The stability of drops forced by both wind and gravity is a fluid mechanics problem relevant to heat exchangers, fuel cells, and aircraft icing. To investigate this phenomenon, drops from 15 micro-liters to 400 micro-liters were placed on the rough aluminum (RA = 3.26 micrometers) floor of a tiltable wind tunnel and brought to critical conditions, when the drop begins to run downstream. Various combinations of drop size, inclination angle, and flow speed were employed. A measurement technique capable of measuring full 3D drop profiles was implemented to investigate the drops' evolution toward runback. The measurement requires the comparison of the speckle pattern captured by an overhead drop image with a corresponding image of the dry surface. Stability limits for 235 drops are measured as functions of drop volume and surface inclination. Drops experiencing airflow alone are found to shed at a Weber number of 8.0 +/- 0.5. From measurement sequences of reconstructed drop profiles, the evolution of contact lines, drop profiles, and contact angle distributions are detailed. Contact line integral adhesion forces are calculated from contact angle distributions and related to the forcing air velocity. Drops whose stability limits are dominated by gravity are found to exhibit significantly different evolution toward runback than those dominated by airflow.
ETD Chair
White, Edward Professor and Associate Department Head
