Apparent Softening of Wet Graphene Membranes on a Microfluidic Platform

Abstract

Graphene is regarded as the toughest two-dimensional material (highest in-plane elastic properties) and, as a consequence, it has been employed/proposed as an ultrathin membrane in a myriad of microfluidic devices. Yet, an experimental investigation of eventual variations on the apparent elastic properties of a suspended graphene membrane in contact with air or water is still missing. In this work, the mechanical response of suspended monolayer graphene membranes on a microfluidic platform is investigated via scanning probe microscopy experiments. A high elastic modulus is measured for the membrane when the platform is filled with air, as expected. However, a significant apparent softening of graphene is observed when water fills the microfluidic system. Through molecular dynamics simulations and a phenomenological model, we associate such softening to a water-induced uncrumpling process of the suspended graphene membrane. This result may bring substantial modifications on the design and operation of microfluidic devices which exploit pressure application on graphene membranes.

Graphene Electromechanical Water Sensor: The Wetristor

“…Therefore, differently from previous reports on electric‐field‐induced electromechanical effects in graphene resonators and switches, our results propose a novel type of electromechanical effect that is induced by water uncrumpling of the graphene membrane. Such action results in a reduction of the membrane wrinkling, which affects transport and optical properties of 2D materials . Finally, the present work also elucidates that the charge transfer process found in the literature between graphene and water/humidity is, without any doubt, substrate‐dependent.…”

“…See Section 1, Supporting Information and ref. for further fabrication details. Figure d shows an optical image of the platform with square windows of 5 µm × 5 µm, separated by 5 µm, with Cr/Au electrical contacts in each side.…”

“…A significant distinction in the membrane height profile is observed when water replaces air in the channel, indicating a morphological modification of the membrane due to the presence of water. By comparing both images we can see a pressure‐induced swelling of the membrane that increases the membrane area, uncrumpling wrinkles and ripples …”

“…In the present work, we develop a microfluidic platform that integrates suspended graphene membrane windows (with electrical contacts) with buried fluid channels to probe the electrical response of a graphene membrane in contact with water. The platform design provides a direct probing of the electrical response of the air/graphene/liquid interface without the presence of any underlying substrate . Our results show a significant change of graphene resistivity (of about 25%) due to the presence of water in the microchannel.…”

“…Our results show a significant change of graphene resistivity (of about 25%) due to the presence of water in the microchannel. The identification of the physical mechanisms behind such strong change in resistivity is not trivial because it occurs simultaneously with a morphological modification of the membrane: water inflates the suspended graphene membranes uncrumpling natural wrinkles, therefore reducing charge scattering . To suppress such uncrumpling effect, we also investigate the graphene membrane covered with thick hexagonal boron nitride (h‐BN) flake, or poly(methyl methacrylate) (PMMA) film.…”