Patterning microfluidic device wettability using flow confinement.
ABSTRACT We present a simple method to spatially pattern the surface properties of microfluidic devices using flow confinement. Our technique allows surface patterning with micron-scale resolution. To demonstrate its effectiveness, we use it to pattern wettability to form W/O/W and O/W/O double emulsions.
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ABSTRACT: UV laser irradiation (λ = 193 nm), below and above damage thresholds, is used to both alter and pattern the surface properties of borosilicate slides to tune and control the contact angle of a water drop over the surface. Large variation exceeding 25° using laser processing alone, spanning across both sides of the original contact angle of the surface, is reported. An asymmetric contact angle distribution, giving rise to an analogous ellipsoidal-like drop caplet, is shown to improve convective self-assembly of silica nanoparticles into straighter optical microwires.Optical Materials Express 02/2013; 3(2). · 2.92 Impact Factor
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ABSTRACT: Herein the water film was introduced to the hydrophilic area on the line patterned surface to solve the contradiction caused by surface roughness (high different wettability has advantage to control the droplet but high roughness for that high wettability difference causes obstruction of droplet moving). Thus the droplet on the water film could not be hindered to line direction but restricted to orthogonal direction, effectively. In addition, droplet behaviors according to droplet volume and line thickness were studied. Droplet fell off the line with narrowing the interface between the droplet and the water film on the line. When the droplet fell off the line, the plate angle was designated as a critical plate angle and it used as an indicator of surface capability to control the droplet. As a result critical plate angle increases as droplet volume decreases and line thickness increases.Journal of the Korean Society for Precision Engineering. 12/2013; 30(12).
Patterning microfluidic device wettability using flow confinement†
Adam R. Abate,‡aJulian Thiele,‡abMarie Weinhartcand David A. Weitz*a
Received 10th March 2010, Accepted 5th May 2010
First published as an Advance Article on the web 21st May 2010
We present a simple method to spatially pattern the surface prop-
erties of microfluidic devices using flow confinement. Our technique
allows surface patterning with micron-scale resolution. To demon-
strate its effectiveness, we use it to pattern wettability to form
W/O/W and O/W/O double emulsions.
Many applications of microfluidic devices require channels with
patterned surface properties.1One such application is the formation
ofmultiple emulsions which consistoflarge dropswith smaller drops
inside.2–4To make these structures requires microfluidic devices with
spatially patterned wettability; this allows the inner drops to be
However, current methods to spatially pattern the wettability of
microfluidic devices are difficult to use and of limited versatility. The
best approach for patterning wettability uses a polymerization reac-
tion that is initiated by exposure to ultraviolet (UV) light.8–11To
spatially control wettability, the microfluidic device is exposed to
a spatially controlled light pattern, imparting a wettability pattern of
the same shape. However, since micron-scale resolution is required,
sophisticated optics and a powerful UV-light source are needed.
Moreover, this method is difficult to use to fabricate many devices
with the same pattern, since this requires precise alignment of the
optical pattern with all devices simultaneously, a technically chal-
lenging procedure. A superior wettability patterning approach would
combine simplicity with high-resolution patterning, and would allow
fabrication of large numbers of devices with identical properties.
In this paper, we present a versatile method for patterning surface
wettability. We use an inert fluid to physically confine a chemical
treatment that alters wettability in selected regions of the device; this
requires only basic equipment and allows high-resolution wettability
patterning. Moreover, since spatial control is achieved by physical
different surface treatments to be used.12,13To illustrate this, we use
photo-initiated and thermal-initiated surface treatments with our
method. To demonstrate the effectiveness of our approach, we use it
and O/W/O double emulsions.
We fabricate our microfluidic devices using soft-lithography in
polydimethylsiloxane (PDMS).14,15Our devices consist of micro-
channels 100 mm in height. To control the wettability of our devices,
a chemical treatment. To accomplish this, we incorporate fluoro-
silanes and methacrylate-silanes into the sol–gel. To prepare the sol–
gel solution we combine 1 mL tetraethylorthosilicate (TEOS), 1 mL
methyltriethoxysilane (MTES), 0.5 mL (heptadecafluoro-1,1,2,2-tet-
rahydrodecyl)triethoxysilane, 2 mL trifluoroethanol and 1 mL
3-(trimethoxysilyl)propyl methacrylate. Before the coating can be
applied the sol–gel must be preconverted by adding an acid catalyst.
