Apparatus for processing liquids or liquid-based substances includes a plurality of volumes at least two of which are defined at least in part by one or more phaseguides inside the volume and/or in a conduit connected thereto for controlling aliquoting of one or more liquids or liquid-based substances inside the volume. Each volume has an upstream and downstream side with respect to meniscus advancement direction via which it may be filled with or emptied of one or more liquids or liquid-based substances. The apparatus also includes at least one common upstream-side conduit connected to supply a liquid or liquid-based substance via a plurality of the inlet or extraction conduits, a plurality of the phaseguides exhibiting a predetermined level of stability and one or more of the phaseguides exhibiting a predetermined different stability compared with the stability of at least one of the other phaseguides whereby to control the preference order in which the volumes fill and/or empty. The stability is determined by the value and radius of an acute angle along a said phaseguide at the downstream side of the phaseguide.