For critical condition
Frictional force provides the centripetal force
\(m{\omega ^2}L = \mu \,R = \mu \,m \times {a_t} = \mu Lm\alpha \)
\(⇒\) \(m{(\alpha t)^2}L = \mu mL\alpha \) \(⇒\) \(t = \sqrt {\frac{\mu }{\alpha }} \) (As \(\omega = \alpha t\))