Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Open Microfluidic and Nanofluidic Systems

16.02.2005


Max Planck scientists develop fundamentals for new microfluidic and nanofluidic devices


Atomic (or scanning) force microscopy images of liquid morphologies on silicon substrates with rectangular surface channels which have a width of about one micrometer. On the left, the liquid does not enter the channels but forms large lemon-shaped droplets overlying the channels (dark stripes). On the right, the liquid enters the channels and forms extended filaments separated by essentially empty channel segments (dark stripes). In the bottom row, several parallel surface channels can be seen in both images; in the top row, there is only one such channel with a single droplet (left) or filament (right). Close inspection of the upper right image reveals (i) that this filament is connected to thin wedges along the lower channel corners and (ii) that the contact line bounding the meniscus of the filament is pinned to the upper channel edges. Image: Max Planck Institute for Colloids and Interfaces


Morphology diagram as a function of the aspect ratio X of the channel and the contact angle q which characterizes the interaction between substrate material and liquid. This diagram contains seven different morphology regimes which involve localized droplets (D), extended filaments (F), and thin wedges (W) in the lower channel corners. The diagram represents a complete classification of all possible wetting morphologies and should be universal, i.e., it should apply to different liquids and substrate materials. Image: Max Planck Institute of Colloids and Interfaces



The labs of the future will be "labs-on-a-chip", i.e., integrated chemical and biochemical laboratories shrunk down to the size of a computer chip. An essential prerequisite for such labs are appropriate microcompartments for the confinement of very small amounts of liquids and chemical reagents. Directly accessible surface channels, which can be fabricated by available photolithographic methods, represent an appealing design principle for such microcompartments and, thus, provide a new route towards open microfluidic and nanofluidic systems. Scientists from the Max Planck Institute of Colloids and Interfaces, the Max Planck Institute of Dynamics and Selforganization and the University of California in Santa Barbara have shown that such open systems are possible in general but only if the geometry of the surface channels is carefully matched with their wettability (PNAS 102, 1848-1852 (2005).

Many research groups around the world work towards the construction of "labs-on-a-chip" in order to integrate chemical and biochemical analyzers on the micrometer or even nanometer scale. These devices will significantly change the way in which research is performed in the life sciences since they offer the ability to work with much smaller reagent volumes, much shorter reaction times, and the possibility of massive parallel processing. In general, this should lead to increased throughput and, thus, to reduced cost of (bio)chemical analysis. In addition, such integrated labs-on-a-chip have many potential applications in biomedicine and bioengineering. In the context of biomedicine, for example, they could provide fast and detailed analysis of blood samples in the physician’s office without the need to wait several days before the sample has been returned from specialized laboratories. Other applications include customized chips for space travel in order to monitor microbes inside spacecraft or to detect life on other planets.


An obvious prerequisite for such miniaturized labs are appropriate microcompartments for the confinement of very small amounts of liquids and chemical reagents. Like the test-tubes in macroscopic laboratories, these microcompartments should have some basic properties: They should have a well-defined geometry by which one can measure the precise amount of liquid contained in them; they should be able to confine variable amounts of liquid; and they should be accessible in such a way that one can add and extract liquid in a convenient manner.

An appealing design principle for such microcompartments is based on open and, thus, directly accessible surface channels which can be fabricated on solid substrates using available photolithographic methods. The simplest channel geometry which can be produced in this way corresponds to channels with a rectangular cross section. The width and depth of these channels can be varied between a hundred nanometer and a couple of micrometer.

At first sight, it seems rather obvious to use such surface channels as microcompartments. However, if one actually tries to fill these channels with a certain liquid, one observes that the liquid often refuses to enter the channels. In fact, as shown in the new PNAS study, liquids at surface channels can attain a large variety of different wetting morphologies including localized droplets, extended filaments, and thin wedges at the lower channel corners. Examples for these morphologies as observed by atomic (or scanning) force microscopy (AFM) are shown in Figure 1.

When the AFM experiments were first performed, it was not known how to produce a certain liquid morphology since there was no systematic theory for the dependence of this morphology on the materials properties and on the channel design. Such a theory has now been developed. This theory addresses the strong capillary forces between substrate material and liquid and takes the ‘freedom’ of contact angles at pinned contact lines into account. Such a contact line is visible in the upper right image in Figure 1. In such a situation, the contact angle is not determined by the classical Young equation but can vary over a wide range of values.

A surprising prediction of the new theory is that the experimentally observed polymorphism of the wetting liquid depends only on two parameters: (i) the channel geometry, i.e., the ratio of the channel depth to the channel width; and (ii) the interaction between substrate material and liquid. One has to distinguish seven different liquid morphologies which involve localized droplets (D), extended filaments (F), and thin wedges (W) at the channel corners. For microfluidics applications, the most important morphology regime is (F) which corresponds to stable filaments. Since this regime covers a relatively small region of the morphology diagram, it can only be obtained if one carefully matches the channel geometry with the substrate wettability. Thus, a water filament in a narrow channel that has a width of 100 nanometer can sustain an overpressure up to 15 atm. In contrast, if the channel had a width of one millimeter, the water filament could only sustain a thousandth part of an atmosphere.

One relatively simple application of the morphology is obtained if the system is designed in such a way that one can vary or switch the contact angle in a controlled fashion. One such method is provided by electrowetting; alternative methods, which have recently been developed, are substrate surfaces covered by molecular monolayers that can be switched by light, temperature, or electric potential.

The experiments described in the PNAS study use a polymeric liquid that freezes quickly and can then be scanned directly with the tip of an atomic force microsope. However, the same morphology diagram should also apply to other liquids and other substrate materials. It should also remain valid if one further shrinks the surface channels and, in this way, moves deeper into the nanoregime. As one reaches a channel width of about 30 nanometer, one theoretically expects new effects arising from the line tension of the contact line, but such nanochannels have not been studied experimentally so far.

The new PNAS study provides an instructive example for the close relation between basic research and technological development in the micro- and nanoregime: open systems with directly accessible surface channels can be used for micro- and nanofluidic applications but only if one carefully matches the channel geometry with the substrate wettability. This constraint is a direct consequence of the strong capillary forces that dominate in the micro- and nanoregime and can be formulated in a quantitative way using the methods of theoretical physics. In general, the development of any new technology requires a systematic understanding of the underlying physics. This latter constraint applies to all length scales: if one wanted to build a robot which walks over water, for instance, a human-like robot is a bad idea while a spider-like robot is a much better choice.
Original work:

Ralf Seemann, Martin Brinkmann, Edward J. Kramer, Frederick F. Lange, Reinhard Lipowsky
Wetting morphologies at microstructured surfaces
PNAS 102, 1848-1852, February 8, 2005, 10.1073/pnas.0407721102

Dr. Bernd Wirsing | EurekAlert!
Further information:
http://www.mpg.de

More articles from Life Sciences:

nachricht What the world's tiniest 'monster truck' reveals
23.08.2017 | American Chemical Society

nachricht Treating arthritis with algae
23.08.2017 | Empa - Eidgenössische Materialprüfungs- und Forschungsanstalt

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Fizzy soda water could be key to clean manufacture of flat wonder material: Graphene

Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.

As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...

Im Focus: Exotic quantum states made from light: Physicists create optical “wells” for a super-photon

Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.

Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...

Im Focus: Circular RNA linked to brain function

For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.

While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...

Im Focus: RAVAN CubeSat measures Earth's outgoing energy

An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.

The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...

Im Focus: Scientists shine new light on the “other high temperature superconductor”

A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.

Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Call for Papers – ICNFT 2018, 5th International Conference on New Forming Technology

16.08.2017 | Event News

Sustainability is the business model of tomorrow

04.08.2017 | Event News

Clash of Realities 2017: Registration now open. International Conference at TH Köln

26.07.2017 | Event News

 
Latest News

What the world's tiniest 'monster truck' reveals

23.08.2017 | Life Sciences

Treating arthritis with algae

23.08.2017 | Life Sciences

Witnessing turbulent motion in the atmosphere of a distant star

23.08.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>