Rheological measurements obtained when the dimensions of the measurement confinement are of at least 2 orders of magnitude larger than the microstructure in the fluid are termed bulk properties.  Rheological measurements in smaller confinements will be different from the bulk measurements.  For many applications, measurements in small confinements are required.

For example, blood in the human body pulses through vessels from 2 cm to 5 microns in diameter.  The most abundant elements in human blood are the red blood cells which are biconcave disks with a diameter of 8 microns and thickness of 2 microns (0.0002 cm x 0.0008 cm).  

Human blood anticoagulated with EDTA was mixed with a solution of Dextran T110 (Mw  = 110,000 Daltons) in isotonic saline for a final Dextran concentration of 1% and final blood hematocrit of 38%.  As a result of the Dextran T110 the blood will strongly aggregate when in a quiescent state, forming aggregates consisting of  many red cells[1].  

Bulk viscosity and elasticity measurements were made in a 0.1 cm i.d (1000 microns) tube at a frequency of oscillation of 2 Hz and at 22 °C with the Vilastic-3 Viscoelasticity Analyzer.  In addition, measurements were made for the same blood and Dextran mixture with the Vilastic-3 and Vilastic Microcapillary System.  The Microcapillary System consisted of a bundle of  microtubes having diameters of 0.0050 cm or 50 microns.   In the microtubes the measured properties are not bulk properties because the diameter of the microtubes are only 6 red cell diameters. 

The figure shows  the bulk viscoelasticity measured in the large diameter tube and in the microtubes.  In the smaller confinement of the microtubes the red cells form smaller aggregates than in the large tube.  This is evident at low shear rates where the viscosity and elasticity measured in the microtubes is lower than was measured in the large diameter tube.  As the shear rate is increased in the large tube, the cells in the aggregates will disassociate into individual cells and as the shear rate is increased further, they will tend to organize into layers forming a "super-fluid" [2].  This is evident by the decreasing bulk viscosity and elasticity with shear rate.  In the microtubes, the viscosity and elasticity modestly decrease with increasing shear rate until a shear rate of 1000/sec when an increase is seen in both values.  This increase in viscosity and elasticity is termed as dilatancy.  The microstructure of the blood is impeded by the confinement of the microtubes preventing the blood from becoming a "super-fluid" as in the bulk. 

In the human body blood must flow in vessels comparable to the microtubes in this example and using only bulk rheological properties to understand or predict the behavior of strongly aggregating blood in the smaller vessels will not be successful.