Foams are systems in which air or some other gas is emulsified in a liquid phase to the point of stiffening. As spreadable topical systems go, medicated foams tend toward the fluid side, but, like some shaving creams, they can be stiffer and approximate to a true semisolid. Like the second type of o/w emulsion that only borders on semisolidity, these derive structure from an internal phase, bubbles of an entrapped gas, so voluminous that it exceeds close spherical packing. Consequently, the bubbles interact with their neighbors over areas rather than points of contact. The interactions are sufficient in many cases to provide a resistance to deformation and something approaching semisolid character. Whipped cream is a common example of this type of system. Here, air is literally beaten into the fluid cream until it becomes stiff. Aerosol shaving creams and certain medicated quick-breaking antiseptic foams are examples of the foams currently found in cosmetic and therapeutic practice. These are supplied in pressurized cans that have special valves capable of emulsifying a gas into the extruded preparations.
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