Nonlinear viscoelastic response of natural polysaccharide-based caffeine microcapsules

Abstract

We report the first account of a methodical study of the mechanical response of aqueous microencapsulated caffeine suspensions under large amplitude oscillatory shear (LAOS) stresses. The core–shell type microcapsules, synthesized via standard single emulsion approach, possess a mixture of natural polysaccharides, namely, starch, calcium alginate and xanthan in the shells, including an outer collagen layer. We characterized the microcapsule size, morphology, diffusion, thermal and mechanical characteristics via microscopy, in vitro release tests, differential scanning calorimetry and oscillatory shear rheology. We observed a broad size distribution, with high encapsulation efficiency and sustained in vitro caffeine release. There is a positive, statistically significant correlation between the microcapsule concentration and oscillation frequency and various representative linear and nonlinear viscoelastic parameters. Under LAOS deformations, we noted intercycle viscoelastic strain softening including a universal elastic softening and weak viscous strain overshoot at higher concentrations, including intracycle elastic strain stiffening and intracycle viscous shear thickening trends, via strain amplitude sweeps, Lissajous Bowditch curves and dimensionless indices based on Chebyshev coefficients. The study presents unique, benchmark experimental data pertaining to nonlinear rheology of microcapsule suspension, which could be validated via numerical simulations. Finally, the microcapsules withstand large-scale mechanical deformations typically suited towards their intended cosmetic application.