  Liquid in grooved capillaries, made by e.g. inserting a plate in a
cylindrical tube, exhibits unusual spreading and flow properties. One example
is capillary rise, where a long, upward tongue on top of the usual meniscus has
been observed along the groove. We attribute the underlying mechanism to a
thermodynamic instability against spreading for a (partial or complete wetting)
liquid in a sharp groove whose opening angle $\alpha$ is less than a critical
value $\alpha_c=\pi-2\theta$. The equilibrium shape of the tongue is determined
analytically. The dynamics of liquid rising is studied in the viscous regime.
When the diameter of the tube is smaller than the capillary length, the center
part of the meniscus rises with time $t$ following a $t^{1/2}$-law, while the
tongue is truncated at a height which grows following a $t^{1/3}$-law. Sharp
groove also facilitates release of gas bubbles trapped inside a capillary under
the action of gravity.
