Graphene-mediated interfacial interactions at polar liquid–air interface in a microfluidic platform

Graphene, a monoatomic two-dimensional layer with high in-plane elasticity and strength, can function as an ultrathin suspended membrane on top of liquid surfaces in a microfluidic platform. Such platforms have promising sensing applications. However, understanding of fundamental forces at such liquid/graphene interfaces remains limited. Furthermore, since suspended graphene membranes on microchannels are prone to defects and wrinkles, nanoscale imaging of this unique interface is essential but remains largely unexplored beyond topography. In this work, we investigate a polar liquid/graphene/air interface fabricated with a graphene membrane on top of microfluidic channels using dynamic atomic force microscopy. We find that suspended graphene on liquid entirely eliminates capillary instabilities, which typically govern tip–liquid interaction. With van der Waals force as a primary factor in tip–graphene/polar liquid interaction, we achieve stable phase imaging, enabling higher resolution imaging than the topography of the liquid/graphene interface across various interaction ranges. Notably, we observe imperfections such as micro/nanoholes in graphene using phase imaging. These imperfections trap a thin water layer and function as a seal for the graphene membrane after the liquid evaporates from the microchannel. This study offers detailed insights into integrating suspended graphene structures into microfluidic device design, paving the way for advanced bio-nanomechanical sensing applications.