The principle of shampooing out of a bottle is to use atmospheric  pressure.

When in use, the air in the liquid storage chamber (that is, the plastic tube that can suck and shampoo the hair) is first discharged by pressing. When the air is exhausted, the plastic foam pump head is pressed again. Since the pump head is released, the spring in the pump head is required. If it is restored to its original state, a vacuum will be formed in the liquid storage chamber and a negative pressure will be generated. At this time, the pressure in the container will be in line with the atmospheric pressure, which will result in a small pressure in the liquid storage chamber and a large pressure in the container. At this time, because of the pressure difference, the atmosphere presses the liquid around the reservoir port into the reservoir and the shampoo is squeezed out.

Learn more about Plastic Foaming Soap Dispenser for cosmetic and bottle from www.talentsplastics.com.