Linear and nonlinear interfacial rheology of responsive microgels at the oil-water interface

Microgelation can significantly enhance the interfacial activity of polysaccharide and the ability to stabilize emulsion, but the interfacial behaviour of microgels and the intrinsic mechanism of stabilizing emulsion is not fully understood. In this study, chitosan microgels (h-CSMs) with uniform size were successfully prepared by electrostatic crosslinking hydrophobically modified chitosan (h-CS) with sodium phytate (SP) using microfluidic technique. The microfluidic conditions for fabricating of h-CSM were optimized: the mass ratio of h-CS: SP = 5: 1 with h-CS concentration of 0.5 mg/ml, and the pressure ratio of Ph-CS (Pa):PSP (Pa) = 1600:1000. The prepared hCSMs were pH-responsive, with a swollen state at pH 5 and a shrunken state at pH 3 and 7. Further, the interfacial adsorption kinetics, and the interfacial rheology particularly the nonlinear interfacial rheology were systematically studied. The instantaneous Si-factor was proposed to describe the nonlinear responses of the microgel interface during an individual dilatational oscillation cycle. The results showed that microgelation significantly improved the interfacial ability of chitosan molecules at the oil-water interface. The h-CSM interface exhibited strain-softening response during extension and strain-hardening response during compression. The interface formed by h-CSMs under the swollen state had higher viscoelastic modulus, lower degree of nonlinearity and higher emulsifying capacity compared to that formed under the shrinking state. Low ionic strength significantly increased the viscoelastic modulus and reduced the nonlinearity and enhanced emulsifying capacity of the h-CSM interface, whereas high ionic strength disrupted the interaction between the h-CSMs and increased the nonlinear of the h-CSM interface due to too strong electrostatic screening effect.