The rapid economic development and increasing consumption of electrical energy requires people to generate more energy. The new energy sources developed are required to be clean and environmentally friendly. Microfluidic energy harvesting device is relatively less known compared with other popular renewable energy sources, such as solar cell and bio-fuels, but can provide such a clean and environmentally-friendly source.

The classical electrokinetic energy conversion mechanism relies on a single stage conversion by forcing liquid through a channel with charged walls. When the net charges inside the electrical double layer (EDL) are transported by water flow, the produced electrical energy can be harvested via connection of electrodes at two ends of a channel.

Different from traditional single phase flow energy conversion, the liquid microjet was used for energy conversion. Applied pressure forces water flow through a micropore, forming a liquid jet. Then the jet broke into droplets, which are absolutely isolated by air. Droplet kinetic energy is converted to electrical energy when the charged droplets decelerate in the electrical field that forms between membrane and target. It operates entirely different with traditional energy conversion from streaming potential, hence we term it “ballistic energy conversion”. Conversion occurs in two stages: first pressure is converted to kinetic energy and subsequently kinetic energy is converted to electrical energy. In the first stage a stream of high-velocity high-charge droplets is produced by forcing water through a micropore. In the second stage the charged droplets travel through an electrical field towards a high-potential target, decelerating them to zero speed. The target acquires its potential by droplet impact. Current is drawn from the target to do useful work. We strongly reduced loss factors by optimizing the setup from the physical model. At present this resulted in an energy conversion efficiency of almost 50% with as main loss factor hydrodynamic friction losses in the micropore.

Now, inspired by Kelvin’s water dropper, we apply the electrostatic charge self-induction mechanism to an inertia-driven (ballistic) energy conversion system, and show the disadvantages of Kelvin’s water dropper do not any more apply. The droplet charge thereby is derived from the same inductive charging mechanism as in Kelvin’s droplet generator, employing two separate systems and cross-connecting targets and induction rings. To prevent overcharging of the droplets by the inductive mechanism and consequent droplet loss by repulsion from the target, we use voltage dividers, trying both a resistor divider and a diode divider, to enable stable energy conversion with a self-induction system. The current rapidly increased once the electrical circuit was connected. With reversely connected diodes, the induction voltage could be properly controlled to a saturated value (about 500V), whereby experimentally maximal 17.9% energy conversion efficiency was obtained with the diode-induced system.

In a few decades from now, droplet-based microfluidics could be the key technology for gathering extensive information from chemical and biochemical analysis, diagnosis to therapeutics. Microfluidics is primary the science and technology dealing with fluidic phenomena on the microscale. The small size of microfluidic devices reduces the amount of samples and reagents needed by several orders of magnitude. The approach in microfluidics research is the scaling down of conventional laboratory equipment and processes which on one-hand reduces the cost and the other accelerate the processes. The fundamental advantage of droplet-based microfluidics is that a continuous stream of a fluid carrying samples is divided into very small volumes ranging from nanoliters to picoliters. Different chemicals and biological samples can be encapsulated and manipulated. In this talk, I will demonstrate different methods that I have developed to actively manipulate and control droplets.

We present a systematical experimental study on droplet deformation under alternating current (AC) electric field in a microfluidic channel. Effects of conductivities and surface tensions on droplet deformation are respectively investigated. It is found that for droplets with low conductivity, droplet deformation depends on both the applied electric field strength and AC frequency. When electric force dominates, droplet deformation decreases with the increasing AC frequency and behaves like a low pass filter. For droplets with higher conductivity, the effects of AC frequency disappear, and droplet deformation stays in the same level at given electric field strength. An equivalent electric circuit model is proposed to explain the frequency dependence of droplet deformation. A force analysis derived from Maxwell stress tensor shows that for droplets of low conductivity, the electric force mainly results from the electric permittivity force, which has distinct magnitudes at different AC frequencies, but for droplets of higher conductivity, the charge force contributes most to the electric charge force, and it depends on the product of the droplet conductivity and the effective elelctric field strength exerted on the droplet. Finally, We categorize a modified electric capillary number to explain the effects of surface tension on droplet deformation under electric field, and the experiment results have good agreement with the theory.

