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Ultra-flexible nanostructures in flow: controlling folding, fracture and orientation in large-scale liquid processing of 2D nanomaterials

Periodic Reporting for period 3 - FlexNanoFlow (Ultra-flexible nanostructures in flow: controlling folding, fracture and orientation in large-scale liquid processing of 2D nanomaterials)

Reporting period: 2019-10-01 to 2021-03-31

Graphene and other atomically thin 2D nanomaterials (e.g. MoS2) are currently being used in a variety of applications to improve material properties, leading in many cases to exceptional improvements. In industrial applications, the flow processing of these highly anisotropic nanoparticles in liquid solvents is present practically everywhere, from processes based on roll-to-roll techniques, to polymer extrusion and spinning. High-performance lubricants, conductive graphene inks for flexible electronics, smart plastics that sense strain, coatings to reduce corrosion, thin films for tactile displays are just some of the applications that will benefit from an improved understanding of how 2D nanomaterials interacts with moving liquids under conditions of large shear rates. Currently such understanding is rudimental, and even experts in the field rely on trial and error approaches to, for example, disperse a graphene powder in a liquid. We also don’t yet know how to produce 2D nanomaterials cheaply on industrial scales by exfoliation in liquids without damaging the nanosheets.

The ERC project FLEXNANOFLOW promises to make a big step in the direction of understanding how 2D nanomaterials exfoliate, deform and interact in liquids, based on a comprehensive theoretical and computational framework that combines multiscale computations and experiments. Such framework is the first of its kind. The project will enable the EU's scientific community to retain and advanced knowledge of these extraordinary materials, and EU's industry to lead the way in the technological exploitation of graphene, a truly European invention.
We have worked primarily along two directions. Firstly, we have investigated how graphene orients in a flow. In a variety of applications, it is crucial to understand this aspect, for instance because the viscosity of a graphene ink depends dramatically on orientation of the suspended nanomaterial, and many material properties (e.g. electrical conductivity) are also strong functions of microstructural orientation. We have found that textbook knowledge on the fluid dynamics of graphene does not enable to explain the orientation of a graphene nanoplatelet in a flow. Indeed, we have found that because graphene tends to have a large slip length (i.e. the liquid does not adhere well to the graphene surface), the application of shear tends to align graphene platelets indefinitely, while conventional theories would predict particle rotation but no instantaneous alignment. Such stable orientation is counter-intuitive, but we have been able to prove that this is a robust result, by combining continuum simulations, theory and molecular dynamics. This result is crucially important. For instance, thanks to the theoretical understanding we have obtained we could better orient this material in reinforced or conductive plastics, and design ink-jet printers for optimal printing of graphene inks.

Secondly, we have investigated how graphene exfoliation works at the level of a single multilayer nanoparticle. Combining multiscale simulations and theory we have been able to quantify thresholds for the shear rate leading to exfoliation, and derive practical expressions that will enable practitioners to, for example, decide the power of a mixer that could lead to optimal exfoliation. These models are the first appearing in the literature that have been rigorously justified by comparison against high-resolution simulations and molecular dynamics. We have also investigated the role of different solvents in reducing inter-layer adhesion and triggering exfoliation in liquids, for the first time evaluating the coupling between adhesion and flow in configurations similar to those occurring in practice. Here the promise is that with the new understanding enabled by the ERC project FlexNanoFlow we will be able to develop scalable methods to produce graphene and other 2D nano materials on the industrial scale (meaning, tons of material!) required by market applications.
Many of our observations are the first appearing in the literature on graphene. Such literature is very rich and ever-expanding, but the fluid dynamics at the level of a single graphene or 2D nanomaterial particle is almost always discussed superficially, preventing a true understanding. We are breaking scientific ground in this area by developing a rigorous theoretical framework which will help other researchers get their heads around the very difficult and coupled fluid-structure interaction occurring at the single-particle level, for instance helping them in the interpretation and rationalisation of experimental results.

The result that we are most excited about is the discovery of the dramatic effect that hydrodynamic slip has on the rotational dynamics of graphene. That a slip length of few nanometers can change the dynamics of graphene particles that can be several microns in length is truly an unexpected result. In the future, we would like to better understand the effects of particle flexibility and increase in particle concentration in the fluid. We are well on track to deliver on these aspects by the end of the project.

We have also pushed scientific boundaries in terms of improving our understanding of the micromechanics of liquid-phase exfoliation. So far we have proposed first-of-their-kind theoretical models for predicting the shear rate leading to exfoliation of graphene. We have proposed a sliding model that should work well with small nanosheets, and a peeling model which should be applied to larger nanosheets. After exploring critical shear rate threshold, what we are now interested in is the time-scale of exfoliation. If I apply a critical shear rate, how long will it take for the particle to exfoliate? At the end of the ERC project we will hopefully have investigated model problems to understand the dynamics of exfoliation, carrying out simulations and conducting experiments on model particles to guide our thought process.

Finally, we will produce new results for the hydrodynamic and capillary interaction of 2D nanomaterials with fluid interfaces. The understanding of the motion of graphene platelets in unbounded bulk liquids we have obtained in the first part of the project constitutes a solid basis for these future investigations.
Shear flow past a multilayer graphene platelet in water (Molecular Dynamics simulation)