Relation between structure and stability of toothpaste with two-step yielding

Rheological measurements are performed to examine the yielding behavior of different commercial and lab-made toothpaste. Several toothpaste are reported to yield via a two-step process which is noted in dynamic oscillatory strain sweeps. The two-step yielding is best seen as two plateaus for storage modulus in a typical plot of storage modulus as a function of applied strain amplitude. Such behavior in structured materials is usually attributed to rupture of the network at lower strains which is followed by the breakup of aggregates at higher strains. In the case of toothpaste, the network and aggregates are formed by particles, such as silica, mediated by polymers, such as xanthan gum and carboxymethyl cellulose. The toothpaste in which such two-step yielding is more pronounced are shown to be less stable, i.e., lose their ability to retain shape when deposited on a toothbrush.