Discontinuous shear thickening in confined dilute carbon nanotube suspensions

Abstract

A monotonic decrease in viscosity with increasing shear stress is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. Here we demonstrate a discontinuous shear-thickening transition on varying shear stress where the viscosity jumps sharply by four to six orders of magnitude in flocculated suspensions of multiwalled carbon nanotubes (MWNT) at very low weight fractions (approximately 0.5%). Rheooptical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shearthinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear-thickening phenomena in confined geometries is pertinent for flow-controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues.A n increase in viscosity of fluids with increasing flow rate termed as "shear thickening" (ST) is of tremendous interest in basic sciences as well as for many applications which include the design of smart materials like soft body armors and shock absorbers (1). Traditionally, the candidates for shear-thickening fluids have been dense suspensions of Brownian/non-Brownian, nonaggregating, monodisperse spherical or rod-like particles (2-6). This trend is further encouraged by recent experimental and theoretical studies which surmised that a strong ST will not be observed in flocculated suspensions because the attractive interactions that drive flocculation increase the yield stress and mask the ST behavior, leading to a monotonic shear-thinning viscosity curve (7-9). Consistent with this picture, a shear-thinning behavior is always seen, for example, in flocculated suspensions of carbon nanotubes (10, 11).The rheophysics of flocculated suspensions formed by colloidal particles with attractive interactions also raises several interesting issues. A universal jamming phase diagram proposed for attractive particles (12) with particle volume fraction Φ, interparticle attraction U, and stress σ as the control parameters is consistent with the nonequilibrium shear diagram for multiwalled nanotubes mapped out using rheooptical studies (11). Here, the nanotubes, which are entangled at low shear stress to exhibit a solidlike behavior, transform to a liquid-like state by dispersing the nanotubes that align along flow to form a nematic phase above a critical shear stress. However, the feasibility of achieving a reentrant jamming phase behavior by tuning the attractive interactions in these systems has not been examined. In fact, ST behavior was not known for flocculated suspensions until recently, where a modest continuous ST (whe...

Temperature and pH effects on the stability and rheological behavior of the aqueous suspensions of smart polymers based on N‐isopropylacrylamide, chitosan, and acrylic acid

“…At temperatures above the LCST of PNIPAM, PNIPAM‐CS showed a rise in apparent viscosity at low shear rates in all the aqueous media, followed by a decline in viscosity at higher shear rates (Figure 7(b,d,f)]. This behavior was also observed for a carbon nanotube suspension at the same concentration 94. The authors attributed the increase in the viscosity at low shear rates to a jammed state.…”

“…The formation of compacted particles can cause resistance to flow, increasing viscosity at low shear rates. When the shear rate applied is sufficiently high, associations between collapsed particles can be destroyed, resulting in shear thinning behavior 94–97. However, as LCST depends on hydrogen bonding ability among hydrophilic segments of the polymer and water molecules, the phase transition is affected by the addition of hydrophilic comonomers such as acrylic acid, and also by different pH environments 98, 99.…”

A nonsmooth model for discontinuous shear thickening fluids: Analysis and numerical solution

“…This suggest that the simulation of shear thickening fluids with very different viscosities could be accelerated using continuation with respect to ν. However, note that we are not primarily interested in the limit case of diverging viscosity ν → ∞ since most experiments with colloidal dispersions in the literature report on a finite increase in viscosity ranging from one to several orders of magnitude [5,10,26] Comparison with linear Stokes flow. In Figure 6.2, we compare the streamlines obtained with a linear Stokes model with those for a shear thickening fluid.…”

“…The behavior of a particle-fluid mixture can be modeled as non-Newtonian fluid with a shear thickening constitutive relation. While such a rheological behavior cannot be derived from first principles, experimental studies suggest that a shear thickening constitutive relation is a reasonable model across a range of different particle sizes, particle shapes, and volume fractions [2,4,5,10,26,34]. Thus, we consider a Stokes fluid in a two-or three-dimensional domain Ω, which shear thickens at high shear strain rates.…”

“…Next, we summarize related work, our main contributions, and the limitations of this work. Related Work: Understanding the phenomena responsible for shear thickening in suspensions is currently the subject of lively debates [2,4,5,10,20,26,35]. Possible explanation include the formation of particle hydroclusters, the expansion of particle collections when flowing past each other and compression-related jamming.…”