 We propose a surface wettability guided assembly (SWGA) approach for high throughput formations of droplets [1]. The SWGA technique is based upon stamping glass slides with PDMS templates to micro-pattern surfaces with predefined hydrophilic and hydrophobic regions. Here, we introduce three applications of droplets by SWGA: i) Tissue engineering. SWGA is used to rapidly bioengineer thousands of heterogeneous cell niches in 3D microgel arrays with quantitative controls over physical, chemical cues and cell populations. Assembly of cell niches on the wettability patterned surface is demonstrated with chemical gradient, cell density gradient and orthogonal gradients of chemical concentration and cell density over individual microgels. We also show spatial organization of heterogeneous cell populations in individual microgels and assembly of a hydrogel sheet with spatially-defined chemical distribution; ii) Earth-on-a-chip. With SWGA technique, a new concept of “earth-on-a-chip” is realized, which enables rapid generating thousands of heterogeneous microearth arrays to study the soil–microbe system and their biogeochemical effects in a precisely controlled microenvironment. We demonstrate utilities of this platform technology by recapitulating natural biogeochemical phenomenon on a chip, including soil colloid assembly, microscale transport of soil organic matter/microbes during wet-dry cycles of soil formation, microbe deposition on  soil/clay mineral, microbial regulated carbon-nitrogen cycling, providing new insights into biogeochemical processes at the microscale; iii) Micro/nano-fabriaction. By SWGA, gold nanoparticles solution self-confines into pre-defined geometries according to the hydrophilic regions. Then, the monodispersed nanoparticles self-assemble into well-defined circuits by coffee-ring-effect. Submicron-height and submicron to microns-width circuit arrays with various structures are precisely generated by varying the PDMS nanofilm patterns. Thousands of circuits with different geometries can be self-assembled simultaneously within 1 min. We belive that this SWGA platform technology may have wide applications in chemistry, biomedicine and engineering.

Abstract

Multiphase microfluidics offers a great number of opportunities in different applications ranging from analytical chemistry, chemical engineering, pharmaceutical and biomedical sciences, to life science. To understand the flow fields within liquid plugs in microchannels, theoretical models are presented. These models offer conveniences for subsequent analyses because transport phenomena can be analyzed directly with the known flow fields.

Different applications are demonstrated with the proposed models, such as the heat transfer in plugs and the chaotic mixing in plugs moving in meandering microchannels. The analyses of heat transfer showed that the heat transfer process can be significantly enhanced by the recirculating vortices as compared to the single phase flow. The study of chaotic mixing showed that rapid chaotic mixing can be achieved in plugs moving in meandering microchannels.

A digital microfluidic platform manipulates droplets on an open surface. Magnetic digital microfluidics utilizes magnetic forces for actuation and offers unique advantages compared to other digital microfluidic platforms. First, the magnetic particles used in magnetic digital microfluidics have multiple functions. In addition to serving as actuators, they also provide a functional solid substrate for molecule binding, which enables a wide range of applications in molecular diagnostics and immunodiagnostics. Second, magnetic digital microfluidics can be manually operated in a "power-free" manner, which allows for operation in low-resource environments for point-of-care diagnostics where even batteries are considered a luxury item. This abstract covers research areas related to magnetic digital microfluidics. Various methods of droplet manipulation using magnetic forces are discussed, ranging from conventional magnetic particle-based actuation to the recent development of ferrofluids and magnetic liquid marbles.In addition, we emphasize applications of magnetic digital microfluidics in biosensing and medical diagnostics, and the current limitations of magnetic digital microfluidics are identified.

In a droplet microfluidics, the generation and stablization of droplets are both important for their applications in chemical or biological fields. In this work, we summarize the surfactants typically used for creation of oil-in-water or water-in-oil droplets. The special requirements for surfactants in stablizing droplets duirng application,such as thermal, biocompatible, ionic properties are discusssed. Moreover,the colloidosomes with nanoparticles as stabilizers for droplets will also been discussed. 

Functional granular materials with typical sizes of 1~1000 µm have received considerable attention for many applications. Generally, the overall functions of these microparticles strongly rely on both of their structures and the properties of their component materials. Thus, the combination of unique structures with functional materials provides an important route for developing advanced functional granular materials. Utilization of emulsions as templates allows producing versatile microparticles, with their size, shape and structure largely depends on those of the emulsions. Emulsion-template synthesis of microparticles allows precise control over their size, shape, composition and structure by tuning those of emulsions via specific emulsification techniques. With excellent control over emulsion drops, microfluidic technique provides a powerful platform for reproducible and scalable production of granular materials with unprecedented control over their structures and compositions. This provides vast opportunities for producing granular materials with the structure-property combination strategy for achieving elaborately designed functions. The controllable architectures of the emulsions and their tunable chemical composition for each separate phase allow for flexible combination of the structure characteristics and material properties for producing microparticles with elaborately tailored functions. In this presentation, we highlight the recent efforts for microfluidic fabrication of granular materials with well-designed functions, along with the development of microfluidic techniques for producing the versatile emulsion templates. We envision that the versatility of microfluidics for microparticle synthesis could open new frontiers and provide promising and exciting opportunities for fabricating brand-new functional microparticles with broad implications for myriad fields.