added the solution may turn cloudy; it is vigorously shaken for
several seconds and placed on a hot plate set to 85?C for 30 s. This
is repeated until the mixture clears, which takes approximately
2 minutes. To coat the channels, we fill them with the sol–gel mixture
immediately after plasma bonding. We then heat the device on a hot
plate set to 180?C; this vaporizes the mixture, depositing a uniform
sol–gel coating on the channel walls. The coating thickness can be
reduced by diluting the sol–gel mixture several times in methanol,
without adversely affecting wettability control. Due to the fluoro-
silanes in the sol–gel, the coated channels are very hydrophobic. To
confirm this wettability, we perform contact angle measurements of
glass slide the water drop beads up, achieving a hydrophobic contact
angle of ?105?, as shown in Fig. 1A. To switch the wettability to
hydrophilic, we use the methacrylate-silanes in the sol–gel. These
silanes contain double bonds, which can be used to graft hydrophilic
microfluidic channels. To confirm this, we perform contact angle
measurements of sol–gel-coated glass slides with water drops in air. The
sol–gel is intrinsically hydrophobic due to the incorporation of fluoro-
silanes, as confirmed by the hydrophobic contact angle of 105?(A). It is
converted to hydrophilic by attaching PAA to the surface using a poly-
merization reaction, as shown by the hydrophilic contact angle after
treatment of 20?(B). The scale bars denote 50 mm.
The sol–gel coating allows us to control the wettability of
aSchool of Engineering and Applied Sciences/Department of Physics,
Harvard University, Cambridge, Massachusetts, USA. E-mail: weitz@
seas.harvard.edu; Tel: +1 617-495-3275
bInstitute of Physical Chemistry, University of Hamburg, Germany
cInstitute of Chemistry and Biochemistry—Organic Chemistry, Freie
Universitaet Berlin, Germany
† Electronic supplementary information (ESI) available: AutoCAD
design of the microfluidic device, and movies of photo-initiated and
thermal-initiated surface treatment as well as movies of W/O/W and
O/W/O double emulsion formation. See DOI: 10.1039/c004124f
‡ Both authors contributed equally to this work.
1774 | Lab Chip, 2010, 10, 1774–1776This journal is ª The Royal Society of Chemistry 2010
COMMUNICATIONwww.rsc.org/loc | Lab on a Chip
polymers to the surface, to make it hydrophilic. For the polymers we
is thus very hydrophilic. To graft the PAA, we fill the channels with
acrylic acid (AA) monomer solution and initiate polymerization; this
creates AA polymers, some of which react with the double bonds on
the sol–gel, grafting them to the surface. This switches the wettability
on a glass slide treated the same way, as shown in Fig. 1B. With the
sol–gel, we can thus control the wettability of our microfluidic
emulsions in microfluidics, and spatially controlled wettability is
essential when forming multiple emulsions. This is because channel
wettability determines the type of drops that a microfluidic device
Thus, a microfluidic device that creates multiple emulsions is a strin-
gent demonstration of the coating technology presented here. To
make double emulsions requires a microfluidic device consisting of
the inlet of the second drop maker, as depicted in Fig. 2A. To make
W/O/W double emulsions, the first drop maker is made hydrophobic
and the second hydrophilic; this allows the first to make water drops
which are encapsulated in oil drops in the second drop maker, as
use our flow-confinement technique to make the second drop maker
hydrophilic. To accomplish this, we inject the reactive monomer
solution into the outlet of the device at 200 mL h?1and the inert fluid
into the inner-phase and middle-phase inlets at 2000 mL h?1; the
continuous phase inlet is left open, to act as the outlet for both
solutions, as indicated in Fig. 2B. This causes the reactive and inert
fluids to meet in the second drop maker, so that a stable interface
magnitude of diffusive to advective transport. If diffusion across the
interface is small compared to the flow velocity, the reaction is
the ratio of advective to diffusive transport at a fluid–fluid interface.
controlled by syringe pumps, d ¼ 100 mm the length of the liquid–
liquid interface in the drop formation region, and D the diffusion
coefficient of the monomer, 1.3 ? 10?9m2s?1. We calculate Pe to be
?300; thus, diffusion is negligible in our system, yielding a sharp
interface that confines the reaction. This interface sets the location at
which the wettability transitions from hydrophobic to hydrophilic.
