Characterizing and modeling wear-recovery behaviors of acid-induced casein hydrogels

Hydrogels are soft materials with applications in multiple industries, but their fundamental wear behaviors are relatively unknown. Studies on hydrogel wear behaviors generally use empirical measurements. Additionally, it can be difficult to measuring hydrogel wear rates by traditional mass loss measurements because wear debris cannot always be removed without disturbing intact sample. Therefore, the objective of study was to develop a method for more precise calculation of soft material wear rates. The method developed compared creep-recovery and wear-recovery behaviors of casein hydrogels to separate deflection of the material under an applied load from the amount of material removed. Casein hydrogels were prepared at different concentrations (8%, 10%, and 15% w/w) and pH (2.3, 3.6, 4.8), and tested under a range of normal loads (0.3, 0.4, 0.5 N). Wear-recovery behaviors were fit to an exponential model. Maximum penetration depth during wear and creep, creep-recovery, and wear-recovery increased with lower casein concentration and higher normal force and pH. Exponential models for casein gel wear-recovery behaviors indicated good fit (R2 > 0.99). This wear measurement method is useful for developing food products with specific oral and industrial processing behaviors.