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Any piece of matter is bounded by surfaces or
interfaces. This
is obvious on the macroscopic scale but applies equally well on the micrometer
and nanometer scale and plays an important role in
micro- and nanofluidics.
Fluid interfaces are particularly interesting since they can easily adapt their shape to
external forces and constraints. If a small liquid droplet is placed onto a rigid substrate, for example,
the droplet can attain a variety of wetting morphologies depending on the
chemical composition and/or surface topography of the underlying substrate. As
one varies a control parameter such as the droplet volume, the droplets undergo
morphological wetting transitions
[1]
as first observed for
water channels on striped substrate surfaces
[2].
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Recent research activities on interfacial phenomena include:
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Wetting of chemically patterned substrates:
Many experimental techniques can be used to prepare substrate surfaces
with several types of surface domains that differ in chemical composition and, thus,
wettability. When a certain amount of liquid is placed on
such a substrate, it undergoes morphological wetting transitions as shown for circular lyophilic
[1]
[3]
and lyophobic
[4]
surface domains as well as for short
[2]
and long
[5]
surface stripes.
Nucleation of liquid droplets at circular surface domains is
governed by two successive energy barriers
[6].
Two recent reviews are
[7] and
[8].
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Wetting of topographically structured substrates:
Morphological transitions also occur for liquids in contact with a
topographically structured substrate as shown for
surfaces with rectangular grooves.
[9]
Both for topographically structured and for chemically patterned surfaces,
the basic mechanism underlying these transitions is provided by
variable contact angles
at pinned contact lines
[7].
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Line tension effects:
When the size of a liquid droplet is smaller than about 100 nanometers,
its shape is affected by the tension of the contact line
[10].
In fact, the
local contact angle depends both on this line tension and on
the line tension gradient as first shown in
[11].
Line tension changes the stability of thin liquid channels (or filaments)
on uniform surfaces
[12]
as well as the morphology of small liquid droplets on chemically
patterned surfaces
[13].
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Tense membranes and vesicles:
When subject to mechanical tension arising, e.g., from osmotic forces,
membranes and vesicles attain shapes that
are very similiar to those of interfaces and droplets.
This similarity applies in particular to
strongly adhering vesicles and wetting droplets
[8].
Furthermore, membranes in contact with several aqueous phases
undergo complete-to-partial wetting transitions
[14],
which are governed by a "hidden" material parameter, the
intrinsic contact angle
[15].
The latter systems can also store a large amount of membrane area in
the form of membrane nanotubes
[16].
For more information, see
interface topics.
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