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News

Open microfluidic and nanofluidic systems

Max Planck Society : 16 February, 2005  (Technical Article)
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.
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‚?TM 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.
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