Microfluidic systems are widely used in fundamental research and industrial applications due to the unique behavior, enhanced control and manipulation opportunities of liquids in constrained geometries. In micrometer-sized channels, electric fields are efficient mechanisms for manipulating liquids, leading to deflection, injection, poration or electrochemical modification of cells and droplets. While PDMS-based microfluidic devices are commonly used due to inexpensive and facile fabrication, they are strongly limited in terms of electrode integration. By using silicon as channel material, state-of-the-art micro-fabrication techniques can be employed to create nearby, well-defined and stable electrodes. Despite the advantages that silicon offers, its opacity has so far prevented its usage in the most important microfluidic applications that all require optical access. To overcome that hurdle, silicon-on-insulator technology in microfluidics is introduced here for the first time to create optical view ports and channel-interfacing electrodes. More specifically, microfluidic channel walls are directly electrified by selective, nanoscale etching to introduce insulation segments inside the silicon device layer, thereby achieving the most homogeneous electric field distributions and lowest operation voltages feasible across microfluidic channels. These ideal electrostatic conditions enable a drastic energy reduction as it is exemplarily demonstrated by picoinjection and fluorescence-activated droplet sorting applications at voltages below 6 V, and 15 V, respectively, paving the road towards low-voltage electric field applications in next-generation microfluidics.