A rotatable microfluidic device that comprises a hydrophilic microchannel structure in which there is a) an upstream microcavity I with a liquid outlet I, b) a microconduit I connected to liquid outlet I, and c) a capillary valve I associated with microconduit I. The inlet end of the microconduit is closer to the spin axis than the outlet end of the microconduit. The difference in radial distance between the inlet end and the outlet end of microconduit I is typically ≧5%, such as ≧10% or ≧100% or ≧500%, of the difference in radial distance between the uppermost part of the upstream microcavity and liquid outlet I.