Shear-induced structuration of confined carbon black gels: steady-state features of vorticity-aligned flocs

Abstract

Various dispersions of attractive particles are known to aggregate into patterns of vorticity-aligned stripes when sheared in confined geometries. We report a thorough experimental investigation of such shear-induced vorticity alignment through direct visualization of carbon black gels in both simple plane shear and rotational shear cells. Control parameters such as the gap width, the strain rate, and the gel concentration are systematically varied. It is shown that in steady states the wavelength of the striped pattern depends linearly on the gap width h while being insensitive to both the gel concentration C and the shear rateγ. The width of the vorticity-aligned flocs coincides with the gap width and is also independent of C andγ, which hints to a simple picture in terms of compressible cylindrical flocs. Finally, we show that there exists a critical shear rateγc above which structuration does not occur and thatγc scales as h −α with α = 1.4 ± 0.1 independently of C. This extensive data set should open the way to quantitative modelling of the vorticity alignment phenomenon in attractive colloidal systems.

Flocculation of microfibrillated cellulose in shear flow

“…From Fig. 9, it can be estimated, however, that the diameter of some of the floc chains was greater than the gap (1 mm), which is different from the carbon nanotube (Lin-Gibson et al 2004;Hobbie and Fry 2006;Ma et al 2007) and carbon black suspensions (Grenard et al 2011). …”

“…This kind of shear-induced formation of vorticity aligned floc cylinders in a confined space has been observed, for example, in carbon nanotube suspensions (Lin-Gibson et al 2004;Hobbie and Fry 2006;Ma et al 2007) and carbon black gels (Grenard et al 2011) with optical microscopy. It has been suggested that the cylindrical flocs undergo a log-rolling movement, and they are a consequence of elastic instability and large negative first normal stress differences Montesi et al 2004) or aggregation confined by the gap size ).…”

Microstructural Rearrangements and their Rheological Implications in a Model Thixotropic Elastoviscoplastic Fluid

“…When combined with multibody hydrodynamic effects in these fluids [11], the resulting microstructure-flow relationship becomes complex and may show long time scale transient behavior and multiple steady states [1,12]. This leads to a wide range of timedependent responses that can also be observed, including microphase separation [13], vorticity aligned structure formation [14][15][16], local rigid plug formation and shear banding [17], plus shear-induced rejuvenation of the particle network [18].…”

“…When combined with multibody hydrodynamic effects in these fluids [11], the resulting microstructure-flow relationship becomes complex and may show long time scale transient behavior and multiple steady states [1,12]. This leads to a wide range of timedependent responses that can also be observed, including microphase separation [13], vorticity aligned structure formation [14][15][16], local rigid plug formation and shear banding [17], plus shear-induced rejuvenation of the particle network [18].Although the general form of the flow curve (relating the shear stress to shear rate) and some transient phenomena of TEVP fluids can be described by detailed continuum models [3,[19][20][21], a clear and fundamental understanding of the underlying mechanisms requires connecting macroscopic material properties to events that happen at the microstructural level. Experimentally, making this connection is very challenging because monitoring of the entire microstructure under flow is essential, together with elaborate protocols to produce well-defined initial conditions [22].…”

Avalanche-like fluidization of a non-Brownian particle gel

“…33,34 Moreover, as an illustration of the impact of confinement on flows of attractive gels, one can mention the spectacular shear-induced structuration observed at moderate shear rates and reported in silica suspensions, 35 gels of anisotropic particles, 36 attractive emulsions 37 and carbon black gels. [38][39][40] In such cases, the gel rearranges to form a striped pattern of logrolling flocs aligned along the vorticity direction, whose origin and formation mechanism are still highly debated. 41 Beyond the effects of bounding walls and confinement, attractive interactions alone are also responsible for long-lasting transients under external shear.…”