Measurements of Contact Angle Variations and Resultant Adhesion Force Acting on a Pining Water Droplet Exposed to a Boundary Layer Airflow over a Flat Plate*

An experimental investigation was conducted to examine the variation characteristics of the contact angles along the entire contact line for a pining droplet exposed to a boundary airflow over a flat surface. A novel digital image project (DIP) technique was developed to achieve non-intrusive measurements of the three-dimensional (3D) shape of the pining droplet under different test conditions (i.e., by varying the droplet volume and incoming airflow speed). While the variations of the contact angles along the entire droplet contact line (i.e., as a function of the azimuthal angle on test plate) were determined quantitatively based on the DIP measurements, the adhesion force acting on pining droplet was estimated by integrating the surface tension forces along the droplet contact line on the test plate. The DIP measurement results were also used to validate/verify four different theoretic/empirical models available in literature in predicting the contact angle variations and the resultant adhesion forces acting on the pining droplets. This work provided here is believed to the first of its kind to achieve non-invasive measurements of the 3D shapes of wind-driven water droplets, thereby, quantifying the variations of the contact angles along the entire droplet contact lines, which is very essential in determining the adhesion forces acting on the wind-driven droplets over solid surfaces.