Experimental Observations on Depletion Before the

Red blood cells (RBCs) are biconcave particles and their detailed shape and size depend on the RBC type. The human RBC may be considered a disc with a diameter of 6.6 pm and a thickness of 2 pm, its volume thus being of the order of 10 pm. The RBCs occupy about 40 to 50 vol % of our blood. 

Demonstrate stacking all red blood cells in a human being (having about 5 L of blood) in a single column provides a RBC cylinder with a height that is of the order of the earth s circumference. 

Enhanced RBC aggregation can be detected for instance by measuring the sedimentation rate. The sedimentation rate varies between 1 and 3 mm/h for healthy blood up to 100 mm/h in case of severe illnesses. The blood sedimentation test, based on monitoring aggregation of red blood cells, became a standard method for detecting illnesses. The relation between pathological condition, RBC aggregation and enhanced sedimentation rate, has been known for at least two centuries, as described in [69, 70]. 

Some authors interpret rouleaux formation as being caused by bridging of RBCs by semm proteins. There is however no evidence for protein adsorption onto RBCs. A smdy on rouleaux formation in mixtures of human RBCs (diameter 6.6 pm) and rabbit RBCs (diameter 7.8 pm) resulted in rouleaux stmctures that consisted (mainly) of only a single type of RBC [71]. This can be explained by a depletion effect (the overlap volume, hence entropy, is maximized if similar RBCs stack onto each other). In case of bridging, however, mixed aggregates are expected, so there is little support for the bridging hypothesis [72]. The general picture is that red blood cells tend to cluster at elevated concentrations of the blood semm proteins, which act as depletants. 

Traube [80] showed that adding plant and seaweed polysaccharides led to a phase separation between an extremely dilute and a very concentrated phase. Since the particles are lighter than the solvent the concentrated phase, with volume fraction 0.5 / 0.8, floats on top. The Iowct phase is clear and hardly contains particles. Baker [81] and Vester [82] systematically investigated the mechanism that leads to what they called (enhanced) creaming. 

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