FIGURE 3.

Measurement of the contact angle. (a) An example of an image of a droplet on a glass slide. (b) The outline extracted from image a by digital image processing. The coordinates of the contacts with the surface (A and B) and the vertex (C) could be determined using a computer. (c) A schematic drawing of the side view of a droplet. The contact angle θ can be calculated from the coordinates of points A, B, and C under the assumption that the droplet is spherical. Here, A and B are the contacts with the surface, C is the vertex, D is the foot dropped perpendicularly from point C to line AB, O is the center of the circular arc ACB, r is the radius of arc ACB, a is the length of line AD, and b is the length of line CD. Contact angle θ can be obtained by solving the following simultaneous equations:

Here, (x_A, y_A), (x_B, y_B), and (x_C, y_C) are the coordinates of points A, B, and C.