Electrowetting has become one of the most widely used tools to manipulate tiny amount of liquids on surfaces. Applications range from lab-on-a-chip devices to adjustable lenses or new types of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both the contact angle reduction as well as the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects -rather than a unique one -is responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can not only give rise to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fiber optics, and biotechnology-related microfluidic devices.3

The explosive growth in our knowledge of genomes, proteomes, and metabolomes is driving ever-increasing fundamental understanding of the biochemistry of life, enabling qualitatively new studies of complex biological systems and their evolution. This knowledge also drives modern biotechnologies, such as molecular engineering and synthetic biology, which have enormous potential to address urgent problems, including developing potent new drugs and providing environmentally friendly energy. Many of these studies, however, are ultimately limited by their need for even-higher-throughput measurements of biochemical reactions. We present a general ultrahigh-throughput screening platform using drop-based microfluidics that overcomes these limitations and revolutionizes both the scale and speed of screening. We use aqueous drops dispersed in oil as picoliter-volume reaction vessels and screen them at rates of thousands per second. To demonstrate its power, we apply the system to directed evolution, identifying new mutants of the enzyme horseradish peroxidase exhibiting catalytic rates more than 10 times faster than their parent, which is already a very efficient enzyme. We exploit the ultrahigh throughput to use an initial purifying selection that removes inactive mutants; we identify ∼100 variants comparable in activity to the parent from an initial population of ∼10 7 . After a second generation of mutagenesis and high-stringency screening, we identify several significantly improved mutants, some approaching diffusion-limited efficiency. In total, we screen ∼10 8 individual enzyme reactions in only 10 h, using < 150 μL of total reagent volume; compared to state-of-the-art robotic screening systems, we perform the entire assay with a 1,000-fold increase in speed and a 1-million-fold reduction in cost.

We describe a highly efficient microfluidic fluorescence-activated droplet sorter (FADS) combining many of the advantages of microtitre-plate screening and traditional fluorescence-activated cell sorting (FACS). Single cells are compartmentalized in emulsion droplets, which can be sorted using dielectrophoresis in a fluorescence-activated manner (as in FACS) at rates up to 2000 droplets s(-1). To validate the system, mixtures of E. coli cells, expressing either the reporter enzyme beta-galactosidase or an inactive variant, were compartmentalized with a fluorogenic substrate and sorted at rates of approximately 300 droplets s(-1). The false positive error rate of the sorter at this throughput was <1 in 10(4) droplets. Analysis of the sorted cells revealed that the primary limit to enrichment was the co-encapsulation of E. coli cells, not sorting errors: a theoretical model based on the Poisson distribution accurately predicted the observed enrichment values using the starting cell density (cells per droplet) and the ratio of active to inactive cells. When the cells were encapsulated at low density ( approximately 1 cell for every 50 droplets), sorting was very efficient and all of the recovered cells were the active strain. In addition, single active droplets were sorted and cells were successfully recovered.