H. Gu

Droplets Formation and Merging in Two-Phase Flow Microfluidics

Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed.

Electrowetting-controlled droplet generation in a microfluidic flow-focusing device

We studied the generation of aqueous microdrops in an oil–water flow-focusing device with integrated insulator-covered electrodes that allow for continuous tuning of the water wettability by means of electrowetting. Depending on the oil and water inlet pressures three different operating conditions were identified that shift upon applying a voltage: stable oil–water interface, drop generation, and laminar water jet formation. Full control over the drop generation is achieved within a well-defined range of inlet pressures, in quantitative agreement with a model based on the additive contributions from electrowetting and the local hydrostatic pressure at the junction. The tuning power of electrowetting is shown to increase upon device miniaturization, which makes this approach particularly attractive for flow control on the sub-micrometer scale.

Electrowetting --A versatile tool for controlling microdrop generation

Abstract.Integrating insulator-covered electrodes into a microfluidic flow focusing device (FFD) we demonstrate enhanced flexibility and control of the flow of two non-miscible liquids based on electrowetting (EW). In the parameters space, determined by liquid inlet pressures, we identify a specific region where drops can only be generated and addressed via EW. In this regime we show that the size distribution and the frequency of drop generation can be controlled by the applied voltage and the width of voltage pulses. Moreover it turns out that with EW the drop size and the frequency can be tuned independently. Finally we show that the same drop generation phenomena can also be observed in the presence of surfactants.

A microfluidic platform for on-demand formation and merging of microdroplets using electric control

We discuss a microfluidic system in which (programmable) local electric fields originating from embedded and protected electrodes are used to control the formation and merging of droplets in a microchannel. The creation of droplets-on-demand (DOD) is implemented using the principle of electrowetting. Combined with hydrodynamic control, the droplet size and formation frequency can be varied independently. Using two synchronized DOD injectors, merging-on-demand (MOD) is achieved via electrocoalescence. The efficiency of MOD is 98% based on hundreds of observations. These two functionalities can be activated independently.

Electrowetting-enhanced microfluidic device for drop generation

We integrated electrowetting into a microfluidic flow focusing device to study drop generation under the influence of electric fields. Using both the dispersed phase inlet pressure and the applied voltage as control parameters, we find that the range of drop sizes and the drop generation rate can be controlled in a much finer way than with hydrodynamics alone. In particular a “conical spray” regime occurring at a voltage of O(50 V) allows for continuous tuning of the (highly monodisperse) drop diameter from ≈ 5 to 50 μm at a fixed continuous flow rate.