Periodic Reporting for period 4 - FlexNanoFlow (Ultra-flexible nanostructures in flow: controlling folding, fracture and orientation in large-scale liquid processing of 2D nanomaterials)
Okres sprawozdawczy: 2021-04-01 do 2023-03-31
Graphene and other atomically thin 2D nanomaterials (e.g. MoS2) are small nanoparticles that are used in a variety of high-tech applications, including new generation batteries, electrical cables, conductive plastics, new-generation food packaging, high-performance tires, biotech devices and others. These materials are in most applications produced and processed in a liquid state, in the form of a mixture of particles suspended in a liquid. When such liquid mixture is processed, the performance of the resulting material depends crucially on how the liquid and its particulate constituents flow. We call this new branch of science the "fluid dynamics of graphene" or "graphene hydrodynamics".
The challenge that the FlexNanoFlow project aims to address is the poor understanding of how graphene and other 2D nanomaterial particles behave when they are processed in liquids. This knowledge is crucially important to model or predict the flow of 2D nanomaterials in coatings, 3D printing techniques, sprays, extrusion processes that are commonly employed to produce items we use every day.
The specific objectives of the project are to use a combination of theoretical, computational and experimental techniques to understand the effect of the flow on how the particles orient when suspended in the liquid, how the particles deform (by folding like sheets of paper) and how they break under the action of the strong shear forces produced by the flow. Finally, we aimed to understand how these particles adsorb at fluid interfaces (for instance the surface of a liquid drop) and the resulting semi-solid particle layer deform under compression.
The overarching conclusion the team has reached thanks to ERC funding is that graphene and 2D nanomaterials do not behave in flow like other commonly employed nanoparticles do. They have very specific behaviour, rooted in their extremely small thickness, surface slip properties, and ability to bend. For example, we have discovered that graphene and certain other 2D nanomaterials do not behave like other anisotropic nanoparticles because they do not rotate in a shear flow. As a result of this absence of rotation, 2D nanoparticles can give an extraordinary effect: a reduction in viscosity of the liquid by adding solid material. No other nanoparticle additive has been demonstrated to give such effect. We have also elucidated the enormous importance that bending deformations have on the breakup of multilayer 2D nanomaterials. Such knowledge is important to develop software tools to quantify the yield of methods to produce 2D nanomaterials on an industrial scale.
The challenge that the FlexNanoFlow project aims to address is the poor understanding of how graphene and other 2D nanomaterial particles behave when they are processed in liquids. This knowledge is crucially important to model or predict the flow of 2D nanomaterials in coatings, 3D printing techniques, sprays, extrusion processes that are commonly employed to produce items we use every day.
The specific objectives of the project are to use a combination of theoretical, computational and experimental techniques to understand the effect of the flow on how the particles orient when suspended in the liquid, how the particles deform (by folding like sheets of paper) and how they break under the action of the strong shear forces produced by the flow. Finally, we aimed to understand how these particles adsorb at fluid interfaces (for instance the surface of a liquid drop) and the resulting semi-solid particle layer deform under compression.
The overarching conclusion the team has reached thanks to ERC funding is that graphene and 2D nanomaterials do not behave in flow like other commonly employed nanoparticles do. They have very specific behaviour, rooted in their extremely small thickness, surface slip properties, and ability to bend. For example, we have discovered that graphene and certain other 2D nanomaterials do not behave like other anisotropic nanoparticles because they do not rotate in a shear flow. As a result of this absence of rotation, 2D nanoparticles can give an extraordinary effect: a reduction in viscosity of the liquid by adding solid material. No other nanoparticle additive has been demonstrated to give such effect. We have also elucidated the enormous importance that bending deformations have on the breakup of multilayer 2D nanomaterials. Such knowledge is important to develop software tools to quantify the yield of methods to produce 2D nanomaterials on an industrial scale.
A large part of the project’s team effort has been to investigate how graphene orients in a flow The team has found that that because graphene tends to have a comparatively large slip length (i.e. the liquid does not adhere well to the graphene surface), the application of fluid shear tends to align graphene platelets indefinitely, while conventional theories would predict particle rotation but no instantaneous alignment. An important implication of this is that suspensions of graphene can have a negative intrinsic viscosity, meaning that one can reduce the viscosity of a liquid by adding solid material to it!
A second major effort has been in understanding exfoliation of multilayer graphene by fluid forces. We have been able to derive mathematical models for the critical shear rate leading to exfoliation. These will be used by industry decide the power of a mixer that could lead to optimal exfoliation without damaging the produced graphene sheets. We have also investigated the role of different solvents in reducing inter-layer adhesion and triggering exfoliation in liquids.
Other important results are:
- The comprehensive analysis via experiments and computer simulations of the conditions for graphene layers to peel off due to the application of a shear flow
- A computational and experimental analysis of sedimentation of polydispersed graphene suspensions.
- The analysis of how graphene and graphene oxide nanoparticles adsorb at a liquid surface, for example the surface of a drop or bubble
- The analysis of how layers of graphene, assembled at a liquid surface, bend when compressed.
- The discovery of sustained flapping mode for thin disks when placed in a shear flow with their face in the plane of the flow.
- The analysis of buckling modes of drops filled with graphene oxide.
