Discontinuous shear thickening in confined dilute carbon nanotube suspensions (original) (raw)

Elongational and shear rheology of carbon nanotube suspensions

Rheologica Acta, 2009

Rheological behavior of concentrated suspensions of chemical vapor deposition carbon nanotubes in uniaxial elongation and simple shear is studied experimentally and theoretically. Nanotubes are suspended in viscous host liquids-castor oil or its blends with n-decane. The elongational measurements are performed by analyzing self-thinning (due to surface tension effect) liquid threads of nanotube suspensions. A quasi-one-dimensional model is used to describe the self-thinning process, whereas corrections accounting for thread nonuniformity and necking are introduced a posteriori. The effects of nanotube concentration and aspect ratio, viscosity of the suspending liquid, and initial diameter of the self-thinning thread in uniaxial elongation are elucidated. The results for uniaxial elongation are compared with those for simple shear. The correspondence in the results of the shear and elongational measurements is addressed and interpreted. The results conform to the Herschel-Bulkley rheological constitutive equation (i.e., power law fluids with yield stress). However, the yield stress in elongation is about 40% higher than in simple shear flow, which suggests that the original Herschel-Bulkley model need modification with the yield stress being a function of the second invariant of the deviatoric stress tensor. The present effort is the first to study capillary self-thinning of Herschel-Bulkley liquids, which are exemplified here by suspensions of carbon nanotubes.

Shear Relaxation Behaviour in Linear Regime of Diluted Carbon Nanotube Suspensions

2011

Carbon nanotubes (CNT) are very long cylinders formed by rolled-up graphene sheets at the nano scale. In addition to their unique structure, interest for CNTs is based on their impressive physical properties [1]. CNTs have been envisaged as component-material in a large range of applications: electrical circuits, nano-probes for near-field microscopes, high-resolution gas-molecules detectors, drug and gene delivery systems, highstrength fibres and high-performance polymer-based composites. A large number of envisaged applications require dissolving CNTs and processing them in liquid phase. Control of those forming processes needs a deep understanding of the rheology of CNT suspensions [2]. In that context, this work tackles the modelling of the rheological behaviour of dilute and low semi-dilute suspensions of treated CNTs into a Newtonian solvent. CNTs have an important influence on the viscous behaviour of their suspensions. For example, dilute and low semi-dilute suspensions of t...

Modeling interactions in carbon nanotube suspensions: Transient shear flow

Journal of Rheology, 2016

Transient shear flow data of untreated multiwalled carbon nanotubes (MWCNTs) dispersed in a Newtonian epoxy matrix are analyzed. A sequence of shearing and rest steps was applied to characterize the transient responses of the suspensions. Stress overshoots appeared at very small deformation during forward and reverse flow experiments and their intensity increased with rest time between two consecutive flows, during which the suspension structure was reconstructed. The transient behavior of the MWCNT suspensions is explained with the help of a recently proposed model [G. Natale et al., AIChE J. 60(4), 1476-1487 (2014)]. The MWCNTs are described as rigid rods dispersed in a Newtonian matrix, and the evolution of the system is controlled by hydrodynamics, rod-rod interactions, and Brownian motion. The force due to the interactions is modeled as a nonlinear lubrication force and the total stress tensor is evaluated introducing a fourth-order interaction tensor. The Fokker-Planck equation is numerically solved for transient simple shear flow using a finite volume method, avoiding the need of closure approximations. The model predictions show that interactions slow down the orientation evolution of the rods. For the first time, the effect of shear rate is directly accounted by the model, which predicts that a critical shear rate is necessary to break down the structure and let the rods orient in the flow direction. In addition, we confronted the model predictions with the rheological data of a glass fiberfilled polybutene [M. Sepehr et al., 48(5), 1023-1048 (2004)], demonstrating its ability to describe the behavior of micro and nano-scale particle suspensions. V

Steady Shear Response of Carbon Nanotube Networks Dispersed in Poly(ethylene oxide)

Macromolecules, 2008

The response of fractal networks of dispersed single walled carbon nanotubes in poly(ethylene oxide) to continuous constant-rate shear flow is examined as functions of shear rate and nanotube concentration. The steady shear viscosity values are strong functions of shear rate and follow a power-law shear-thinning character, while the nanotube concentration dependence of the viscosity (at a fixed shear rate) is somewhat weaker than the scaling of the equilibrium modulus values. For dispersions with nanotube concentrations corresponding to the semidilute regime, the stress response to constant-rate continuous shear from rest demonstrates a stress maximum that decays to a steady value at long times. The stress maximum and steady shear behavior can be reconciled in terms of the changing structure at the mesoscale for the fractal networks. On the other hand, the transient development of the stress during the start-up experiments can be qualitatively reconciled in the context of a cluster dynamics model.

