3.3. Water holding capacity by centrifugation

WHC was determined by centrifugation at different relative centrifugal forces to apply a range of pressures to the equilibrated gels, similar to Kocher and Foegeding (1993), and Paudel et al. (2015) (Kocher and Foegeding, 1993, Paudel et al., 2015). Samples were gently taken from pre-swollen gels using a biopsy punch with a diameter of 4 mm. The samples were placed in centrifugal filters with a pore size of 0.2μm (Pall Centrifugal Devices). An Eppendorf centrifuge with a swinging bucket rotor was used to ensure the pressure was applied perpendicularly to the surface of the sample. A centrifugation time of 1h was found to be sufficient to reach a constant gel weight. During centrifugation, the expelled fluid was collected in the bottom compartment of the tube without being in contact with the sample, and the amount of water released was determined by weighing.

Assuming a linear gradient in ϕ and Πext, the relative centrifugal force can be converted to an average pressure applied on the sample as:

ρ¯ is the mean density of the sample, gactual is the relative centrifugal force at the sample location, and H is the sample height. Since the amount of polymers is assumed to remain constant during centrifugation, the change in sample height as a result of the moisture loss can be calculated as:

ϕinit is the initial polymer volume fraction, Hinit is the initial sample height, and ϕ¯ is the average polymer volume fraction.