Microbubbling viscous liquids and suspensions

Using a T-junction together with a cross flow technique, we have carried out a detailed study on the formation of near-monodisperse microbubbles in liquids with viscosities in the range of 5-950 mPa s. The data collected were analysed in the context of the classical momentum equation for viscous liquid flow to propose an analytical equation correlating dimensionless viscosity ratio ($\mu _{l}$/$\mu _{g})$ to the ratio of liquid pressure to gas pressure (P$_{l}$/P$_{g})$ required to generate bubbles. This equation is useful in predicting P$_{l}$/P$_{g}$ for microbubbling a liquid having a known viscosity. Our experimental results show that in the liquids investigated, the ratio of P$_{l}$/P$_{g}$, which is a function of dynamic equilibrium of pressure of liquid and gas at the T-junction, is decreasing proportional to dimensionless viscosity ratio. We calculated radial pressure for a given liquid pressure (P$_{l})$ to establish that for liquid viscosities $\ge 48.5$mPa s the radial velocity of liquid, which is responsible for imposing radial pressure on the gas-jet, dominates the mechanism of microbubble pinch-off. In contrast, in the low viscosity regime ($\le 48.5$mPa s), deceleration of the gas stream from the initial velocity is largely the cause of pinch-off of microbubbles. We made ceramic liquid foams using the technique.