A Method To Estimate the Average Shear Rate in a Bubble Column Using Liposomes

We describe a novel approach to estimate average shear rate in an external-loop airlift bubble column, based on the shear stress-induced structural change in liposomes. As a shear-sensing probe, the 5(6)-carboxyfluorescein-containing liposome (CFL) was suspended at 40 degrees C in 50 mM Tris-HCl/50 mM NaCl buffer of pH 7.4 containing 0.3 wt % carboxymethyl cellulose. The CFL was 180 +/- 1 nm in mean diameter and physically stable under shear flow. The permeability coefficient (P-CF) of the dye molecules through the CFL membrane increased linearly as the shear rate (gamma) increased up to 1425 s(-1), as revealed using a cone-and-plate device. The P-CF value was then determined for the identical CFL suspension in the airlift operated at superficial gas velocity (U-G) values up to 0.03 m/s. The shear rate in the airlift could be reasonably estimated through correlating gamma with U-G, using their quantitative dependence on P-CF, obtained as above. The gamma value of 2.5 x 10(3) s(-1) was obtained at U-G = 0.03 m/s for the airlift.