A natural shear-thickening water-soluble polymer from the fronds of the black tree fern, Cyathea medullaris: Influence of salt, pH and temperature

A water-soluble polymer was extracted from the fronds of the black tree fern (Cyathea medullaris or “Mamaku” in Māori) and characterized under various physico-chemical conditions. The rheological properties (in the semi-dilute region) and particle size and charge (in the dilute region) were investigated under various salt (0-1 M NaCl), pH (1-12) and temperature conditions (5-80 °C), using rheometry and dynamic light scattering techniques. A 7% (w/w) Mamaku crude extract showed strong shear-thickening properties at high salinity levels (1 M NaCl) and over the whole range of pH (1-12). However, the thickening properties disappeared above 50 °C. Apparent viscosity remained constant over the pH range 3-9, although the particle size systematically decreased with increasing pH. Overall, Mamaku solutions showed very good salt and pH resistance and exhibited strong temperature dependency. Hydrophobic and electrostatic interactions are not likely to be the cause of the shear-thickening phenomenon observed. Other forces such as hydrogen bonding may play a dominant role on the formation of shear-induced associations.