Fluiddynamische Instabilitäten haftender Tropfen unter Gravitation, mechanischer Schwingungsanregung und aerodynamischer Krafteinwirkung

The dynamic wetting of liquid droplets on surfaces under the influence of external forces is of great importance in many technical fields. Many interesting questions about the fluid dynamic instabilities associated with the superposition of forces have not yet been clearly understood. Hence, the fragmentary description of droplet physics requires a detailed analysis of the individual phenomena. The present experimental investigation provides the first contribution towards gaining an understanding of the influence of superposition of airflow, downward force of gravity and surface vibration on the droplet behavior. The objective of this work is to develop models to predict the critical velocity for the droplet detachment.

A test bench is set up and various measurement techniques are used to perform the investigation of adhering droplets on surfaces under superposition. For the analysis of the wetting properties and rheology of the droplet, different liquid solutions of water-glycerin and water-ethanol are compared. Moreover, two differently wetted materials are contrasted and several droplet volumes are measured. For further investigations, the excitation parameters frequency and acceleration, the surface inclination and the airflow velocity are modified.

The obtained results show, that contact forces are compensated when the airflow is superimposed with both the mechanical surface vibration and the downward force of gravity. In the range of the first two eigenfrequencies of the droplet as well as with increasing the excitation acceleration or the inclination of the surface, a decline of the critical velocity can be expected. The droplet detachment process is facilitated by a low surface energy of the material and a small contact angle hysteresis.