If an air bubble of diameter 2 mm rises steadily through a liquid of density \(2000kg/{m}^{3}\) at a rate of \(0.5cm/s\)…

For steady rise, viscous force balances buoyant force:

\(6\pi \eta rv=\frac{4}{3}\pi {r}^{3}\rho g\)

\(\eta =\frac{2{r}^{2}\rho g}{9v}\)

\(r=1mm={10}^{-3}m,v=0.5cm/s=5\times {10}^{–3}m/s\)

\(\eta =\frac{2{\left({10}^{-3}\right)}^{2}\times 2000\times 10}{9\cdot 5\times {10}^{-3}}=\frac{2\times {10}^{-6}\times 20000}{45\times {10}^{-3}}\)

\(=\frac{0.04}{0.045}\approx 0.88Pa=8.8\) poise