Moving-wall Structures Research Articles

A microfluidic platform for formation of double-emulsion droplets

A new method for actively controlling the number of internal droplets of water-in-oil-in-water (W/O/W) double-emulsion droplets was demonstrated. A new microfluidic platform for double-emulsion applications has been developed, which integrates T-junction channels, moving-wall structures, and a flow-focusing structure. Inner water-in-oil (W/O) single-emulsion droplets were first formed at a major T-junction. Then the droplets were sub-divided into smaller uniform droplets by passing through a series of secondary T-junctions (branches). The moving-wall structures beside the secondary T-junctions were used to control the number of the sub-divided droplets by selectively blocking the branches. Finally, double-emulsion droplets were formed by using a flow-focusing structure downstream. Experimental data demonstrate that the inner and outer droplets have narrow size distributions with coefficient of variation (CV) of less than 3.5% and 5.7%, respectively. Double-emulsion droplets with 1, 2, 3, and up to 10 inner droplets have been successfully formed using this approach. The size of the inner droplets and outer droplets could be also fine-tuned with this device. The development of this new platform was promising for drug delivery applications involving double emulsions.

A droplet-based microfluidic system capable of droplet formation and manipulation

Formation of emulsion droplets is crucial for a variety of industrial and scientific applications. This study presents a new droplet-based microfluidic system capable of generating tunable and uniform-sized droplets and subsequently deflecting these droplets at various inclination angles using a combination of flow-focusing and moving-wall structures. A pneumatic air chamber was used to activate the moving-wall structures, located nearby the outlet of the flow-focusing microchannels, such that the sheath flows can be locally accelerated. With this approach, the size of the droplets can be fine-tuned and sorted without adjusting the syringe pumps. Experimental data showed that droplets with diameters ranging from 31.4 to 146.2 μm with a variation of less than 5.39% can be generated. Besides, droplets can be sorted upwards or backwards with an inclination angle ranging from 0° to 53.5°. The development of this emulsion system may be promising for the formation and collection of emulsion products for applications in the pharmaceutical, cosmetics and food industries.

Droplet Formation Utilizing Controllable Moving-Wall Structures for Double-Emulsion Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The formation of microscale single- and double-emulsion droplets with various sizes is crucial for a variety of industrial applications. In this paper, we report a new microfluidic device which can actively fine-tune the size of single- and doubleemulsion droplets in liquids by utilizing controllable movingwall structures. Moreover, various sizes of external and internal droplets for double emulsions are also successfully formed by using this device. Three pneumatic side chambers are placed at a T-junction and flow-focusing channels to construct the controllable moving-wall structures. When compressed air is applied to the pneumatic side chambers, the controllable moving-wall structures are activated, thus physically changing the width of the microchannels. The size of the internal droplets at the intersection of the T-junction channel is then fine-tuned due to the increase in the shear force. Then, the internal droplets are focused into a narrow stream hydrodynamically and finally chopped into double-emulsion droplets using another pair of moving-wall structures downstream. For single emulsions, oil-in-water droplets can be actively fine-tuned from 50.07 to 21.80 <formula formulatype="inline"><tex>$\mu\hbox{m}$</tex> </formula> under applied air pressures from 10 to 25 psi with a variation of less than 3.53%. For a water-in-oil single emulsion, droplets range from 50.32 to 14.76 <formula formulatype="inline"><tex>$\mu\hbox{m}$</tex></formula> with a variation of less than 4.62% under the same applied air pressures. For double emulsions, the sizes of the external and internal droplets can be fine-tuned with external/internal droplet diameter ratios ranging from 1.69 to 2.75. The development of this microfluidic device is promising for a variety of applications in the pharmaceutical, cosmetics, and food industries. <formula formulatype="inline"><tex>$\hfill$</tex></formula>[2007-0230] </para>

A tunable microflow focusing device utilizing controllable moving walls and its applications for formation of micro-droplets in liquids**Preliminary results of the current paper were presented at the 2007 IEEE International Conference of Nano/Micro Engineered and Molecular Systems (IEEE NEMS 2007), Bangkok, Thailand, 16–19 January, 2007.

