Effect of interfacial viscoelasticity on shear-induced deformation of MXene-covered droplets

Interfacially assembled conductive 2D nanoparticles can endow the interface with viscoelastic properties that can be leveraged to shape multifunctional materials into 3D architectures such as high internal phase pickering emulsions or bijels. To rationally tailor such microstructures, the relations between the interfacial assembly and the flow-induced structure dynamics should be known. In the current work, we focus on the deformation dynamics of single droplets covered with 2D MXene nanosheets subjected to shear flow. The MXene nanosheets are synthesized in house from the layered ceramics Ti₃AlC₂, using the MILD method. MXene-covered water droplets in a silicone oil matrix were investigated whereby the interfacial assembly is promoted by electrostatic interactions between the MXenes and either mono- or bi-amine terminated PDMS in the oil phase. The interfacial assembly is characterized by means of interfacial shear and dilatational rheology, using respectively a double-wall ring attached to a stress-controlled rheometer and pendant drop experiments. An interfacial network with a yield stress and limited frequency-dependency of the viscoelastic moduli is found whereby only the shear response is dominated by the particle-particle interactions. When using amine-terminated surfactants with a different number of charged heads, the interfacial network structure and resulting viscoelasticity is altered, with the bi-amine terminated surfactant leading to higher moduli but a slightly more brittle network. The shear-induced dynamics of MXene-covered droplets is investigated in a counter-rotating shear flow cell. The MXenes-covered droplets act as microcapsules, starting to deform only after a yield point, and exhibiting limited deformation as well as remaining deformation after flow cessation. Our analysis indicates that this behaviour is mainly dominated by the interfacial shear rheology whereby both the shear yield stress as well as the residual stiffness beyond yielding play a role.