Chapter 3 85 aforementioned structures, while also being esthetically pleasing. The hair care formulation scientist has an ever-increasing variety of surfactants available in the formulation toolbox, and so these structures can be obtained via a wide range of concoctions. Nonetheless, attaining such stable structures is not a trivial task, due to the presence and interactions of so many ingredients in the typical formulation. Therefore, with historical knowledge involving many established ingredients already being relatively well-understood, it is a brave formulation chemist that opts to cut a new pathway. Moreover, it is also probably prudent to arrive at these structures in the most cost-effective manner. For these reasons, it is imperative to understand how the surfactant structure, together with interactions with other molecules alters the nature of the aggregate structures. Oily Soil Removal Mechanisms The principal function of a shampoo is to remove oily soil from the hair. In this context it is advisable to consider the different mechanisms by which surfactant compositions can remove oily soils. There are several principal detergency mechanisms for removing oily soils: “roll-up,”25 emulsification, penetration, and solubilization. In the roll-up mechanism, the detergent solution causes a steady increase in the contact angle of the oil at the oil/fiber/ aqueous interface (Figure 10). The oil droplet is rolled up on the surface, and when the contact angle reaches 180 degrees the resultant interfacial force that is holding it to the surface is overcome by the wetting tension of the Figure 10. In this mechanism the oil contact angle at the oil/water/fiber interface steadily increases until it “rolls up” and floats off of the solid surface. This mechanism was first reported by N. K. Adams.