A method for measuring properties of a fluid including placing a quantity of fluid in a container; inducing flow in the fluid wherein the flow is substantially streamlined in at least a measuring region of the container by constricting flow, and wherein the fluid is recirculated through the measuring region. A mixing region can be created separate from the measuring region sufficient to substantially mix the fluid. Free stream particulates in the fluid can be measured in the streamlined region. The invention is of particular interest in the assessment of blood platelet function. The method provides specific localized regions of thorough mixing that enable reproducible platelet aggregation, and also provides specific localized regions of streamlined flow that enable certain modalities of assessing aggregation. Both of these regions of flow are induced such that damage to platelet aggregates and other blood components as well as undesirable agglomeration on device surfaces is minimized.