Pressure-Driven Filling of Closed-End Microchannel: Realization of Comb-Shaped Transducers for Acoustofluidics

We demonstrate the complete filling of both deionized water (DI water) and liquid metal (eutectic gallium-indium, EGaIn) into closed-end microchannels driven by a constant pressure at the inlet. A mathematical model based on gas diffusion through a porous polydimethylsiloxane (PDMS) wall is developed to unveil the physical mechanism in the filling process. The proposed theoretical analysis based on our model agrees well with the experimental observations. We also successfully generate traveling surface acoustic waves by actuating interdigitated microchannels filled with EGaIn. Our work provides significant insights into the fabrication of liquid electrodes that can be used for various acustofluidics applicationsAustralian Research Council DE170100600National Natural Science Foundation of China Grants No. 11472094, No. 11772259, No. U1613227, No. B1703

Automated droplet measurement (ADM): an enhanced video processing software for rapid droplet measurements

This paper identifies and addresses the bottlenecks that hamper the currently available software to perform in situ measurement on droplet-based microfluidic. The new and more universal object-based background extraction operation and automated binary threshold value selection make the processing step of our video processing software (ADM) fully automated. The ADM software, which is based on OpenCV image processing library, is made to perform measurements with high processing speed using efficient code. As the processing speed is higher than the data transfer speed from the video camera to permanent storage of computer, we integrate the camera software development kit (SDK) with ADM. The integration allows simultaneous operations of the video transfer/streaming and the video processing. As a result, the total time for droplet measurement using the new process flow with the integrated program is shortened significantly. ADM is also validated by comparing with both manual analysis and DMV software. ADM will be publicly released as a free tool. The software can also be used on a video file or files without the integration with the camera SDK.Singapore. Ministry of Education (Tier 2 Grant 2011-T2-1-0-36

Influence of interfacial gas enrichment on controlled coalescence of oil droplets in water in microfluidics

Interfacial gas enrichment (IGE) of dissolved gases in water is shown to govern the strong attraction between solid hydrophobic surfaces of an atomic force microscopy (AFM) colloidal probe and a solid substrate. However, the role of IGE in controlling the attraction between fluid-fluid interfaces of foam films and emulsion films is difficult to establish by AFM techniques due to the extremely fast coalescence. Here, we applied droplet-based microfluidics to capture the fast coalescence event under the creeping flow condition and quantify the effect of IGE on the drainage and stability of water films between coalescing oil droplets. The amount of dissolved gases is controlled by partially degassing the oil phase. When the amount of dissolved gases (oxygen) in oil decreases (from 7.89 mg/L to 4.59 mg/L), the average drainage time of coalescence significantly increases (from 19 ms to 50 ms). Our theoretical quantification of the coalescence by incorporating IGE into the multilayer van der Waals attraction theory confirms the acceleration of film drainage dynamics by the van der Waals attractive force generated by IGE. The thickness of the IGE layer decreases from 5.5 nm to 4.9 nm when the amount of dissolved gas decreases from 7.89 mg/L to 4.59 mg/L. All these results establish the universal role of dissolved gases in governing the strong attraction between particulate hydrophobic interfaces

Droplet-based microfluidics

In this thesis, the background of both microfluidics and droplet-based microfluidics are first introduced and presented. Next, the objectives and scopes are defined to give a clear understanding on the research topic. The literature of droplet formation in different configurations is then reviewed thoroughly through many different past works. Basic characteristics of the nature of ferrofluids and applications are also reviewed. Firstly, the temperature dependence of droplet formation in a flow focusing microfluidic device is investigated. Both DI water and nanofluids were used to generate droplets at different temperature. The heat generated by the integrated micro heater changes the droplet formation process and increases the size of the droplet due to the dominance of viscosity over interfacial tension. At constant flow rates, three different droplet formation regimes were observed in both fluids. Experimental results also demonstrated that water-based nanofluid exhibits similar characteristics as DI water. Furthermore, a change in the nanofluid flow rates has little effect on the size of the droplet formed.MASTER OF ENGINEERING (MAE

Generation and manipulation of monodispersed ferrofluid emulsions : the effect of a uniform magnetic field in flow-focusing and T-junction configurations

This paper demonstrates the use of magnetically controlled microfluidic devices to produce monodispersed ferrofluid emulsions. By applying a uniform magnetic field on flow-focusing and T-junction configurations, the size of the ferrofluid emulsions can be actively controlled. The influences of the flow rates, the orientation, and the polarity of the magnetic field on the size of ferrofluid emulsions produced in both flow-focusing and T-junction configurations are compared and discussed

Microfluidic flow-focusing in ac electric fields

International audienceWe demonstrate the control of droplet sizes by an ac voltage applied across microelectrodes patterned around a flow-focusing junction. The electrodes do not come in contact with the fluids to avoid electrochemical effects. We found several regimes of droplet production in electric fields, controlled by the connection of the chip, the conductivity of the dispersed phase and the frequency of the applied field. A simple electrical modelling of the chip reveals that the effective voltage at the tip of the liquid to be dispersed controls the production mechanism. At low voltages (≲ 600 V), droplets are produced in dripping regime; the droplet size is a function of the ac electric field. The introduction of an effective capillary number that takes into account the Maxwell stress can explain the dependance of droplet size with the applied voltage. At higher voltages (≳ 600 V), jets are observed. The stability of droplet production is a function of the fluid conductivity and applied field frequency reported in a set of flow diagrams

Editorial for the Special Issue on the Insights and Advancements in Microfluidics

We present a total of 19 articles in this special issue of Micromachines entitled, ”Insights and Advancements in Microfluidics.”[...

Insights and Advancements in Microfluidics

Microfluidics has developed rapidly over the past three decades. Relentless diagnostic, medical and chemical applications have been demonstrated in various applications, plateforms and tools. Have microfluidics realized its full potential? Or is it only a leveraging academic tool? In this Special Issue, we focus on both insights and advancements in microfluidics. We invite emerging investigators and pioneers to contribute commentaries, perspectives and insightful reviews on related topics. The various insights from esteemed colleagues will be collated. We will also discuss technological breakthrough of original works in both short communications and full papers. The main idea is to stimulate the community and to provide an unique collection of insightful papers. We will also cover various topics ranging from 3D printing, paper-based microfludics to conventional polymer-based microfluidics which contributes to the technological advancements