...it hits a water-repellant surface. In this case, compare what happens to a drop of water when it hits glass:
to water hitting a fabric that has been treated with a special nano-scale coating that causes the water to bead up:
The thin, spread-out film of water that forms on glass occurs because the water droplets "wet" the surface of the glass (through adhesion). The water molecules do not "stick" to the specially treated fabric and surface tension holds the water droplet together in little spheres. This is sometimes referred to as the Lotus Effect because the micro structures of the lotus leaf also cause water to form tiny spherical droplets instead of adhering to the surface.