Terence D. Blake

Experimental evidence of nonlocal hydrodynamic influence on the dynamic contact angle

The dynamic contact angle formed when a liquid curtain impinges onto a moving solid is measured for aqueous glycerol solutions in different flow regimes. It is usually assumed that the dynamic contact angle is simply a function of the contact-line speed and the material properties of the contacting media. The new results show that this is not the case. For a given gas/liquid/solid combination and a given contact-line speed, the dynamic contact angle can be varied by varying the flow rate of the liquid and/or the curtain height, that is by varying the flow field near the contact line. The possibility of attributing this effect merely to free-surface bending and interpreting the results in terms of the so-called “apparent” contact angle is discussed and ruled out on the basis of some general qualitative arguments and analysis of the characteristic length scales involved. A probable connection between the observed effect and the physical mechanism of interface disappearance and formation incorporated in a re...

Wetting: Static and Dynamic Contact Lines

Wetting is basic to coating. Initially air contacts the solid, and during coating the liquid displaces the air from the moving solid surface so that none is visible in the coated film. Thus, coating is a process of dynamic wetting. For uniform coating, the wetting line must remain straight and advance steadily. At sufficiently high speeds, however, the wetting line becomes segmented and unsteady as a thin air film forms between the solid and liquid. The air film disrupts the uniformity of the coated film, and often air bubbles appear in the coating. Dynamic wetting failure limits coating speed.