Pseudo-laminarization of surfactant solutions in capillary flows

Abstract

Wormlike micelle surfactant solutions showed turbulent drag reduction effects in pipe flows. It was well-known that SIS (Shear Induced Structure), reported in previous studies, made them. However, inner diameters (characteristic length) of pipes used in the previous studies were in the order of millimeter. Thus, shear rates were limited to the order of 10 4 s −1 at a maximum. In the present study, flow properties of water and wormlike micelle surfactant solutions were investigated at higher shear rates using capillaries whose inner diameters ranged from 133 µm to 2.87 mm. Because the observed Reynolds number estimated for the micron-sized capillaries ranged from 10 2 to 10 4 , drag reduction effects were observed as a pseudo-laminarization, which is a phenomenon that transition from laminar flows to turbulent flows with the increase in the Reynolds number delays. Viscosity measurements using a capillary method indicated that the surfactant solution used in the present study had non-Newtonian viscosity. Therefore, the Reynolds number of the surfactant solution flow was estimated by the generalized Reynolds number. By using a jet thrust method, elastic properties of wormlike micelle surfactant solutions were measured in the corresponding to the viscosity measurement. For water, the resultant pressure drops (the frictional coefficient of pipes) agreed with both the prediction of laminar flows and the Blasius expression. diameters. In other words, these results suggested that the pseudo-laminarization was occurred in the capillary flows. Moreover, the relationship between the shear rate at which the surfactant viscosity asymptotically approached to the water one and the transition to turbulent flows was discussed to clarify their correlation.