fabricating-high-viscosity-droplets-using-microfluidic-capillary

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure

The generation of monodisperse droplets with high viscosity has always been a challenge in droplet microfluidics. Here, we demonstrate a phase-inversion ...

Tsinghua University

We demonstrate a microfluidic method to first generate double emulsion droplets containing two different inner drops, and to then control the internal coalescence of the encapsulated drops. The advantages of the core-coalescence method are illustrated by fabricating high viscosity particles and by demonstrating the dissolution of cell membranes.

Reactions in double emulsions by flow-controlled coalescence of encapsulated drops.

The dynamics and acoustic emission of two interacting cavitation bubbles exposed to strong acoustic fields with a frequency of 515 KHz are investigated numerically in this paper. After comparing the dynamics of a single bubble excited by the given pressure waves, bubbles with ambient radii of 2 and 5 μm were chosen to be studied to discuss the influence of the mutual bubble-bubble interaction on the dynamic behaviors and acoustic emission of the bubbles. The results show that, aside from the external driving pressure waves, the interaction between the bubbles imposes an extra nonlinear effect on the oscillations of the bubbles and that the dynamics of the smaller bubble could be suppressed gradually with the enhancement of this mutual interaction by decreasing the distance between the bubbles. Moreover, the improvement in the oscillation nonlinearity of the bubbles due to the change in the ambient circumstance could readily be observed from the frequency spectra of the bubbles' acoustic emission, which interprets the change by exhibiting an appropriate development of the subharmonics, the ultraharmonics, and the broadband component.

Frequency spectrum of the noise emitted by two interacting cavitation bubbles in strong acoustic fields.

We demonstrate an approach to prepare zirconium dioxide (ZrO2) microspheres by carrying out a diffusion-induced sol-gel reaction inside double emulsion droplets. A glass capillary microfluidic device is introduced to generate monodisperse water-in-oil-in-water (W/O/W) double emulsions with a zirconium precursor as the inner phase. By adding ammonia to the continuous aqueous phase, the zirconium precursor solution is triggered to gel inside the emulsions. The double emulsion structure enhances the uniformity in the rate of the sol-gel reaction, resulting in sol-gel microspheres with improved size uniformity and sphericity. ZrO2 ceramic microspheres are formed following subsequent drying and sintering steps. Our approach, which combines double-emulsion-templating and sol-gel synthesis, has great potential for fabricating versatile ceramic microspheres for applications under high temperature and pressure.

Fabrication of ceramic microspheres by diffusion-induced sol-gel reaction in double emulsions.

Lubrication plays a crucial role in reducing friction for transporting heavy objects, from moving a 60-ton statue in ancient Egypt to relocating a 15,000-ton building in modern society. Although in China spoked wheels appeared ca. 1500 B.C., in the 15th and 16th centuries sliding sledges were still used in transporting huge stones to the Forbidden City in Beijing. We show that an ice lubrication technique of water-lubricated wood-on-ice sliding was used instead of the common ancient approaches, such as wood-on-wood sliding or the use of log rollers. The technique took full advantage of the natural properties of ice, such as sufficient hardness, flatness, and low friction with a water film. This ice-assisted movement is more efficient for such heavy-load and low-speed transportation necessary for the stones of the Forbidden City. The transportation of the huge stones provides an early example of ice lubrication and complements current studies of the high-speed regime relevant to competitive ice sports. 

Ice lubrication for moving heavy stones to the Forbidden City in 15th- and 16th-century China.

An approach based upon sonication-microfluidics is presented to fabricate nanoparticle-coated microbubbles. The gas-in-liquid slug flow formed in a microchannel is subjected to ultrasound, leading to cavitation at the gas-liquid interface. Therefore, microbubbles are formed and then stabilized by the nanoparticles contained in the liquid. Compared to the conventional sonication method, this sonication-microfluidics continuous flow approach has unlimited gas nuclei for cavitation that yields continuous production of foam with shorter residence time. By controlling the flow rate ratios of the gas to the liquid, this method also achieves a higher production volume, smaller bubble size, and less waste of the nanoparticles needed to stabilize the microbubbles. 

