Dynamic contact angles

The dynamic contact angles of various liquids on a gelatin‐subbed polyester tape were investigated by plunging a tape into a pool of liquid, in the manner of Perry and of Burley and Kennedy. The effect of the upper fluid was studied by replacing the air normally present by immiscible oils. A fair correlation was found at the point of air entrainment, relating the capillary number, μV/σ, to a physical properties number, gμ4/ρσ3. A dimensional correlation, relating the air entrainment velocity to the viscosity to the −0.67 power, was even better. At various velocities, the dynamic contact angle (or the dynamic contact angle minus the static angle) could be related to the capillary number, the physical properties number, and to density and viscosity ratios. Again, the dimensional correlations were better than the dimensionless ones, perhaps caused by an omission of a significant dimensionless group due to our inability to choose a suitable characteristic length. With any one system, the data could be expressed as θ = kN Cab above some minimum velocity, or, covering all velocities, as (θ − θs) = kN Cab. With air as the upper fluid, all the data at one angle lie within one decade, with capillary numbers at air entrainment in the range of 0.6–1.3.