Grenard, V., Taberlet, N., & Manneville, S. (2011). Shear-induced structuration of confined carbon black gels: steady-state features of vorticity-aligned flocs Soft Matter DOI: 10.1039/C0SM01515F

Conventional rotational rheometers allow test procedures of varying shear strain/stress, temperature and time, corresponding to multiple “sweep” tests. Modern rheometers are indeed designed to accurately vary these parameter according to command. Besides these conventional sweepable variants, comparison among different geometries (in bulk materials and surface) was also done by many researchers, though such experiments can only be done discretely; you cannot do a “geometry sweep”.

There are also cases when the gap confining the sample plays an significant role in its rheological propertes.

One of such cases is merely a form of experimental error. When we interpret the experimental result from a plate-plate geometry we assume that the two plates are perfectly concentric and parallel. Practically the two plates must have an extent of imperfection, and a tilted height between the two parallel always exists, although it is often negligible compared to the most used gap values. When the gap gets smaller until a value comparable to the tilted height of the plate-plate geometry, the inevitable effect is a gap dependency of the result. Procedures have been proposed by many authors to model the effect of this nonparallel imperfection on the result of viscometry. A common experimental technique to probe this effect is to run a series of tests at different gaps with a Newtonian fluid. In this case the gap dependency gives information of equipment rather than the tested samples.

There should be interesting gap dependence rheology that are not resulted from instrument imperfection but the confinement effect of the tested sample. Clasen and McKinley developed a device specifically for measuring complex fluids (which are often heterogeneous with micron-scaled structrures) between very small gaps, and indeed observed gap-dependent viscosity and yielding phenomena at a shear rate much lower than conventional rheometers can reach. The gap-dependent results are specifically from a very small gap limit.

Is there gap-dependent phenomena in the gap range of conventional rheometry? Recently I noticed the reported observation of shear alignment of carbon black suspensions, which show a gap-dependence topology. Carbon black particles suspended in a light mineral oil tend to align into parallel stripe of flocs under shear field. Counterintuitively, the stripe’s dimension is independent of shear rate, particle fraction, and location of observation. The only significant factor that found to affect the stripe width is the gap of shear geometry. Particularly, a power law dependence was observed. The simplicity of physics here is in itself beautiful. And the finding is also indicative in manufacture technique of width control stripes of anything. However, the author did not measure the effect of such shear induced structuring on the viscoelasticity of the suspension. It is equally interesting to know whether or not the stripe structures affect the viscosity of the bulk suspension.