Life question: dirty hair + shampoo why not foam?

If the hair is not too dirty, add shampoo to rub a lot of bubbles, but if the hair is not washed, this time may not add a lot of shampoo Bubble, this is why?

to put it simply, it is because there is not enough surfactant.

Are you prepared to look better in little bride dresses during your next event? We have a huge range of styles and cuts to choose from.

bubbles themselves do not occur spontaneously but depend on rubbing when washing hair. However, due to the effect of surface tension, if it is only water, bubbles will be easily broken and cannot be foamed. The surfactant in shampoo reduces the surface tension and stabilizes the Bubble.

at the same time, it is these surfactants that wash off the grease from your hair. The oil from the hair is not easily washed off by water alone, while one end of the surfactant is lipophilic and easy to combine with grease, and the other end is hydrophilic, which can also be well dispersed in the water so that the oil can be taken away.

as can be seen from the figure above, the presence of surfactants is required on both bubbles and surfaces that disperse dirt. But the shampoo used at one time is limited, if there is a lot of dirt to disperse, the surfactants will be consumed, and they may not have free hands to stabilize the instant noodles. At this time, it also produces the feeling that it will not foam even if it is scrubbed.

but it should be noted that not all surfactants are very good at foaming, and it is not necessarily that Bubble has better cleaning power. Some detergents are specially designed to be low Bubble.

the oil is dispersed into small oil droplets by surfactant, which is the process of emulsification. Add some water to the oil pan and stir it with detergent, does it become like milk? by the way, the phrase "blending of water and milk" is very scientific. Milk (or whatever) has a lot of fat in it. But it can be easily mixed with water, this is because milk is a "water-in-oil" emulsion system, as shown in the above diagram, water is on the outside, a continuous phase. At this time, adding more water will not cause delamination, and if you add a little oil, even if it can be compatible with the one inside, it will not be able to "blend" with the emulsion system. If the word "blending of milk and water" is rewritten in academic language, it is:  the emulsion system can be diluted by its external phase.