Exploitation:
- In collaboration with colleagues from Trinity College Dublin, we are developing hollow capsules made of graphene oxide to be used as surface area enhancers
- We are working to patent a new centrifuge camera that will enable one to quickly quantify sedimentation of nanoparticles and other colloidal media. The device will include sedimentation models that have been developed as part of this ERC project.
- We are working with French and US rheologists to verify whether graphene can be used to produce graphene additives that are “viscosity neutral”, i.e. the addition of graphene additives does not incur in an increase in viscosity and friction
Dissemination:
The team has produced more than 18 papers (other 4 in preparation), published in the leading fluid mechanics journals (Journal of Fluid Mechanics, Physical Review Fluids), colloidal science and physical chemistry journals (Langmuir, Soft Matter, J. of Physical Chemistry), as well as high-impact interdisciplinary journals (Nature Communications) . The team has regularly presented at international conferences in fluid dynamics, rheology and graphene applications.
A second major effort has been in understanding exfoliation of multilayer graphene by fluid forces. We have been able to derive mathematical models for the critical shear rate leading to exfoliation. These will be used by industry decide the power of a mixer that could lead to optimal exfoliation without damaging the produced graphene sheets. We have also investigated the role of different solvents in reducing inter-layer adhesion and triggering exfoliation in liquids.
Other important results are:
- The comprehensive analysis via experiments and computer simulations of the conditions for graphene layers to peel off due to the application of a shear flow
- A computational and experimental analysis of sedimentation of polydispersed graphene suspensions.
- The analysis of how graphene and graphene oxide nanoparticles adsorb at a liquid surface, for example the surface of a drop or bubble
- The analysis of how layers of graphene, assembled at a liquid surface, bend when compressed.
- The discovery of sustained flapping mode for thin disks when placed in a shear flow with their face in the plane of the flow.
- The analysis of buckling modes of drops filled with graphene oxide.
Exploitation:
- In collaboration with colleagues from Trinity College Dublin, we are developing hollow capsules made of graphene oxide to be used as surface area enhancers
- We are working to patent a new centrifuge camera that will enable one to quickly quantify sedimentation of nanoparticles and other colloidal media. The device will include sedimentation models that have been developed as part of this ERC project.
- We are working with French and US rheologists to verify whether graphene can be used to produce graphene additives that are “viscosity neutral”, i.e. the addition of graphene additives does not incur in an increase in viscosity and friction
Dissemination:
The team has produced more than 18 papers (other 4 in preparation), published in the leading fluid mechanics journals (Journal of Fluid Mechanics, Physical Review Fluids), colloidal science and physical chemistry journals (Langmuir, Soft Matter, J. of Physical Chemistry), as well as high-impact interdisciplinary journals (Nature Communications) . The team has regularly presented at international conferences in fluid dynamics, rheology and graphene applications.
The team is prticularly excited about 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 unexpected.
We have also pushed scientific boundaries in terms of improving our understanding of the micromechanics of liquid-phase exfoliation, proposing first-of-their-kind theoretical models for predicting the shear rate leading to exfoliation of graphene.
For suspensions, we have proven that when particles are stacked at close proximity the threshold at which a single particle buckle in a simple shear flow is reduced by a surprisingly large factor (of almost 10). This observation, which we have demonstrated both in simulations and experiments, indicates that the rheology of flexible graphene sheets will be particularly sensitive to increase in solid concentration.
Finally, we will produce new results for 2D nanomaterials at fluid interfaces. Beyond state of the art is the demonstration that monolayers of graphene sheets compressed at a fluid interface have a buckling wavelength comparable to the length of the sheet, while all previous models would predict a much larger wavelength. This results paves the way for studies on the role of particle shape on the buckling modes of particles adsorbed at fluid interfaces.
Because we have used techniques that are rarely used in combination, such as molecular dynamics (atomistic) and continuum (boundary integral) simulations, and because we have insisted in the theoretical understanding of our observations, most of our results - listed in the previous section on “Work performed from the beginning of the project” - are beyond the state of the art and of the unique, unexpected character that is typical of ERC-funded projects.
We have also pushed scientific boundaries in terms of improving our understanding of the micromechanics of liquid-phase exfoliation, proposing first-of-their-kind theoretical models for predicting the shear rate leading to exfoliation of graphene.
For suspensions, we have proven that when particles are stacked at close proximity the threshold at which a single particle buckle in a simple shear flow is reduced by a surprisingly large factor (of almost 10). This observation, which we have demonstrated both in simulations and experiments, indicates that the rheology of flexible graphene sheets will be particularly sensitive to increase in solid concentration.
Finally, we will produce new results for 2D nanomaterials at fluid interfaces. Beyond state of the art is the demonstration that monolayers of graphene sheets compressed at a fluid interface have a buckling wavelength comparable to the length of the sheet, while all previous models would predict a much larger wavelength. This results paves the way for studies on the role of particle shape on the buckling modes of particles adsorbed at fluid interfaces.
Because we have used techniques that are rarely used in combination, such as molecular dynamics (atomistic) and continuum (boundary integral) simulations, and because we have insisted in the theoretical understanding of our observations, most of our results - listed in the previous section on “Work performed from the beginning of the project” - are beyond the state of the art and of the unique, unexpected character that is typical of ERC-funded projects.