Small-Scale Vane Rheometry

Vane rheometry continues to become more well-known and popular. Heterogeneous samples, particularly those comprising solid fillers, highly structured materials that yield, and materials that slip, are some general classes in which vane spindles are useful when other geometries do not provide acceptable measurements. Although vanes may be inserted into sample containers and tests run directly, the shear rates may be undefined as material behavior varies across large spindle-edge-to-container-wall gaps. Additionally, some customer applications require small volumes of sample. One such example is the periodic testing of small amounts of valuable pharmaceutical, gel-like products. Therefore, the use of small vanes, in small cylindrical containers, having both a reasonably narrow gap for relatively well-defined shear rates and conservation of precious samples, is rheologically useful geometry for rheological analysis.

Figure 2 - Vane-in-cylinder configuration in a small sample adapter. a) The accessory mounted on the viscometer, and b) close-up of vane immersed in water in the accessory.

Figure 1 - Vane-in-cylinder geometry. a) Side-view schematic, and b) end-on view, showing how the rotating four-bladed vane sweeps out a cylinder of sample material and deforms the rest of the bulk material.

Experimentalists—whether they are technicians, chemists, engineers, or others— often prefer to test small quantities of materials. There are various reasons, such as smaller amounts to clean up, sample cost, and availability of limited quantities. If viscosity is needed as a function of shear rate, then an additional constraint—well-defined geometry—is added. Viscosity-shear rate curves are often required for pump and process instrumentation sizing. Furthermore, if the materials slip and/or have yield behavior, then traditional smooth geometries, such as coaxial cylinder, cone-plate, or parallel-plate, may have difficulty obtaining meaningful results.

One possible solution is to use “vane-in-cylinder” geometry. The vane blades stick out into and “grab” the sample. The cylindrical sample chamber provides a finite boundary. This well-defined geometry permits shear rates to be calculated, at least to reasonable approximations (see Figure 1a and b). In one particular instance, pharmaceutical samples, retained for aging stability studies, are periodically taken and tested over time. The use of a small vane-in-cylinder geometry allows a small sample quantity of precious material to be tested while minimizing slip. Thus, the remaining “retain” may be kept, well-sealed, in its original container, such as a squeezable tube.

Figure 2a and b are photographs of one vane-in-cylinder configuration used in a small sample adapter. The vane spindle is connected with a quick-connect accessory to the viscometer/rheometer coupling, and the sample chamber is held in a water jacket. The jacket, in turn, may be connected to a bath providing temperature control.

Figure 3 - Vane-in-cylinder results for petroleum jelly.

Figure 4 - Test results for a viscous lotion using vane-in-cylinder geometry.

Petroleum jelly or petrolatum is a commercially important material. It is often sold neat, or used as a base for various ointments or creams. These materials often exhibit significant slip. Therefore, vanes may provide better data than smooth geometries. Figure 3 shows data acquired by testing petroleum jelly.

The rising “viscosity” value actually reflects increasing wind-up of the instrument torque sensor as the motor rotates, while the sample resists the vane rotation. However , once the torque reaches a maximum, the material yields. The yield point is the point at which the solid-like material begins to flow like a liquid, that is, the sample structure breaks down, and this results in the viscosity decreasing over time after the yield point. This will therefore be referred to as an apparent viscosity.

Figure 4 shows the behavior of a viscous lotion tested with vane-in-cylinder geometry as well. The behavior of the lotion is in some ways similar to that of the petroleum jelly. The material yields and the measured viscosity decreases over time as the structure breaks down.

Figure 5 - Cut, disposable sample chamber immersed in peanut butter.

The operator may be concerned about irreversibly destroying structure before testing in some materials. There is an alternative to removing sample from its original container and scooping/packing it into a sample chamber. The closed end may be cut off from a disposable sample chamber, for example, and this hollow cylinder is then inserted vertically down into the sample. The spindle is then centered over this area and lowered into the material. This geometry again provides vane rotation within a known, well-defined, circular boundary. This in turn allows testing at known shear rates. Figure 5 shows an example in which a cut chamber is immersed in peanut butter. Results from this type of test are shown in Figure 6.

Figure 6 - Test results for vane-in-cylinder geometry using a cut, disposable sample chamber immersed in peanut butter.

Some materials have such severe structural breakdown and/or slip after yield that the apparent viscosity may eventually become almost negligible afterward. Other materials, such as the lotion and peanut butter discussed above, may have sufficient structure after yielding so that their apparent viscosity approaches a steady-state or “nearly constant” value. This may be referred to as the “steady-state viscosity after yield.” In sum, vane-in-cylinder geometry proves useful for testing small amounts of materials within a well-defined geometry. It provides meaningful data for materials that may exhibit slip and for those that may yield.

Dr. Moonay is Sales Engineer—Rheology Laboratory Supervisor, Brookfield Engineering Laboratory, Inc., 11 Commerce Blvd., Middleboro, MA 02346, U.S.A.; tel.: 508-946-6200, ext. 144; e-mail: d_moonay@brookfieldengineering.com; www.brookfieldengineering.com. The author thanks his colleagues, Luis Botelho (Sales Engineer) for performing the experiments per his request, and David A. DiCorpo (General Manager—Laboratory Sales) for suggesting the cut-end disposable chambers to provide a well-defined boundary.

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