Gel electrophoresis is one of the most applied and standardized tools for separation and analysis of macromolecules and their fragments in academic research and in industry. In this work we present a novel approach for conducting on-demand electrophoretic separations of DNA molecules in open microfluidic (OM) systems on planar polymer substrates. The approach combines advantages of slab gel, capillary-and chip-based methods offering low consumable costs (< 0.1$) circumventing cost-intensive microfluidic chip fabrication, short process times (5 min per analysis) and high sensitivity (4 ng/mu L dsDNA) combined with reasonable resolution (17 bases). The open microfluidic separation system comprises two opposing reservoirs of 2-4 mu L in volume, a semi-contact written gel line acting as separation channel interconnecting the reservoirs and sample injected into the line via non-contact droplet dispensing and thus enabling the precise control of the injection plug and sample concentration. Evaporation is prevented by covering aqueous structures with PCR-grade mineral oil while maintaining surface temperature at 15 degrees C. The liquid gel line exhibits a semi-circular cross section of adaptable width (similar to 200-600 mu m) and height (similar to 30-80 mu m) as well as a typical length of 15-55 mm. Layout of such liquid structures is adaptable on-demand not requiring time consuming and repetitive fabrication steps. The approach was successfully demonstrated by the separation of a standard label-free DNA ladder (100-1000 bp) at 100 V/cm via in-line staining and laser induced fluorescent end-point detection using an automated prototype.