Unidirectional liquid spreading is of significant interest for a wide range of applications, such as microfluidic devices, self-lubrication, controllable chemical reaction, and biomedical. Although the surfaces with wettable gradient or asymmetric nanowires could harness the liquid to spread unidirectionally, the spreading is remarkable slow over a short distance and just occurs with specific surface chemistry property. Nature inspired one-dimensional fibers from spider silks and cactus spines give a way of combining surface energy gradient and curvature gradient to drive the droplet directionally, while they are difficult to be applied as two-dimensional surfaces anticipated in microfluidics devices, biomedical devices and so on. Recently, continuous uni-directional liquid spreading with fast speed was firstly discovered on the peristome of Nepenthes alata as shown in Fig.1, which possesses superhydrophilic hierarchical microgrooves and duck-billed microcavities with arc-shaped edge and gradient wedge corner [1].

Inspired from the surface structure on peristome, a novel bio-inspired uni-directional liquid spreading surface was built via two-step and inclined UV exposure photolithography as shown in Fig.2. The controlling of its uni-directional liquid spreading was realized by changing surface wettability and structural features of microcavities as shown in Fig.3, and the underlying mechanisms were experimentally demonstrated [2,3]. Moreover, gradient Taylor rise was built to support the mechanism of uni-directional liquid transport on basis of traditional Taylor rise. Combining this structure surface with thermoresponsive material, a novel smart temperature-controlled uni-directional liquid spreading surface was carried out, which can be used in medical and microfluid devices [4].

As another typical natural surface with unique water transport function, tree frog toe pads possess superior wet friction as shown in Fig.4. This wet friction also comes from the characteristic surface structure i.e. hexagonal pillar. Liquid on the toe pad of tree frog demonstrated directional spreading from surrounded channels to the top of hexagonal pillars during separating from substrate. In this process, liquid film crushes into pieces and each pillar was left with a drop on it. This effect efficiently enhanced the friction under wet condition and can be applied in medical devices, such as surgical grasper [5].

Finally, all of these investigations were applied in the minimally invasive surgical devices to demonstrate their efficiency. On basis of uni-directional liquid transport mechanism, bio-inspired anti-adhesion electrosurgical knife was proposed and developed to prevent soft tissue adhesion. Experimental results shows that the anti-adhesion force of bio-inspired electrosurgical knife is reduced about 80% as compared with conventional electrosurgical knife. Bio-inspired anti-slipping grasper was also developed under the inspiration of tree frog toe pads. The wet friction force was improved about 20%, and the soft tissue deformation was clarified to be decreased about 100 times.

Herein I present two works of using droplets to study the mechanism of “on water” reactions and to perform large-scale organic synthesis. A series of organic reactions such as Diels-Alder cycloadditions have been found to proceed dramatically faster in a heterogeneous mixture of the reactants and water than in the homogeneous mixture. It is not clear whether the interfacial chemical species or the hydrodynamic effects or both are responsible for the rate acceleration.[1]

In the first work, we produced droplets containing diethyl azodicarboxylate (DEAD) and quadricyclane to study the interface effect on organic reactions (Figure 1a).[2] We confined the droplets in the glass capillaries (Figure 1b) to minimize the hydrodynamic effects, and analyze the “on-water” reaction to find out which factor, the catalysis by the free OH groups at the interface, the hydrodynamic effects, or both, is responsible for the rate acceleration. The cycloaddition reaction process was recorded by a CCD camera. The results showed the reaction proceeded in three steps (Figure 2). The comparison of the reaction in droplet and the bulk emulsion (Table 1) showed that the organic-water interface effectively accelerated the “on-water” reaction to the same level as in bulk solution, indicating that the organic-water interface was the major catalysis factor, and the hydrodynamic effects were negligible during the “on-water” reaction.[3]

In the second work, we produced charged droplets for the large-scale synthesis of isoquinoline. Previously, Richard and co-workers used electrospray to generate charged droplets and analyze the reaction process by mass spectrometry [4]. They found the surface protons of the charged droplets efficiently catalyzed conversion of the reactant benzalaminoacetal to the product isoquinoline. Herein, we combined the ultrasound and electric field to continuously generate large amounts of charged droplets (Figure 3), which served as the micro-reactors for the isoquinoline synthesis. The synthesis result was evaluated by mass spectrometry (Figure 4), indicating the high efficiency of the droplet-based synthesis.