Examples of Foam having gas as it's dispersed phase.
Answers
Soap Suds, Whipped Cream etc.
Explanation:
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Answer:
shaking it can emulsify[1] the propellant into the liquid. When you press the button, the liquid comes out, and any dissolved or emulsified propellant in the liquid will expand, creating bubbles in the liquid. This is very imporant in things like Redi-Whip topping, because you want these bubbles in the cream to give you that fluffy, whipped texture. If the solubility of the gas is low for the liquid, then you'll have the propellant just pushing out the liquid, but the liquid won't contain enough propellant within it to produce the nice texture. You just get liquid mush.
Likewise for some other aerosols, like spray paint or hair spray. If the propellant has low solubility, the emulsification of the liquid and propellant means that more propellant comes out with the liquid, meaning lower droplet sizes. For paint, that means fewer gobs and less clogging, and for hairspray it means a faster drying allotment with more even distribution.
If you have something that doesn't need to be shaken, it's likely that either the propellant is highly soluble in the high-pressure liquid (like with shaving cream) or that you're not trying to produce a liquid at all, and the contents are uniform (like with canned air).
[1] Edit: Eric Pepke is largely correct; it's not (usually) an emulsion (though it may be, if the pressure is sufficient to make the propellant a liquid). If the propellant is a gas under the high pressure in the can, then it should be called a foam, not an emulsion. I was using the term loosely and incorrectly. However, he also seems to be suggesting that the idea of it being an emulsion or not is a product of degree. That's not the case. Emulsion just means that both phases are liquid. But a colloid (using a general term that refers both to aerosols, foams, and emulsions alike) can be a transient state. It doesn't need to be a long-lasting colloid to count. Peanut butters like Skippy etc. that keep the oils and solids uniform and don't separate (like Adam's Natural or Laura Scudder) are not the only forms colloids take, and the oil and water of Italian salad dressing is a good example of a more transient emulsion that lasts only from the time you shake it up to the time you pour it out.
The point here is that you're trying to disperse a propellant phase into a liquid product phase, such that once the pressure drops, the dispersed propellant phase expands. To make an aerosol, the dispersed phase becomes the continuous phase. For whipped cream, the continuous phase remains the continuous phase.
Explanation: