Droplet Production and Handling in Microchannels Using Electric Fields

Droplet-based protocols in microfluidic devices have found numerous applications in such different areas as bioanalytics, chemical synthesis, drug delivery etc. Droplets can either be produced in a continuous stream or on-demand. Employing an active technique via applying external sources such as temperature, acoustic, magnetic or electric fields, potentially in combination with a passive technique, could enhance the utility and controllability of droplet generation. Among these approaches, probably the most versatile and flexible one is based on the application of electric fields, because electric actuation tends to be faster and requires less complex components than mechanical actuation. This thesis addresses electrically manipulation of droplets inside microchannels generated both on-stream and on-demand along with some particular applications such as using the droplets as biological reaction compartments or as carriers to transfer tiny amounts of dissolved species. For the on-stream case, the effect of DC electric fields on the generation of droplets of water and xanthan gum solutions in sunflower oil at a microfluidic T-junction is experimentally studied. The electric field leads to a significant reduction of the droplet diameter, by about a factor of 2 in the case of water droplets. The droplet size is tuned by varying the electric field strength, an effect that can be employed to produce a stream of droplets with a tailor-made size sequence. Compared to the case of purely hydrodynamic droplet production without electric fields, the electric control has about the same effect on the droplet size if the electric stress at the liquid/liquid interface is the same as the hydrodynamic stress. The focus of the thesis, however, is the manipulation of droplets generated on-demand via electric fields. In the first scenario for droplets being utilized in the context of artificial genetic circuits in biological systems as outlined by the LOEWE CompuGene project (managed by TU Darmstadt), a method is presented allowing to produce monodisperse droplets with volumes in the femtoliter range in a microchannel on demand. The method utilizes pulsed electric fields deforming the interface between an aqueous and an oil phase and pinching off droplets. Water and xanthan gum solutions are considered as disperse-phase liquids, and it is shown that the method can be applied even to solutions with a zero-shear rate viscosity more than 104-times higher than that of water. The droplet formation regimes are explored by systematically varying the pulse amplitude and duration as well as the salt concentration. The dependence of the process on the pulse amplitude can be utilized to tune the droplet size. To demonstrate the applicability of the electric-field-driven droplet generator, it is shown that the droplets can be used as versatile biological reaction compartments. It is proven that droplets containing a cell-free transcription–translation system execute gene transcription and protein biosynthesis in a timely and programmable fashion. Moreover, it is verified that biomolecules inside the aqueous droplets such as small RNAs can be diffusionally activated from the outside to induce a ligand-driven biochemical switch. In another scenario of using droplets as carrier, adding and subtracting the smallest amounts of liquid in a well-controlled manner is a key step. A principle is demonstrated allowing the transfer of tiny amounts of dissolved species to an aqueous femtoliter droplet reciprocating between two aqueous reservoirs (or interfaces) under the influence of a DC electric field. Mass transfer is shown to be size selective and adaptive, for example, via tuning the viscosity of the surrounding oil phase or the electric-field strength. A map of the dynamic regimes is provided, indicating under which conditions the reciprocating droplet motion occurs. A model based on diffusive mass transfer is formulated that describes the amount of species taken up and transferred by the droplet. Interestingly, in some cases, the droplets reciprocating between two aqueous interfaces show simultaneously volume losses (at most contacts with the reservoirs) under certain conditions, a phenomenon called ‘partial coalescence’. Accordingly, a scaling model is provided allowing the prediction of daughter droplet size during partial coalescence. Overall, the results significantly help to facilitate the handling, production and manipulation of femtoliter droplets