Xize Niu

Pillar-induced droplet merging in microfluidic circuits

A novel method is presented for controllably merging aqueous microdroplets within segmented flow microfluidic devices. Our approach involves exploiting the difference in hydrodynamic resistance of the continuous phase and the surface tension of the discrete phase through the use of passive structures contained within a microfluidic channel. Rows of pillars separated by distances smaller than the representative droplet dimension are installed within the fluidic network and define passive merging elements or chambers. Initial experiments demonstrate that such a merging element can controllably adjust the distance between adjacent droplets. In a typical scenario, a droplet will enter the chamber, slow down and stop. It will wait and then merge with the succeeding droplets until the surface tension is overwhelmed by the hydraulic pressure. We show that such a merging process is independent of the inter-droplet separation but rather dependent on the droplet size. Moreover, the number of droplets that can be merged at any time is also dependent on the mass flow rate and volume ratio between the droplets and the merging chamber. Finally, we note that the merging of droplet interfaces occurs within both compressing and the decompressing regimes.

Efficient spatial-temporal chaotic mixing in microchannels

A chaotic micro mixer with multiple side channels is designed and investigated, in which fluid can be stirred by pumps through the side channels. By stretching and folding fluid in the main and side channels, chaotic mixing can be achieved. A simple mathematic model is derived to understand the movement of particles in the microchannel. Spatial trajectories of fluid particles are projected to Poincare sections by mapping. The route from the quasi-period to chaos is revealed to be destruction of KAM curves and shrinkage of the quasi-periodic areas. Lyapunov exponents (LE) are used as the mixing index and the criteria to evaluate the chaotic behavior of the system. We found that LE is closely related to the amplitude and frequency of stirring and can be used to optimize our design and operation. From the relationship of LE and striation thickness, the minimal mixing length required can be estimated, which is much shorter than that needed in passive mixer design.

A microdroplet dilutor for high-throughput screening

Pipetting and dilution are universal processes used in chemical and biological laboratories to assay and experiment. In microfluidics such operations are equally in demand, but difficult to implement. Recently, droplet-based microfluidics has emerged as an exciting new platform for high-throughput experimentation. However, it is challenging to vary the concentration of droplets rapidly and controllably. To this end, we developed a dilution module for high-throughput screening using droplet-based microfluidics. Briefly, a nanolitre-sized sample droplet of defined concentration is trapped within a microfluidic chamber. Through a process of droplet merging, mixing and re-splitting, this droplet is combined with a series of smaller buffer droplets to generate a sequence of output droplets that define a digital concentration gradient. Importantly, the formed droplets can be merged with other reagent droplets to enable rapid chemical and biological screens. As a proof of concept, we used the dilutor to perform a high-throughput homogeneous DNA-binding assay using only nanolitres of sample.

Real-time detection, control, and sorting of microfluidic droplets.

We report the design and implementation of capacitive detection and control of microfluidic droplets in microfluidic devices. Integrated microfluidic chip(s) with detectioncontrol circuit enables us to monitor in situ the individual volume of droplets, ranging from nanoliter to picoliter, velocity and even composition, with an operation frequency of several kilohertz. Through electronic feedback, we are able to easily count, sort, and direct the microfluidic droplets. Potential applications of this approach can be employed in the areas of biomicrofluidic processing, microchemical reactions as well as digital microfluidics.

A microfluidic approach for high-throughput droplet interface bilayer (DIB) formation

We present a simple, automated method for high-throughput formation of droplet interface bilayers (DIBs) in a microfluidic device. We can form complex DIB networks that are able to fill predefined three dimensional architectures. Moreover, we demonstrate the flexibility of the system by using a variety of lipids including 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC).

Droplet-based compartmentalization of chemically separated components in two-dimensional separations

We demonstrate that nanolitre-sized droplets are an effective tool in coupling two-dimensional separations in both time and space. Using a microfluidic droplet connector, chemically separated components can be segmented into nanolitre droplets. After oil filtering and droplet merging, these droplets are loaded into a second dimension for comprehensive separations.

Opportunities for microfluidic technologies in synthetic biology

We introduce microfluidics technologies as a key foundational technology for synthetic biology experimentation. Recent advances in the field of microfluidics are reviewed and the potential of such a technological platform to support the rapid development of synthetic biology solutions is discussed.

Generation and manipulation of “smart” droplets

We report the generation and manipulation of electrorheological (ER) droplets that exhibit the giant ER effect. The experiments were carried out on specially designed microfluidic chips, in which the ER droplets were generated by using the microfluidic flow-focusing approach. Both the size and formation rate of these droplets can be controlled through digitally applied electrical signals. The principle of droplet manipulation is based on the electrical responsiveness of ER droplets and hence the denotation of “smart” when the electrical signals can be triggered by sensing/control devices. Due to the unique characteristics of the GER effect, the smart droplets can deform and even stop the microfluidic channel flow under an applied electric field. The pressure difference induced by the smart droplets inside the micro-channel is controllable by varying the field strengths, droplet sizes and particle concentrations in the GER suspension. By trapping and timed release of smart droplets in different micro-branch channels, we demonstrate that the smart droplets generated upstream cannot only be stored or displayed in the desired downstream channel(s) and thereby offer the potential of micro-droplet display, but also be useful in counting, flow directing and sorting the desired number of passive droplets sandwiched between two smart droplets. Such capabilities of smart droplets will enable the programmable control of discrete processes in bio-analysis, chemical reactions, digital microfluidics, and digital droplet display.

Electro-Coalescence of Digitally Controlled Droplets

In this paper we describe a universal mechanism for merging multiple aqueous microdroplets within a flowing stream consisting of an oil carrier phase. Our approach involves the use of both a pillar array acting as a passive merging element, as well as built-in electrodes acting as an active merging element. The pillar array enables slowing down and trapping of the droplets via the drainage of the oil phase. This brings adjacent droplets into close proximity. At this point, an electric field applied to the electrodes breaks up the thin oil film surrounding the droplets resulting in merging.

Active microfluidic mixer chip

We report the design and fabrication of a chaotic mixer based on the electrorheological (ER) fluid-controlled valves. The flow in the main channel is perturbed by liquid flow in orthogonal side channels, driven by hydrodynamic pulsating pumps. Each pulsating pump consists of a chamber with diaphragm plus two out-of-phase ER valves operating in a push-pull mode. All the valves, pumps, and mixing channels are integrated in one polydimethylsioxane chip. Mixing characteristics in the main channel are controlled by the strength and frequency of external electric fields applied on the ER fluid.