Determination of aggregation force in rouleaux by fluid mechanical technique

The shear stress needed to disaggregate red cell rouleaux was quantified in a flow channel under microscopic observation. Measurement were made on two-cell rouleaux formed in 4g% dextran with a mean molecular weight of 74,500. Stepwise increases in shear stress caused a progressive peeling of the top cell of the rouleau, and the percentage of separation as estimated from the exposed area of the bottom cell showed a sigmoidal relation with shear stress. The 50% separation occurred at a shear stress of 0.25 ± 0.01 dyn/cm² (mean ± SEM), which is approximately one-half of the value obtained in rotational measurements on bulk suspensions where the orientation of red cells is more random. This technique of studying stress-disaggregation relationships in a flow channel has the advantages that (a) the applied shear stress is parallel to the rouleau interface and (b) the degree and process of disaggregation can be followed directly by microscopic observation.