A drop, also known as a droplet, is a tiny column of liquid confined entirely or almost entirely by free surfaces. A pendant drop is created when liquid collects at the bottom end of a tube or other surface boundary, resulting in a hanging drop. Condensation of a vapor or atomization of a larger amount of liquid can also produce drops.

Because liquids have surface tension, they produce droplets.

Allowing the liquid to flow slowly from the bottom end of a vertical tube of small diameter is a simple technique to produce a drop. The liquid’s surface tension forces it to dangle from the tube, producing a pendant. When the drop reaches a particular size, it loses its stability and detaches. Surface tension holds the falling liquid together as well.

Because they produce drops and exhibit droplet behavior, several substances that appear to be solids may be demonstrated to be highly viscous liquids. Pitch, a material that resembles solid bitumen, is proved to be a liquid in the classic pitch drop experiments. Pitch in a funnel creates droplets over time, each one taking roughly ten years to develop and break off.

A drop of liquid is suspended by surface tension from the end of a tube or any other surface in the pendant drop test. The force owing to surface tension is proportional to the length of the liquid-tube interface, with the proportionality constant referred to as display style gamma. The force due to surface tension is given because the length of this border is the circumference of the tube.

The adherence of a drop to a solid can be classified into two types: lateral adhesion and normal adhesion. Lateral adhesion is similar to friction (though lateral adhesion is a more accurate term tribologically) and refers to the force required to slide a drop on the surface, namely the force required to detach the drop from its position on the surface only to translate it to another position on the surface. The adhesion necessary to detach a drop from a surface in the normal direction, i.e., the force required to cause the drop to fly away from the surface, is known as normal adhesion. The Centrifugal Adhesion Balance may be used to test both types of adhesion (CAB). To produce any ratio of lateral and normal forces, the CAB employs a mix of centrifugal and gravitational forces. It can, for example, provide a normal force with no lateral force to cause the drop to fly away from the surface in a normal direction, or it can generate a lateral force with no normal force (simulating zero gravity).

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