Generating Gas Bubbles in Water with Hydrodynamic Flow-Focusing Based Droplet System

By AZoNano.com Staff Writers

Topics Covered

Introduction
System Setup
Experimental Results
Conclusion
About Dolomite Microfluidics

Introduction

This article discusses the application of hydrodynamic flow-focusing based Small Droplet System from Dolomite to generate gas bubbles in liquid.

The Small Droplet System microfluidic setup is user friendly and reliably generates monodisperse gas bubbles with a Small Quartz Droplet Chip, making it ideal for advanced applications in areas, including medical diagnostics, research and development, petrochemicals, and materials science.

With chemical stability, the wetted components provide optical access for visual diagnostics. Dolomite’s sophisticated processes precisely fabricate the chip with feature size down to 8µm. This chip based system is proven to be capable of generating bubbles down to 6µm diameter with a narrow size distribution.

Dolomite P-Pumps control the flow rates to provide pulse-free flow, which is an ideal and reliable technique for controlling bubble size. The combination of the Small Droplet System and the Small Quartz Droplet Chip can adjust the volume fraction of the gaseous phase from zero to one.

System Setup

The Small Droplet System is a microfluidic device engineered to generate tiny droplets in the size range of 2-30µm. It achieves stable bubble generation, thanks to its near-circular microchannel profile at the junction and hydrophilic coating.

The Small Quartz Droplet Chip (Hydrophilic) is equipped with a Top Interface 4-way and a Linear Connector 4-way to interface the fluidic connection between the chip and the tubing. Sketch of the Small Quartz Droplet Chip and in-depth view of the junction are shown in Figure 1. The red box represents the region of interest (ROI), which is the focus of imaging.

Figure 1. Left: Sketch of Small Quartz Droplet Chip (Part No. 3200152). Right: Detailed view of the junction with dimensions.

The droplet phase is nitrogen and the carrier phase is 1% SDS solution in water. Nitrogen and water are delivered to the chip using two Mitos P-Pumps. PEEK tubing and FEP tubing are employed for fluid delivery across the system.

Schematic showing assembly of standard components is depicted in Figure 2. Flow resistors are prepared by cutting tubing to appropriate lengths and are connected in the configuration as represented in Figure 3.

Figure 2. Schematic showing assembly of standard components

Figure 3. Tubing details used in the connections used for the bubble generation setup

The single FEP tube is split into two separate FEP tubes leading to the Linear Connector 4-way by the T-Connector ETFE with the carrier water.

The 2-way In-line Valve is an EFTE shut-off valve used in the PEEK tubing transferring the droplet phase nitrogen to the Linear Connector 4-way.

After passing through the Small Quartz Droplet Chip and Top Interface 4-way, the fluid is collected in a microcentrifuge tube. For visualizing the Small Quartz Droplet Chip, a High Speed Camera and Microscope System is used.

Experimental Results

The carrier water’s flow rates are shown in Figure 4, with inclusion of the effects owing to the variation caused by the gas pressure change. However, the spread is still narrow. The backflow is minimized using the PEEK tubing with ID of 25µm. The change in relative pressure alters the relative flows. This results in smaller or larger bubbles.

Table 1 summarizes the correlation between the size of the bubble and the quantity of gas present in it. The bubble size will vary when it passes downstream of the pressure gradient. The amount of gas present in a bubble can be calculated for ideal gas using the relationship, A PV = nRT.

Figure 4. Range of carrier flow rates achieved during test. The spread in the data represents the variation due to gas pressures in the range of 3 to 5 bar.

Table 1. Size to volume conversion

Bubble Diameter (micrometers) Bubble Volume (picoliters)
5 0.065
10 0.524
15 1.767
20 4.189

The bubble size can be estimated through pixel analysis of images. The channel depth strongly influences the size of stable bubbles. The bubble size needs to be carefully calculated as there is a change in the size of the bubbles that are leaving the chip. Figure 5 shows the difference in bubble size with variations in pressure ratio.

The stability of the bubble s is dictated by the pressure ratio (Pgas/Pcarrier), which is controlled with P-Pumps. Very large bubbles or backflow are the results of extreme ratios. Hence, it is possible to generate a wide range of bubble sizes by controlling the pressure ratio.

Figure 5. Left: Dependence of bubble size on water pressures for gas pressure of 5 bar, and 6 bar respectively. Right: Dependence of bubble size on gas pressure at water pressure of 3.5 bar.

Bubble generation relies on the pressure balance at the junction. As can be seen in Figure 5, the increasing carrier pressure at constant gas pressure changes the pressure ratio at the junction and increases the flow of the carrier water. This lowers the flow rate of the gas, yielding smaller and slower bubbles.

Conversely, the increasing gas pressure at constant carrier water pressure results in larger and faster bubbles. The bubble generation rate is roughly 10-1000 bubbles per second. Higher bubble generation rates can be achieved with higher total flows. Larger bubbles are the result of increasing nitrogen pressure and smaller bubbles are the result of lowering nitrogen pressure.

Conclusion

This article demonstrates the ability of the Small Quartz Droplet Chip to rapidly generate monodisperse gas bubbles in the diameter range of 6-40µm. Gas bubble generation relies on the on the pressure balance at the junction of the Small Quartz Droplet Chip and different possibilities can be accomplished by simply controlling P-Pump pressures.

With a broad range of bubble size and generation frequency and the ability to produce gas bubbles with consistent characteristics make the Small Quartz Droplet Chip suitable for applications, including merging bubbles, single file bubbles, polydisperse bubbles and foam generation. The Small Quartz Droplet Chip is compatible with existing Dolomite pumps, tubing, valves, camera, interfaces, connectors and microscope systems, thus providing a versatile solution to rapidly perform measurement analyses.

About Dolomite Microfluidics

Dolomite is a world leader in Productizing Science™ and an innovator in creating microfluidic devices and solutions. We sell the coolest microfluidic products around the world, often working with partner companies to extend the range of technology available to our customers. Productizing Science™ means creating marketable and commercially successful products from scientific discovery, and Dolomite excels in commercialising microfluidic products which exceed expectations.

We offer modular, standard microfluidic systems benefiting a wide range of applications, always adhering to the principles of having multiple functionalities, scalability, user-friendly design and a cost-effective, flexible solution for our customers.

Moreover, we offer Productizing Science™ as a service, which is a product development & manufacturing partnership creating microfluidic solutions for problems which span an extremely wide range of applications. Customers come to Dolomite with their technical challenges, and Dolomite helps solve these problems using its extensive background technology.

Dolomite also designs & manufactures a wide range of world leading standard components such as OEM products, microfluidic connectors & interfaces, chips, pumps, valves, detectors, sensors & accessories. Finally, we offer design consultancy to create customized chips or connectors and/or a prototyping service for the supply of glass, metal or polymer devices, and custom microfluidic connectors.

This information has been sourced, reviewed and adapted from materials provided by Dolomite Microfluidics.

For more information on this source, please visit Dolomite Microfluidics.

Date Added: May 21, 2014 | Updated: May 22, 2014
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