Lab on a Chip Blog

Surface tension is an important property of all liquids and influences many of their characteristics, such as droplet formation. Surface tension at the interface of two different liquids is interfacial tension. An example of this is the separation of oil and water in solution.

There are other solutions of dissimilar liquids that have extremely small interfacial tensions, 1000 times less than that of oil and water. Measuring this accurately by traditional techniques, such as force tensiometry methods, is a problem. Although many more methods, have been proposed to improve upon this, a simple and accurate technique is yet to be found.

Howard Stone has now led a team at Princeton University and Harvard University, USA, in producing the first microfluidic tensiometer with the capability of measuring ultralow interfacial tension. Their development uses only a small amount of reagents and magnetic particles followed by application of a mathematical model to calculate the interfacial tension.

The measurement is taken by observing the distance between the center of the magnetic particle and the flowing interface. The paramagnetic beads may pass through the interface or be trapped by it due to the balance of magnetic and interfacial forces – only when this ratio passes a threshold value do the particles pass through.  The method uses these threshold values to then calculate the ultralow interfacial tension.

This simple, miniaturised tensiometer has application in many different systems, with advantages for the applications of ultralow interfacial tension including enhanced oil recovery and microemulsions.

Microfluidic ultralow interfacial tensiometry with magnetic particles
Scott S. H. Tsai, Jason S. Wexler, Jiandi Wan and Howard A. Stone
DOI: 10.1039/C2LC40797C

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Point-of-care diagnostics is the aim for much medical research. Often the problem is the ease of preparation of the biological sample, which usually requires multiple steps before the diagnostic test can even begin. Automation of sample preparation has been explored and another option is pressure-driven microfluidics. However, this requires complex external equipment to run the chip only suitable for use in the laboratory away from the patient.

To get true point-of-care diagnostics, sample preparation has to be fast, simple and cheap.

Stationary microfluidics is now being explored.  Liquid is present in fixed positions on the chip and it is magnetic particles that move between the points holding liquid. This often involves separating the liquids using an oil barrier, but this has disadvantages of over-complicating the device by having more than one liquid (the test solution), meaning an additional very careful pipetting step. Not all test solutions or all diagnostic tests would be compatible with this oil barrier, for example, purification of protein. Oil-free versions are now being designed, usually “open droplet” devices, but this again needs accurate pipetting and risks contamination of the sample. Using a large tube limits the miniaturisation capability.

Menno Prins has led a team based at Philips Research and Eindhoven University of Technology, The Netherlands, to solve this problem. They demonstrate technology called Magneto–Capillary Valve (MCV) technology. There is no need for an oil phase.  The confinement of the liquid occurs by capillary forces and they can be separated by a gas. The device operation is a balancing act between magnetic and capillary forces.

In this HOT article, they give numerous device architectures and clearly demonstrate the advantages of this technique in purifying and enriching nucleic acids and proteins. They look in detail at how the device works and its applicability for a wide range of problems in point-of-care-diagnostics.

This research is best read in detail, and as this HOT article is free to access for the next 4 weeks*, you can give it a read now by clicking on the link below:

Magneto-capillary valve for integrated purification and enrichment of nucleic acids and proteins
Remco C. den Dulk, Kristiane A. Schmidt, Gwénola Sabatté, Susana Liébana and Menno W. J. Prins
DOI: 10.1039/C2LC40929A

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Researchers led by Aloke Kumar at the Oak Ridge National Laboratory, USA, and Thomas Thundat at the University of Alberta, Canada, used a porous microfluidic device to study biofilm streamer development in porous media.

The microfluidic method allowed close observation and measurement of streamers developing between microposts embedded in a microchannel. The formation of streamers in the device forming a web connecting different microposts was strongly influenced by the hydrodynamics.

Higher flow rates resulted in thicker structures and higher numbers of streamers as they grew much faster. At very high flow rate, their formation was only transient as they were destabilized after forming. The fluid flow seems to be the cause of both the formation and destabilization of the streamers. Interestingly, streamers formed parallel to the flow direction in regions of high flow rate and transverse to it in regions of low flow rate. In contrast with a previous report, they observe streamers distributed throughout the height of the device, attributed to secondary flow in the corners of the device. Carrying out numerical simulations, Kumar and Thundat et al. showed that secondary flows in the z-direction do not have a large role to play.

Development of streamers is complex and a great deal of detail remains to be elucidated, however, this Lab on a Chip article indicates that streamer formation might lead to mature microbial structures under the right conditions.

This HOT article is free to access* on our site for the next four weeks, so why not download the paper here:

A web of streamers: biofilm formation in a porous microfluidic device
Amin Valiei ,  Aloke Kumar ,  Partha P. Mukherjee ,  Yang Liu and Thomas Thundat
DOI: 10.1039/C2LC40815E

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