Annual Transactions of the Nordic Rheology

The small-deformation behaviour of Newtonian emulsion drops covered with a viscoelastic layer of adsorbed proteins (globular proteins and random coil protein) is investigated in shear flow and compared to emulsions prepared with a low molecular weight surfactant (SDS). Drop deformation experiments are performed with dilute emulsions in the rheometer using light microscopy as well as rheo-SALS [1, 2]. The relaxation behavior of emulsions with protein-stabilized interfaces are compared with those prepared using SDS [3, 4]. To obtain detailed information about the interfacial stress boundary condition of the drops, all proteins and surfactants are characterized using the following interfacial rheological properties: (i) transient and equilibrium interfacial tension or interfacial pressure; (ii) interfacial shear viscosity, dynamic storage and loss modulus, transient relaxation modulus, and creep compliance; in particular, combined time/frequency sweep data and stress relaxation loop tests are presented for the proteins; (iii) dynamic interfacial dilatational moduli; transient dilatational modulus after step deformation. An attempt is made to interpret the macroscopic drop deformation behaviour with the microrheology of the interfacial layers. The results show direct evidence for the important role of in-plane interfacial stresses of a viscoelastic protein network on the macroscopic drop deformation in comparison to the equilibrium interfacial