A drop hangs before it falls
The balance between surface tension and weight at the tip.
A drop hangs at the end of a tube or nozzle because surface tension is pulling around the rim of the opening hard enough to support the drop's full weight, and it only falls once the drop has grown heavy enough for gravity to win that contest.
A collar of tension around the rim
The ring where emerging liquid meets the rim of an opening is under tension, acting like a small elastic collar holding the hanging mass up. As more liquid arrives the drop swells and its weight climbs, while the most that collar can pull upward stays fixed, set by the liquid's surface tension and the circumference of the opening. The drop keeps growing for as long as the collar can support it and detaches the instant weight exceeds that fixed limit, which is why a hanging drop looks stable right up until it is not.
A dripping tap is the same event slowed down enough to watch. Water gathers at the spout, bulging larger and seeming to defy gravity for longer than intuition expects, held by nothing more solid than the tap's wet rim and the water's own stretched surface. Then it lets go all at once, and the cycle begins again from an empty spout.
The outlet chooses the drop size
Because the collar acts along the circumference of the opening, that circumference sets the heaviest drop the opening can hold. A wider nozzle offers a longer ring and supports a heavier drop, which is why a dripping garden hose produces fatter drops than a fine hypodermic needle under the same water. For water from an ordinary dropper, the old rule of thumb is about twenty drops to the millilitre, or roughly 50 microlitres a drop. That can be larger than the entire volume a small-volume instrument is trying to dispense. An open tip left to release liquid under gravity is therefore dispensing whatever single drop its opening happens to support, a volume decided by geometry and surface tension rather than by the operator.
The same link can be run backwards as a measurement. Counting the drops that fall from a tube of known bore while a known volume drains through it gives a working estimate of the liquid's surface tension, since drop size at a given opening is tied directly to how strongly that liquid's surface pulls together. This drop-weight method predates any of the instruments this set is about.
Taking the release away from gravity
Anything built to deliver a precise small volume has to override this natural drop-and-release behaviour. It can wipe the tip against a surface to strip the drop free before it reaches its natural size, keep the outlet small enough that even a full natural drop stays within tolerance, or control the moment of detachment directly. A tip that dispenses well below the natural drop volume for its outlet is fighting this mechanism on every cycle, since the last of the liquid would prefer to keep hanging. That preference feeds the retention covered earlier in this set, and it explains why a wipe against the side of the receiving vessel, rather than a longer pause or a harder push, is usually what persuades a stubborn drop to let go.