Shampoo and Conditioner Science 86 oil and aqueous solutions on the fiber surface. Roll-up is favored by fibers that are oleophobic and hydrophilic.26 The removal of oily soil by detergent compositions is not necessarily predictable due to the wide variation of the surface properties of hair that arise from prior treatments and weathering. Moreover, the transport of the detergent solution to the fiber surface can occur by three different routes: (i) along the fiber surface, (ii) through a previously applied permeable surface treatment, or (iii) through the body of the fibers (Figure 11). Roll-up of oily drops on fibers occurs when the contact angle exceeds a critical value and this causes the oily drop to adopt an unstable axially asymmetric attachment on one side of the fiber.27 The rate of roll up depends also on the viscosity of the oily soil, and mechanical action is often necessary to dislodge viscous oily soils from the fiber surface. In some cases the oil forms a viscous emulsion when contacted by the detergent composition, and the resulting viscous soil can be difficult to remove from the fiber. “Perfect” hair is covered by a covalently attached monolayer of 18-methyleicanosoic acid (18-MEA) which confers hydrophobicity on the hair. Modern grooming techniques and weathering removes this layer of 18-MEA,28 Removal of the layer of 18-MEA results in hair becoming macroscopically hydrophilic.29 The roll-up mechanism, therefore, should be expected to become more prominent on damaged rather than pristine hair. If the fiber is initially completely coated in oil, or if the fiber itself is hydrophobic, the detersive solution cannot easily reach the oil/ Figure 11. In the roll-up mechanism, the detergent solution can be transported to the fiber/oil interface along the fiber surface, through a permeable coating on the fiber, or through the fiber itself.