This study reports a new microfluidic device capable of fine-tuned sample-flow focusing and generation of micro-droplets in liquids by controlling moving wall structures. Two microfluidic components including an ‘active microchannel width controller’ and a ‘micro chopper’ can be used to fine-tune the width of the hydrodynamically pre-focused stream and subsequently generate micro-droplets. In this study, a basic concept of a ‘controllable moving wall’ structure was addressed and applied as the active microchannel width controller and the micro chopper to generate the proposed function. Pneumatic side chambers were placed next to a main flow channel to construct the controllable moving wall structures. The deformation of the controllable moving wall structure can be generated by the pressurized air injected into the pneumatic side chambers. The proposed chip device was fabricated utilizing polymer material such as PDMS (polydimethylsiloxane) to provide the flexibility of the controllable moving wall deformation. The microfluidic chip device with dimensions of 2.5 cm in width and 3.0 cm in length can be fabricated using a simple fabrication process. Experimental data showed that the deformation of the controllable moving wall structure can be adjusted by applying different air pressures, so that the width of the main flow channel can be controlled accordingly. By utilizing the proposed mechanism, the pre-focused dispersed phase stream could be actively focused into a narrower stream, and well-controlled micro-droplets with smaller diameters could be generated. The stream width can be reduced from 30 µm to 9 µm and micro-droplets with a diameter of 76 µm could be generated by utilizing the proposed device. In addition, to generate micro-droplets within smaller diameters, uniform size distribution of the micro-droplets can be obtained. According to the experimental results, development of the microfluidic device could be promising for a variety of applications such as emulsification, nano-medicine and droplet-based microfluidics.

Active micro-mixers utilizing moving wall structures activated pneumatically by buried side chambers**Preliminary results were presented at The Asia-Pacific Conference of Transducers and Micro-Nano Technology (APCOT), June 25–28, 2006.

In this study, a new active micro-mixer utilizing moving wall structures has been demonstrated. Rapid and reliable fabrication techniques involving standard SU-8 lithography and a polydimethylsiloxane (PDMS) replication process were employed for the formation of this micro chip device. The moving wall structures are activated pneumatically by buried side chambers which deform the channel walls and generate a rapid mixing of the confluent sample streams. The deformation of the moving wall structure can be controlled by the applied air pressure. A maximum deformation of 96.5 µm (about 96.5% of the channel width) can be achieved at an applied pressure of 50 psi for a wall structure with a width of 50 µm and a thickness of 100 µm. In this study, two pairs of moving wall structures were used for active mixing of the samples. Two dynamic operation modes, namely symmetric and asymmetric wall activation, are employed to alternately perturb the flow field and to generate a significant mixing effect. Mixing efficiency as high as 93.6% and 96.4%, respectively, can be achieved for these two modes. The effect of the operation frequency was also investigated. Experimental results indicate that the mixing efficiency increases with increasing operation frequency. However, once the operation frequency is higher than the frequency response of the device, the mixing efficiency drops sharply since the moving wall cannot be completely deflected within one actuation cycle. A numerical simulation was employed to investigate the mixing mechanism and to identify how the moving wall affected the sample flow field. Numerical data are in reasonable agreement with the experimental data with a variation less than 5%. Experimental results and numerical data indicate that the developed chip device can mix two confluent samples successfully. The development of active micro-mixers is a crucial component in many microfluidic applications.

Micro-droplet formation utilizing microfluidic flow focusing and controllable moving-wall chopping techniques**The preliminary results of the current paper had been presented at the 2006 IEEE International Conference of Nano/Micro Engineered and Molecular Systems (IEEE NEMS 2006), Zhuhai, China, 18–21 January 2006.

The formation of micro-scale droplets in liquids is crucial for many applications. This paper reports a new microfluidic chip capable of generating tunable micro-droplets in liquids by using the combination of two microfluidic techniques, microfluidic flow focusing and a controllable moving-wall chopper. The microfluidic chip can generate droplets with tunable sizes. Dispersed phase sample flow is first hydrodynamically focused into a narrow stream by using neighboring sheath flows containing continuous-phase samples. A new chopping microstructure called ‘controllable moving walls’, which is a pair of side chambers orthogonally placed next to the sample flow channel, is used to generate micro-droplets. The moving-wall structures can be deformed by external air pressure to cut the pre-focused stream into segments. By controlling the air injection frequency and air pressure of the side chambers, tunable micro-droplets with a relatively uniform diameter can be formed accordingly. Experimental data show that a maximum deformation of 62.5 µm could be achieved at a pressure of 30 psi for a moving wall with a cross section of 100 µm × 50 µm. By controlling the flow rate between the dispersed and continuous phases with a range from 2 to 20, micro-droplets with a relatively uniform size ranging from 10 µm to 120 µm can be successfully generated. The micro-droplets have a much more uniform size compared to previous studies. This new microfluidic device can be promising for a variety of applications such as emulsification, nano-medicine and droplet-based microfluidics.