To explain the working principle of a rubber dropper.
A rubber dropper consists of a small glass or plastic tube with a hollow rubber bulb at one end and a nozzle at the other end. Droppers are handy for drawing up and dispensing small quantities of liquids in drops.
Initially the pressure inside the rubber dropper is same as the pressure outside. When the bulb is pressed, air from the nozzle of the dropper escapes from it and partial vacuum is created inside the nozzle. Pressure inside the dropper decreases. When the bulb is pressed keeping the nozzle inside water, the air escapes in the form of air bubbles. On releasing the bulb, water rushes into the nozzle to fill this vacuum. This is because of the much greater air pressure on the surface of water, pushes the water into the dropper. The harder we press, the larger the amount of air that is pushed out and the greater the vacuum that is created. As a result, more water is pushed in.
The water does not escape out of the dropper when taken out as water pressure inside the dropper equals the atmospheric pressure. Water is held inside the dropper against atmospheric pressure. On pressing the bulb, the liquid pressure rises and the water will spill out of the dropper once it exceeds atmospheric pressure.
The rubber droppers with measurements marked on them are used for transferring required amount of liquids. It is typically used as medicine dropper in taking prescribed amount of medicine. Similarly students and scientists in laboratories use it to obtain the right amount of chemicals. The droppers are very effective and widely used because of their precision. The same mechanism is applied in various cases such as drinking milkshake with a straw, working of a syringes and pichkaris used during Holi etc.