Rise of pulsating bubbles in fluids with a yield stress

Liquids important to the process industries sometimes have a yield stress, i.e., they only start flowing if a certain level of shear stress is reached . This can limit processes which require mass transfer to or from bubbles d ispersed in the liquid, because babbles below a certain size can get stuck in the liquid. One possibility of making such bubbles rise is investigated in this work. It consists of subjecting the liquid containing the bubbles t o an oscillating external pressure. The deformation outside the babbles cau ses the yield stress to be overcome and bubbles to rise which would otherwi se have remained stationary. This effect has been investigated both theoret ically and experimentally. An analytical model for the rise of pulsating bu bbles in a Bingham medium, commonly used to model fluids with a yield stres s, is derived. Experimental results Sor an aqueous gel show that the effect discussed does exist and is in good qualitative agreement with the model. MSC (1991): 76D99.