Experimental study of foam rheology in straight capillary tubes

A fully automated capillary tube viscometer with four different interchangeable capillary tubes was constructed to study the rheological behavior of anionic and non-ionic foams flowing as constricted single lamellae in the experimental tubes. Pressure drop and flow rate over the tubes data were recorded as functions of time and these data further analyzed to obtain characteristic flow curves, foam quality, (G), ranging from 75-97%, where G = V_g/(V_g + V₁). The study has shown that the flow behavior of foam in capillary tubes is complex and no individually unique flow curve may be used to describe all the results obtained for the foaming solution studied. In this multiparameter problem, however, most of the data could be empirically described by a Bingham plastic flow model, in the standard fluid equation of the form: t = t_y + k(su/sr)ⁿ, where k and n were constants for a given flow curve, dependent on a range of experimental variables. It has been found that the yield stress increased with increasing foam quality and was also a function of the capillary tube diameter d. For anionic foams the yield stress decreased while in non-ionic foams the opposite effect was observed as the diameter of the tube increased. Similarly, the apparent foam viscosity (equation here) increased with both foam quality and tube diameter, but decreased with increase in shear rate. Finally, when anionic and non-ionic foaming solutions were compared the apparent viscosity of the anionic foam under similar conditions was found to be approximately twice that of the non-ionic foam. The need for a deeper physico-chemical understanding of lamellae flowing in capillary tubes is identified, so that proper design techniques for such systems may be developed.