Experimental Study of Pressure Loss and Rheo-Optical Behavior of CTAB/NaSal Aqueous Solution under Elongational Flow

Abstract

In a previous paper we clarified the behavior of pressure losses ΔP for aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in an elongation dominant flow through slots: the flow curves relating ΔP to the apparent elongational rate consisted of three characteristic regions. In particular, ΔP increased largely in a specific region II, and this characteristic was suggested to be closely connected to the flow induced structure (FIS). However, it could not be asserted that FIS solely causes the large increase in ΔP, because vortices were also found in the flow upstream of the slot in region II and such vortices are known to induce large pressure drops frequently. In the present experiment, we used a nozzle providing almost the same constant elongational rate as the slot in order to eliminate the influence of vortices on ΔP, estimated elongational rates of the upstream flow, compared ΔP for both the slot and the nozzle, and concluded that vortices existing in the region upstream of the slot do not give a significant effect on ΔP. Next, the image of small angle light scattering (SALS) was examined to clarify the developing process of FIS: A butterfly-type and streak-type combined pattern in SALS occurs in region II, indicating the generation of FIS; the streak-type pattern degenerate at higher elongational rates in region III, meaning a partial collapse of FIS generated in region II; change in FIS depends not only on strain rate but also on strain in elongational flow. Moreover it was found that FIS shows a step-like development in the upstream flow of constant elongational rate.