Sonication-microfluidics for fabrication of nanoparticle-stabilized microbubbles.

Porous carbon particles have been widely used in many areas including energy storage. Production of carbon microspheres in an efficient, controlled, and low-cost manner, however, is challenging. Here, we demonstrate a microfluidic approach to generate porous carbon particles using inexpensive precursors and show that the size of the particle and pores can be tuned by adjusting the deionized (DI) water content in droplets and preheating temperature. The developed strategy offers an effective approach to control the production of porous carbon spheres with a well-defined diameter, narrow size distribution and pore size. 

Microfluidic production of porous carbon spheres with tunable size and pores.

Drag reduction was found in polydimethylsiloxane (PDMS) microchannels when the flow was pulled by depressurization at the inlet, and it was attributed to the formation of the bubbles on the PDMS surface. The formed bubbles were examined by atomic force microscopy (AFM), and the resultant effective slip length was measured by microparticle image velocimetry (μPIV). The drag reduction was found to decrease as the bubbles grew and detached from the surface, causing a pulsatile flow in the microchannel.

Drag Reduction by Bubble-Covered Surfaces Found in PDMS Microchannel through Depressurization.

Emulsions-liquid droplets dispersed in another immiscible liquid-are widely used in a broad spectrum of applications, including food, personal care, agrochemical, and pharmaceutical products. Emulsions are also commonly present in natural crude oil, hampering the production and quality of petroleum fuels. The stability of emulsions plays a crucial role in their applications, but controlling the stability without external driving forces has been proven to be difficult. Here we show how heterogeneous surface wettability can alter the stability and dynamics of oil-in-water emulsions, generated by a co-flow microfluidic device. We designed a useful methodology that can modify a micro-capillary of desired heterogeneous wettability (e.g., alternating hydrophilic and hydrophobic regions) without changing the hydraulic diameter. We subsequently investigated the effects of flow rates and heterogeneous wettability on the emulsion morphology and motion. The experimental data revealed a universal critical timescale of advective emulsions, above which the microfluidic emulsions remain stable and intact, whereas below they become adhesive or inverse. A simple theoretical model based on a force balance can be used to explain this critical transition of emulsion dynamics, depending on the droplet size and the Capillary number-the ratio of viscous to surface effects. These results give insight into how to control the stability and dynamics of emulsions in microfluidics with flow velocity and different wettability.

Altering Emulsion Stability with Heterogeneous Surface Wettability.

Mass-transfer-induced multistep phase separation was found in emulsion droplets. The agent system consists of a monomer (ethoxylated trimethylolpropane triacrylate, ETPTA), an oligomer (polyethylene glycol diacrylate, PEGDA 700), and water. The PEGDA in the separated layers offered partial miscibility of all the components throughout the multistep phase-separation procedure, which was terminated by the depletion of PEGDA in the outermost layer. The number of separated portions was determined by the initial PEGDA content, and the initial droplet size influenced the mass-transfer process and consequently determined the sizes of the separated layers. The resultant multilayered emulsions were demonstrated to offer an orderly temperature-responsive release of the inner cores. Moreover, the emulsion droplets can be readily solidified into onionlike microspheres by ultraviolet light curing, providing a new strategy in designing particle structures. 

Mass-Transfer-Induced Multistep Phase Separation in Emulsion Droplets: Toward Self-Assembly Multilayered Emulsions and Onionlike Microspheres.

Fluids containing high concentration polymers, sols, nanoparticles, etc., usually have high viscosities, and high-viscosity fluids are difficult to be encapsulated into uniform droplets. Here we report a surface-controlled breakup method to generate droplets directly from various aqueous and nonaqueous fluids with viscosities of 1.0 to 11.9 Pa s and a dispersed-to-continuous viscosity ratio up to 1000, whereas the volume fraction of droplets up to 50% can be achieved. It provides a straightforward method to encapsulate high viscosity fluids, in a well-controlled manner in the rapid developing droplet-based applications, including materials synthesis, drug delivery, cell assay, bioengineering, etc.

