Caused by adhesion the water runs along the glass and does not fall straight.

Adhesion is caused by intermolecular forces between the interface of materials. An example for adhesion is glueing materials together.

In 1950 F. P. Bowden and D. Tabor produced a collection of knowledge on friction and lubrication, where it must be noticed that most results given in the book were obtained by themselves. The book The friction and lubrication of solids has become the standard work on friction and lubrication for a couple of decades. Based upon their knowledge about friction, Bowden and Tabor presented a simple model for friction on a micrometer scale: The Bowden and Tabor adhesion model or plastic junction model. The model assumes that friction is proportional to both the real area of contact and a mean lateral force per unit area, the so-called shear strength.

Since friction is proportional to the real area of contact in this model as is adhesion, the model may be called adhesion model. The energy loss in the friction mechanism is described as plastic deformation of the asperities. Thus, it also may be called plastic junction model. The understanding of friction at the micrometer scale has been reduced to an understanding of two new quantities: shear strength and area of contact.