Rheology of carbon nanotube dispersions

2006

We report on rheological properties of a dispersion of multi-walled carbon nanotubes in a viscous polymer matrix. Particular attention is paid to the process of nanotubes mixing and dispersion, which we monitor by the rheological signature of the composite. The response of the composite as a function of the dispersion mixing time and conditions indicates that a critical mixing time t * needs to be exceeded to achieve satisfactory dispersion of aggregates, this time being a function of nanotube concentration and the mixing shear stress. At shorter times of shear mixing, t < t * , we find a number of non-equilibrium features characteristic of colloidal glass and jamming of clusters. A thoroughly dispersed nanocomposite, at t > t * , has several universal rheological features; at nanotube concentration above a characteristic value nc ∼2-3 wt% the effective elastic gel network is formed, while the low-concentration composite remains a viscous liquid. We use this rheological approach to determine the effects of aging and re-aggregation.

The rheological modelling of carbon nanotube (CNT) suspensions in steady shear flows

International Journal of Material Forming, 2008

This paper is concerned with the rheological modelling of both chemically treated and untreated carbon nanotube (CNT) suspended in a Newtonian epoxy resin. CNT suspensions generally exhibited shear-thinning characteristic-the apparent viscosity decreases as shear rate increases-when subject to steady shear flows. Chemically treated CNT suspensions with little optical microstructure were found to exhibit a less significant shear-thinning effect compared with untreated CNT suspensions where clear optical aggregates were observed. In the case of treated CNT suspensions, the shear-thinning characteristic could be described using a Fokker-Planck based orientation model. The model assumed that the treated CNTs behaved as high aspect ratio rods and that shear flow was able to align the CNTs in the flow direction, thereby resulting in a decrease in the shear viscosity. Despite the success in describing the rheological response of treated CNT in steady shear flows, the orientation model failed to explain the more pronounced shear-thinning effect observed in untreated CNT suspensions having a hierarchy of aggregate structures. A new model called the aggregation/orientation (AO) model was formulated by modifying the Fokker-Planck equation. The AO model considered elements of aggregation as well as CNT orientation and it was capable of capturing the steady shear response of untreated CNT suspensions.

Viscoelasticity of Single Wall Carbon Nanotube Suspensions

Physical Review Letters, 2004

We investigate the viscoelastic properties of an associating rigid rod network: aqueous suspensions of surfactant stabilized single wall carbon nanotubes (SWNTs). The SWNT suspensions exhibit a rigidity percolation transition with an onset of solidlike elasticity at a volume fraction of 0.0026; the percolation exponent is 2:3 0:1. At large strain, the solidlike samples show volume fraction dependent yielding. We develop a simple model to understand these rheological responses and show that the shear dependent stresses can be scaled onto a single master curve to obtain an internanotube interaction energy per bond 40k B T. Our experimental observations suggest SWNTs in suspension form interconnected networks with bonds that freely rotate and resist stretching. Suspension elasticity originates from bonds between SWNTs rather than from the stiffness or stretching of individual SWNTs.

Anisotropy of Sheared Carbon-Nanotube Suspensions

Physical Review Letters, 2005

We measure the anisotropy of sheared carbon-nanotube suspensions for a broad range of concentration, aspect ratio, and strain rate using a variety of methods. Our measurements highlight the importance of excluded-volume interactions in the semidilute regime, with scaling in terms of a dimensionless shear rate. Our results also suggest that such interactions might be exploited to fractionate carbon nanotubes by length in simple shear flow.

A review of the microstructure and rheology of carbon nanotube suspensions

2008

The present review is concerned with the way that the incorporation of carbon nanotubes (CNTs) into a fluid matrix can modify the microstructure and rheology of the resulting suspensions. Some background to CNT manufacture and in particular methods of dispersing them into a suspension is presented for a range of different systems, where effective dispersion of CNTs remains a delicate and open issue. Steady shear, linear viscoelasticity, nonlinear viscoelasticity and extensional responses are classified for a range of different CNT/ matrix combinations together with their associated microstructure. The rheological modelling of certain CNT/matrix systems is reviewed, with particular attention given to the authors' work on modelling CNT suspension behaviour using Fokker-Planck advection-diffusion modelling.

The microstructure and rheology of carbon nanotube suspensions

International Journal of Material Forming, 2008

This paper gives a brief overview of microstructure and rheology that have been observed for a range of carbon nanotube (CNT) suspensions. In general, untreated CNT suspensions show a much higher level of observable optical microstructure reflecting their preference to aggregate; they also show higher levels of viscoelasticity over treated CNT suspensions. An unexpected Helical Band texture for untreated CNTs is reported together with a series of parallel plate optical observations showing a broad spectrum of behaviour for different shear conditions. Both steady shear and linear viscoelastic data are presented for treated and untreated systems.