Aerodynamically driven wall-bounded drop motion and rivulet formation are two specific cases of gas-liquid flows involving surfaces. These multiphase flows are relevant in many fields, e.g. process engineering, printing, ice accretion, car soiling and exterior water management on vehicles. As the basic physics of these flow phenomena are not entirely understood, further experimental and numerical research is desirable. The present study approaches the topic using generic experiments performed in a wind tunnel. The wind tunnel provides a fully turbulent, two-dimensional channel flow with optical access throughout the test section.

In the first set of experiments single water drops with varying volumes on four different substrates are exposed to the flow in the test section. For a constant gas flow velocity a constant drop motion is observed. The motion is mainly governed by the pressure (drag) induced by the gas flow, by surface tension and the capillary forces associated with the substrate contact angle hysteresis. An appropriate scaling has been found to describe the dimensionless drop velocity (capillary number) in terms of a dimensionless flow attack velocity, taking into account the surface wetting properties. The model allows for the prediction of capillary numbers from the attack velocity and it agrees very well with the experimental observations.

A detailed analysis of slow, constantly in a stick-slip manner, moving drops is used to investigate the critical contact angle for moving contact lines.

The second set of experiments investigates the interaction between drops and grooves of variable width. A model predicting whether a drop is absorbed by the groove or passes over the groove is presented.

The final experiments investigate rivulet formation. Through bore holes in the previously used substrates a constant liquid volume flow is injected. For each substrate different behavioral regimes of the resulting rivulets are mapped and described.