2.3.2. Viscosity and surface tension
The viscosity of the WPI-XG mixtures is determined for each
batch using the AR-1000N rheometer (TA Instruments, USA) with
cone-plate geometry (60 mm diameter, 4 cone angle) at 25 C.
Shear rate _ c ranged between 0.2 s1 and 2800 s1. The viscosity
of the solutions is affected by the XG concentration, and the addition
of sodium chloride decreases the viscosity. These solutions exhibit
a shear thinning behavior as seen in Fig. 2 and can be fitted
with a power law for shear rates between 0.3 and 700 s1.
l ¼ K _cn1 ð1Þ
where s is the shear stress, K is the flow consistency index and n the
structure index.
The interfacial tension r of the test solutions (3 wt% WPI) was
measured with a K12 tensiometer (Krüss GmbH, Germany) using
the Wilhelmy’s plate method at 20 C during 3 h. With a constant
WPI concentration, the interfacial tension is 43 ± 1 mN m1 at long
time (equilibrium value) for all the solutions. Neither xanthan gum
nor sodium chloride changes the interfacial tension. The composition
and properties of the solutions are gathered in Table 2.