Experimental Study on Pressure Loss of CTAB/NaSal Aqueous Solution through Slots and a Capillary

Abstract

Aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) were made to flow through two-dimensional slots and a capillary, and pressure losses ΔP were measured under various conditions to investigate its flow properties. In the results with slot flow, the flow curves relating ΔP to the apparent elongational rate consisted of three characteristic regions. In the first region of low flow rate, the flow curve showed a gentle slope with the pressure loss increasing with elongational rate. In the second region of intermediate flow rate, ΔP increased sharply, which was thought to be caused by flow induced structure (FIS). In the third region of relatively high flow rate, the slope of the flow curve became gentle again. Here, it should be noted that ΔP in the third region was much higher than that in the first region; for example, the third region provides one order of magnitude larger ΔP than the first region for the concentration of CTAB (C_d) = 3×10^-2 mol/L solutions, and it gives two or three orders of magnitude larger ΔP than the first region for C_d = 5×10^-4 mol/L solutions. Furthermore, the flow field was visualized using reflex powders and a laser light sheet in slot flows. Observations of the flow field indicated that there are three types of flow field corresponding to the three regions of the flow curve. The first region showed a Newtonian-like flow field. The second region exhibited vortices and a contraction flow upstream of the entrance. In the third region, the vortices and the contraction flow fluctuated with increase in elongational rate. On the other hand, experiments regarding the pressure loss in capillary flow for C_d = 3×10^-2 mol/L solutions showed the same character as slot flow. For C_d = 5×10^-4 mol/L solutions, however, the character of the capillary flow curve was completely different from that of slot flow, and the flow curve of capillary flow was almost a normal type of Newtonian viscous flow. Consequently, we postulated that a considerable FIS is generated in the flow around the inlet of the capillary, but it collapsed in the downstream capillary flow for such dilute solutions as C_d = 5×10^-4 mol/L.