Microfluidic Generation of High-Viscosity Droplets by Surface-Controlled Breakup of Segment Flow.

Ventricular assist devices (VADs) can effectively improve the survival rate of patients with end-stage heart failure, but the hemolytic complications induced by long-time VAD support have received wide attention recently. The conventional evaluation method of the hemolytic properties of VADs by the indicator of plasma free hemoglobin (PFH) concentration is used but not sensitive enough to meet the needs of the actual examinations. In this study, an experimental method was applied for the evaluation of the injuries and damages caused by VADs to erythrocytes by both indicators of PFH and lactic dehydrogenase (LDH) in the in vitro hemolysis assay of VADs. The changes of LDH and PFH concentrations in plasma with the shear stress and the exposure time under a fixed shear stress were measured and analyzed to investigate the sensitivity and accuracy of the evaluation of erythrocyte damage by LDH. Furthermore, through 24 h in vitro hemolysis tests, the changes of LDH and PFH concentrations in blood samples were measured in combination with the microscopic histological changes and ultrastructural changes of erythrocytes, to assess the possibility of LDH evaluating the hemolysis of VADs. The changes of the concentration of LDH were more sensitive than those of PFH to different shear stress and exposure times, especially lower stress. Meanwhile, in the 24 h in vitro hemolysis assay, the PFH concentration in the blood samples showed no significant changes in the first 8 h, while the LDH concentration increased significantly in the first 3 h, which was consistent with the morphological changes of erythrocytes. Compared with the changes of the PFH concentration, the LDH concentration is sensitive to the damage of erythrocytes caused by VADs. It was considered that LDH could be applied as an additional indicator in the evaluation of erythrocyte damage and hemolytic properties of VADs in combination with the normalized index of hemolysis, for the more accurate assessment of the blood compatibility of VADs.

Lactic Dehydrogenase in the In Vitro Evaluation of Hemolytic Properties of Ventricular Assist Device.

Margination of stiffened red blood cells has been implicated in many vascular diseases. Here, we report the margination of stiffened RBCs in vivo, and reveal the crucial role of the vessel geometry in the margination by calculations when the blood is seen as viscoelastic fluid. The vessel-geometry-regulated margination is then confirmed by in vitro experiments in microfluidic devices, and it establishes new insights to cell sorting technology and artificial blood vessel fabrication.

Margination of Stiffened Red Blood Cells Regulated By Vessel Geometry.

Phase separation occurs in emulsion droplets containing poly (ethylene glycol) diacrylate (PEGDA), glycerol, and ethanol to form a glycerol-in-PEGDA structure, and the phase separation process is found to depend on the droplet size. The mechanism of this size-dependent phase separation is dependent on the droplet sizes changing the phase separation time by changing the evaporation speed of mutual solvent ethanol, and the relationship between the separation time T and the droplet diameter D is derived as T≈D , which has been validated by experiment results. According to this finding, the structures of the droplets can be designed by applying UV curing at different stages of the phase separation, and the monodispersity of droplets is necessary to achieve polymerized particles with the same structure.

Size-Dependent Phase Separation in Emulsion Droplets.

Prolonged storage will alter the biophysical properties of red blood cells (RBCs), and it decreases the quality of stored blood for blood transfusion. It has been known that less deformable aged RBCs can be separated by margination, but the recognition of the storage time from the separation efficiency of the stiff RBCs is still a challenge. In this study, we realized enhanced separation of aged RBCs from normal RBCs by controlling the channel cross section and demonstrated that the storage time can be deduced from the percentage of the separated RBCs in the stored RBCs. This separation technology helps to reveal the regulation of time on the RBC aging mechanism and offer a new method to separate stiffened cells with high efficiency.

Haosheng Chen

State Key Laboratory of Tribology

Tsinghua University

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure

Haosheng Chen

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}State Key Laboratory of Tribology

Tsinghua University

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure

State Key Laboratory of Tribology