The use of liquid nitrogen for the storage of biological samples should be done with caution. This popular coolant boils quickly when it touches surfaces that are hotter (or less cold), which decreases the efficiency of the procedure. Additionally, when a substance boils at temperatures below the freezing point of water, an effect occurs where an insulating cover of nitrogen bubbles forms on the cooled object, known as the Leidenfrost effect.
This effect can be partially compensated by immersing objects in a mixture of solid and liquid nitrogen, rather than just the element in its liquid form, thus achieving faster cooling. It should not be neglected, that people who handle liquid nitrogen should do with all precautions and safety standards to avoid any accident.