number in liquid-liquid-solid systems is important in a number of applications. For instance, it is one of the ways used to predict water penetration into a porous medium. The value of the contact angle changes when the plate is moved in or out of the pool of liquid and this angle is called the dynamic contact angle. At large plate velocities air is entrained into the liquid or the liquid is entrained which forms the simplest version of coating flows. There have been attempts to relate the dynamic contact angles to the plate velocities. The Cahn-Thermo device is used to measure the excess force needed to plunge a plate vertically into a liquid-liquid interface or pull it out. This force is used to calculate the dynamic contact angle. We have used polydimethylsiloxane (PDMS) with different viscosities for the upper phase and water for the lower phase. An algebraic expression that predicts the dynamic contact angles has been derived using the concept that the total viscous dissipation is equal to the surface work in the contact line region.  This theory, together with another hydrodynamic theory, was compared to the experiments on the advancing contact angles.  The advancing contact lines agree with the theory, and one adjustable parameter that is recovered is found to depend inversely on the viscosity ratio.  The receding contact angles showed significant scatter.  A closer photographic examination showed the system to be unstable.