YIELD STRESS MEASUREMENTS OF AQUEOUS FOAMS IN THE DRY LIMIT

This paper reports measurements of yield stress of aqueous foams appro aching the dry foam limit using a pendulum device. Traditionally, the vane rheometer has been used to measure the yield stress in liquids th at exhibit wall slip. However, using the simple and inexpensive pendul um technique, shear rates many orders of magnitudes lower can be achie ved. The pendulum was used to observe the change in yield stress for t he foam as the gas fraction and bubble size increased. The local gas f raction in the foam was found by measuring the sonic velocity, and the bubble size was determined photographically. Strong support is found for the existence of a true yield stress in aqueous foams at the dry f oam limit. Yield stress results, once scaled by alpha/[R], agree well with data from previous studies. (C) 1998 The Society of Rheology. [S0 148-6055(98)01006-2].