drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. … Drops may also be formed by the condensation of a vapor or by atomization of a larger mass of solid.


Surface tension

Drop of water bouncing on a water surface subject to vibrations
The pendant drop test illustrated.

Liquid forms drops because the liquid exhibits surface tension

A simple way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter. The surface tension of the liquid causes the liquid to hang from the tube, forming a pendant. When the drop exceeds a certain size it is no longer stable and detaches itself. The falling liquid is also a drop held together by surface tension.

Viscosity and pitch drop experiments

Some substances that appear to be solid, can be shown to instead be extremely viscious liquids, because they form drops and display droplet behavior. In the famous pitch drop experimentspitch – a substance somewhat like solid bitumen – is shown to be a liquid in this way. Pitch in a funnel slowly forms droplets, each droplet taking about 10 years to form and break off.

Pendant drop test

In the pendant drop test, a drop of liquid is suspended from the end of a tube or by any surface by surface tension. The force due to surface tension is proportional to the length of the boundary between the liquid and the tube, with the proportionality constant usually denoted {\displaystyle \gamma }. Since the length of this boundary is the circumference of the tube, the force due to surface tension is given by

{\displaystyle \,F_{\gamma }=\pi d\gamma }

where d is the tube diameter.


Drop adhesion to a solid

The drop adhesion to a solid can be divided into two categories: lateral adhesion and normal adhesion. Lateral adhesion resembles friction (though tribologically lateral adhesion is a more accurate term) and refers to the force required to slide a drop on the surface, namely the force to detach the drop from its position on the surface only to translate it to another position on the surface. Normal adhesion is the adhesion required to detach a drop from the surface in the normal direction, namely the force to cause the drop to fly off from the surface. The measurement of both adhesion forms can be done with the Centrifugal Adhesion Balance (CAB). The CAB uses a combination of centrifugal and gravitational forces to obtain any ratio of lateral and normal forces. For example, it can apply a normal force at zero lateral force for the drop to fly off away from the surface in the normal direction or it can induce a lateral force at zero normal force (simulating zero gravity).


Whether standing in the shower, spilling the morning coffee or going to work in the rain, each day typically begins with water droplets splashing off a solid surface. In fact these phenomena are so common that they often go unnoticed. However, the basic physics that governs the dynamics of water droplets is extremely rich, and understanding these events in detail has important scientific and technological consequences.