Chapter 3 83 shampoos, in part because they are easier to apply, but also for aesthetic reasons a thicker formula is generally perceived as being more-luxurious. The desired rheology is achieved from formulations that contain worm-like micelles. Wormlike micelles do, however, show “non-polymeric” behavior at certain shear rates when the shear stress becomes independent of the shear rate and the relaxation time becomes monodisperse.22 This has been explained on the basis that the entanglements can be broken and reformed as the rod-like micelles disassemble and then reassemble upon passing through each other.23-24 Systems like these have been dubbed “phantom networks” by Cates to signify that one micelle flows through another just as we imagine a phantom would pass through a wall. The phantom network behavior may explain why shampoos can show viscoelasticity without the “stringiness” observed in entangled polymer solutions. At higher concentrations, the rodlike micelles mutually repel and this favors alignment into a nematic phase. At still higher concentrations the aligned rods pack in a hexagonal array to form hexagonal phase liquid crystals (Figure 8). Hexagonal phase has the properties of a clear ringing gel that is birefringent in polarized light. Figure 7. Ionic surfactant micelles change shape as a function of ionic strength and surfactant concentration.