Modulating patterns of two-phase flow with electric fields

Abstract

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.

Controlled generation of droplets using an electric field in a flow‐focusing paper‐based device

“…The tip of the Taylor cone is stretched and becomes a narrow filament, and further broken into small droplets because of the Rayleigh instability. We further analyzed the influences of the hydrodynamic stress and the Maxwell stress in the droplet formation process based on a dimensionless number, electric Bond number [16,39] (Bo e = εDE 2 /σ ), which means the ratio of electric field force to interfacial tension force. ε, D, σ, and E indicate the aqueous phase permittivity (7.1 × 10 -10 F/m), the hydraulic size of the channel at the junction (1.4 mm), the aqueous-oil interfacial tension (about 42 mN/m), and electric field strength.…”

“…The flow focusing geometry is usually utilized to produce droplets [14] since the continuous and dispersed phases flow coaxially through a contraction region. Although the droplet dimension relies on the geometry, the fluid properties, and flow velocities of the inner and outer phases [15], an electric field provides another method to change droplet diameter [16]. Kumar et al [17,18] studied the droplet motion including droplet deformation, squeezing deceleration, and pinning, after applying an electric field.…”

A Fluorescence-Activated Single-Droplet Dispenser for High Accuracy Single-Droplet and Single-Cell Sorting and Dispensing

“…Precise control of the formation of an oil droplet containing one aqueous droplet of interest at the open end of the capillary is the key step in accurately dispensing single droplets and cells for downstream analysis. To achieve this, we used electrohydrodynamic actuation and applied high voltage to the capillary holder [33][34][35] to exert a controllable force on the emulsion droplets, thereby forming a double emulsion (aqueous-in-oil-in-air droplet) for dispensing into the multi-well plate. Although the mechanism for generating double emulsions using electrohydrodynamic actuation is somewhat different than electrospray 35 , we note electrospray-based droplet formation has been demonstrated as an efficient technique for the sampling and ejection of single cells or single-cell contents, such as the preparation and analysis of single cells by probe electrospray ionization mass spectrometry (PESI-MS) and single-cell analysis using MALDI-MS analysis [36][37] .…”

AC electrified jets in a flow-focusing device: Jet length scaling

“…In this situation, if the outer viscous stresses on the jet interface are of the order of the surface tension confinement forces, a slender jet appears which eventually breaks up into uniform droplets. 5,[11][12][13][14][15] Liu et al 16 applied DC electric fields to control the droplet production in a flow focusing device. They observed very long jets for high applied voltages and a suitable flow rate ratio between the two phases.…”