Drop (liquid)

A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant drop. Drops may also be formed by the condensation of a vapor or by atomization of a larger mass of liquid.

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.

In the pendant drop test, a drop of liquid is suspended from the end of a tube 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 $\gamma$ $\gamma$ . Since the length of this boundary is the circumference of the tube, the force due to surface tension is given by

where d is the tube diameter.

The mass m of the drop hanging from the end of the tube can be found by equating the force due to gravity ( $F_{g}=mg$ $F_{g}=mg$ ) with the component of the surface tension in the vertical direction ( $F_{\gamma }\sin \alpha$ $F_{\gamma }\sin \alpha$ ) giving the formula

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