We derive a model for voltage-induced wetting, so-called electrowetting, from the principle of virtual displacement. Our model includes the possibility that charge is trapped in or on the wetted surface. Experimentally, we show reversible electrowetting for an aqueous droplet on an insulating layer of 10 µm thickness. The insulator is coated with a highly fluorinated layer impregnated with oil, providing a contact-angle hysteresis lower than 2 • . Analyzing the data with our model, we find that until a threshold voltage of 240 V, the induced charge remains in the liquid and is not trapped. For potentials beyond the threshold, the wetting force and the contact angle saturate, in line with the occurrence of trapping of charge in or on the insulating layer. The data are independent of the polarity of the applied electric field, and of the ion type and molarity. We suggest possible microscopic origins for charge trapping.

We demonstrate control of fluid motion in three-dimensional structures with thousands of microchannels. Fluids are manipulated via an electrocapillary pressure, originating from electrostatic control of the solid/fluid interfacial tension in the microchannels. Reversible fluid displacement has been achieved for all channel orientations with respect to gravity. The velocities of several centimeters per second are nearly two orders of magnitude higher than the velocities demonstrated by other electrofluidic actuation principles.

The realization of biomolecular detection assays for diagnostic purposes is technologically very challenging because such tests demand full integration for ease of use and need to deliver a high analytical performance with cost-effective use of materials. In this article an optomagnetic immunoassay technology is described based on nanoparticles that are magnetically actuated and optically detected in a stationary sample fluid. The dynamic control of nanoparticles by magnetic fields impacts the key immunoassay process steps, giving unprecedented speed, assay control and seamless integration of the total test. The optical detection yields sensitive and multiplexed assays in a low-cost disposable cartridge. We demonstrate that the optomagnetic technology enables high-sensitivity one-step assays in blood serum/plasma and whole saliva. Drugs of abuse are detected at sub-nanogram per millilitre levels in a total assay time of 1 min, and the cardiac marker troponin I is detected at sub-picomole per litre concentrations in a few minutes. The optomagnetic technology is fundamentally suited for high-performance integrated testing and is expected to open a new paradigm in biosensing.