The conformation of drag reducing micelles from small-angle-neutron-scattering experiments

Abstract

Small-angle-neutron-scattering measurements (SANS) were done with a dilute solution of the surfactant n-tetradecyltrimethylammoniumbromide (TTAB) with sodium salicylate (Na Sal) in pipe and channel flow. The solvent used was heavy water. The anisotropy observed in the SANS-curves can be related to the drag reduction behaviour of the surfactant solution. The breakdown of the drag reducing properties above the critical wall shear stress is accompanied by a strong decrease in the scattering anisotropy. However, in this flow region the scattering curves reveal a very similar micellar structure to that found in the fluid at rest. This shows that the micelles exist above the critical wall shear stress.

Secondary flows in turbulent surfactant solutions at maximum drag reduction

“…This result is consistent with investigations by SANS (small angle neutron scattering) on tetradecyltrimetylammoniabromide and salicilate of sodium in D 2 O at maximum drag reduction (Bewersdorff and Gyr [10], Bewersdorff et al [11], as well as Lindner et al [12]). These measurements showed that the micelles are fully aligned in the flow direction at maximum drag reduction, which leads to a suppression of transverse velocity fluctuations, transverse transport of turbulent momentum, and of the secondary flow.…”

The behaviour of drag-reducing cationic surfactant solutions

“…Even in pipes of larger diameters Virk's [34] maximum drag reduction asymptote can be approached [7,9,12,13,32]. This agrees with [7] which showed that the time constants involved in the formation of rod-like micelles are on the order of 103-104s for the surfactants used in [35]. This agrees with [7] which showed that the time constants involved in the formation of rod-like micelles are on the order of 103-104s for the surfactants used in [35].…”

“…In a surfactant solution at rest these rod-like micelles are oriented statistically [35], i.e., meaning they have no preferred orientation. 1), whereas the diameter of the rods is about 3.5 nm as estimated by small-angle-neutron scattering [35]. 1), whereas the diameter of the rods is about 3.5 nm as estimated by small-angle-neutron scattering [35].…”

“…The turbulent friction behaviour depends on the size, number, and surface charge of the micelles [7,23]. Recent small angle neutron scattering experiments [35] in a turbulent channel flow demonstrated that micelles are not destroyed by excess critical wall-shear stress, but that their orientation and superstructures break down instead. Recent small angle neutron scattering experiments [35] in a turbulent channel flow demonstrated that micelles are not destroyed by excess critical wall-shear stress, but that their orientation and superstructures break down instead.…”

The Structure of the Drag Reduced Turbulent Flow over a Backward Facing Step — an experimental study

“…The chosen geometry of a backward facing step supplements investigations of other turbulent flows, e.g. the plane mixing layer (Kwade 1982) or pipe flow (Bewersdorff et al 1986). …”