This communication describes the controlled generation of double emulsions in the gas phase, which was carried out using an integrated emitter in a PDMS (poly(dimethylsiloxane)) microfluidic chip. The integrated emitter was formed using a molding approach, in which metal wires with desirable diameters were used as emitter molds. The generation of double emulsions in air was achieved with electrohydrodynamics actuation, which offers controllable force exerting on the double emulsions. We developed this capability for future integration of droplet microfluidics with mass spectrometry (MS), where each aqueous droplet in the microchannel is introduced into the gas phase as a double emulsion for subsequent ionization and MS analysis.

We report a microfluidic device, using segmented flow in a two-phase system of immiscible liquids, which delivers aqueous droplets into a modified commercial mass spectrometer. The interface coupling the microfluidics to the mass spectrometer achieves up to 96% sample transfer efficiency to the vacuum chamber. Sample ionization is assisted by multipass infrared laser beam in the interface. The system achieves low femtomole detection limits of several analytes ranging from drugs to proteins. Sample ionization in this segmented-flow sampling was found to be remarkably insensitive to the presence of buffer salts and other matrices.

This paper describes the use of electro-hydrodynamic actuation to control the transition between three major flow patterns of an aqueous-oil Newtonian flow in a microchannel: droplets, beads-on-a-string (BOAS), and multi-stream laminar flow. We observed interesting transitional flow patterns between droplets and BOAS as the electric field was modulated. The ability to control flow patterns of a two-phase fluid in a microchannel adds to the microfluidic tool box and improves our understanding of this interesting fluid behavior. 1,2 In recent years, there has been increasing interest in two-phase flow and droplet microfluidics, owing to their potential for providing a high-throughput platform for carrying out chemical and biological analysis and manipulations.3-8 Although droplets may be generated in many different ways, such as with electric fields or extrusion through a small nozzle, 9-12 the most common microfluidic methods are based on the use of either T-junctions or flow-focusing geometries with which uniform droplets can be formed at high frequency in a steady-state fashion. 13,14 Various operations, such as cell encapsulation, droplet fusion, splitting, mixing, and sorting, have also been developed, and these systems have been demonstrated for a wide range of applications, including cell analysis, protein crystallization, and material synthesis.