  Microcapsules are commonly used in applications ranging from therapeutics to
personal care products due to their ability to deliver encapsulated species
through their porous shells. Here, we demonstrate a simple and scalable
approach to fabricate microcapsules with porous shells by interfacial
complexation of cellulose nanofibrils and oleylamine, and investigate the
rheological properties of suspensions of the resulting microcapsules. The
suspensions of neat capsules are viscous liquids whose viscosity increases with
volume fraction according to a modified Kreiger-Dougherty relation with a
maximum packing fraction of 0.73 and an intrinsic viscosity of 4. When
polyacrylic acid (PAA) is added to the internal phase of the microcapsule,
however, the suspensions become elastic and display yield stresses with
power-law dependencies on capsule volume fraction and PAA concentration. The
elasticity appears to originate from associative interactions between
microcapsules induced by PAA that resides within the microcapsule shells. These
results demonstrate that it is possible to tune the rheological properties of
microcapsule suspensions by changing only the composition of the internal
phase, thereby providing a novel method to tailor complex fluid rheology.
