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In this study, the interfacial flow dynamics involving a chemical reaction that produces viscoelastic material at the interface between two liquids is experimentally investigated, and the material is evaluated using interfacial large amplitude oscillatory shear (LAOS) measurements. The flow dynamics indicates fingering patterns at low injection flow rates and fracturing patterns at high flow rates in Hele-Shaw cells, where a more viscous xanthan gum solution is displaced by the less viscous Fe(NO3)3 solution with various concentrations of Fe(NO3)3. The threshold flow rate value of such a transition is different for various concentrations of Fe(NO3)3. Although such a transition without chemical reactions has been discussed, the factors responsible for the transition remain unclear. The flow dynamics in Hele-Shaw cells is considered to flow under large deformation, which exceeds the small amplitude oscillatory shear condition but is under the LAOS condition. Therefore, LAOS measurement of the viscoelastic interface is performed for various concentrations of Fe(NO3)3. Using the characteristic properties extracted from the LAOS measurements, the elastic and viscous forces of the viscoelastic interface are evaluated. We show the transition from fingering to fracturing patterns when the elastic force exceeds a certain value. These findings highlight that rheology under large deformation of the viscoelastic interface plays a crucial role in interfacial flow, where viscoelastic materials are produced by chemical reactions at the interface. In addition, this study should be an example of the successful elucidation of physical phenomena by interfacial LAOS, which has been reported in a very limited number of studies.