Why do raindrops falling on a windowpane not always run down straight?

A water drop on a vertical windowpane is pulled down by gravity and should run down straight if there is no other force. Adhesive forces between the glass and water keep a small drop stuck to the glass surface. There is also the surface tension acting at the free surface between the drop and air. Due to surface tension the free surface behaves like a stretched membrane and this gives shape to the drop. Similar surface energies are present between the windowpane and water and also between windowpane and air. These three surface energies fix the contact angle of the drop surface at the contact line. Contact angle, surface tension and gravity decide the shape of the drop. As a drop rolls down there is a hairpin shaped contact line on the windowpane. Most of the water is concentrated at the head with a tail of thin film between the two boundaries of the contact line. When the drop rolls down further there is an increase in the surface area. Creating more surface area needs additional work. If there is already a water-film or drop, this additional work is not needed and water takes this path of least resistance. If such a film downstream does not exist, the complex shape of the drop's head affects the direction of progress. This direction need not always be straight. Surface waviness, dust particles acting as barriers, impurities present, which locally change the contact angle and surface tension, thus modify the direction in which the water stream progresses. Path of raindrops on a windowpane is also affected by the wind.

Source: The Hindu