Attempt of the quantitative evaluation of flow properties of agar microgel suspensions using a weak-gel model

It is important to quantitatively evaluate the flow behaviour of complex fluids such as paste-like foods. In this study, we investigated the flow properties of a pastelike food model using a weak-gel model. As a model of pastelike food, agar microgel suspensions were prepared using a water-in-oil (W/O) emulsion system. Because the microgels prepared by this method have a spherical shape, their flow properties can be evaluated quantitatively. The steady state viscosity and dynamic modulus of the suspensions were measured with the volume fraction of the microgel particles ranging from 0.68 to 0.80. Because the flow curves showed a pseudo-plastic flow, they were analysed using the Herschel-Bulkley equation to obtain characteristic flow parameters, namely, the viscosity coefficient (k), apparent yield stress (σy), and the Herschel-Bulkley index (n). The frequency dependence of the complex modulus was analysed using the weak-gel model to obtain two parameters, namely the coordination number (z), which relates to the relaxation behaviour, and gel strength (Af). A sudden increase in both σy and z was observed when the volume fraction was close to 0.77. Moreover, k and Af were also found to undergo a sudden increase at the same volume fraction. This value is expected to be the critical volume fraction at which the microgel particles reach a random close packing state. The results showed that we can make quantitative evaluation of the flow behaviour of a complex fluid under a small deformation by using the weak-gel model.