Our technique can also create the inverse wettability pattern, to form
weinjectthereactive andinertfluids, asshown in Fig. 2C;thismakes
the first drop maker hydrophilic and the second hydrophobic, as
illustrated in Fig. 2D. Other injection strategies can also be used to
pattern more complex devices, as discussed in the ESI.†
Since spatial control of the hydrophilic treatment is achieved by
physically confining the reaction, our approach is very general with
though many other reactions are possible.12,13,18For the photo-initi-
ated reaction, we use a monomer solution consisting of 5.8 mol L?1
AA in ethanol. To initiate the reaction, we incorporate 2-hydroxy-2-
methylpropiophenone (Darocur? 1173) as a photo-initiator at
22.6 mol%, relative to the amount of AA. Under exposure to UV
light, these molecules release radicals that initiate polymerization of
the AA. The monomers covalently bond, forming polymers; some of
these polymers attach to the double bonds on the surface, attaching
them to the surface. The device is exposed to light everywhere, but
attachmentof the polymers occursonly in the lower portion, because
the other regions are blocked by the inert fluid, as shown in Fig. 3A.
Forthe thermal-initiated reaction,we use AA in water at5.8 mol L?1
concentration; however, rather than a photo-initiator we use
a thermal initiator. We use APS at 1.50 mol% with TEMED at
3.7 mol% asan accelerant, both in relation to the amountofAA.We
inject the solutions as before, but this time initiate the reaction by
placing the device on a hot plate setto 80?C. Again, even though the
device is heated everywhere, the reaction is confined to the lower
portion ofthe device by the inert fluid, as shown in Fig. 3B. To verify
that this allows us to spatially control grafting of PAA, we image the
meniscus between HFE-7500 fluorocarbon oil and deionized water
them at the wettability crossover; this allows us to image the shape of
control where PAA is grafted.
To demonstrate that flow patterning provides the control needed
to form double emulsions, we use it to pattern devices to form both
W/O/W and O/W/O double emulsions. As fluids for the double
emulsions, we use HFE-7500 fluorocarbon oil with the ammonium
salt of Krytox? 157 FSL at 1.8% by weight as the surfactant; for the
as the surfactant. To form W/O/W double emulsions, we use flow
upper portion is hydrophobic and the lower portion hydrophilic (A). To
create this wettability pattern, we inject a reactive surface treatment
solution into the device outlet and an inert blocker solution into the inner
and middle-phase inlets (B). Where the two solutions meet a sharp
interface forms, due to laminar flow conditions; this sets the cross-over
between the treated and un-treated regions. To form O/W/O double
emulsions, we invert the pattern (C); this is achieved by switching the
inlets into which the reactive and inert solutions are injected (D).
To form W/O/W double emulsions requires a device in which the
This journal is ª The Royal Society of Chemistry 2010Lab Chip, 2010, 10, 1774–1776 | 1775
confinement to make the first drop maker hydrophobic and the
second hydrophilic. We inject the fluids into the first, second, and
third inlets at 1000, 900 and 1500 mL h?1, respectively; this allows the
first drop maker to produce water drops in oil and the second to
encapsulate the water drops in larger oil drops, forming W/O/W
double emulsions, as shown in Fig. 4A. To produce O/W/O double
emulsions, we simply invert the wettability pattern, as shown in
Spatial control of wettability is necessary for a variety of appli-
cations of microfluidic devices. In contrast to other wettability
patterning methods which require precise alignment of an optical
pattern with the microfluidic device, our method requires only that
fluids are injected in the correct configuration; this makes our
approach simple and very scalable. This should be useful for
applications that require fabrication of large numbers of devices
with identical properties, as needed in scale-up. It should also be
useful for patterning the functional properties of devices for bio-
logical applications, as in cancer-cell screening applications in which
cells must pass through certain regions of the device but be captured
This work was supported by the NSF (DMR-0602684), the Harvard
MRSEC (DMR-0820484), and the Massachusetts Life Sciences
Center. JT received funding from the Fund of the Chemical Industry
(Germany) which is gratefully acknowledged.
Notes and references
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means, our approach is general with respect to the surface chemistries
that can be used to control wettability, which we demonstrate by using
(A) a UV-initiated reaction and (B) a thermal-initiated reaction. Because
the same flow pattern is used for both reactions, the resulting wettability
patterns are the same. To confirm these patterns, we image the meniscus
between HFE-7500 fluorocarbon oil and deionized water under static
conditions in the channel. Due to the different wettability properties in
the upper and lower junctions, a meniscus forms between them at the
wettability crossover; this allows us to image the shape of the crossover,
as shown in (C). The scale bars denote 100 mm.
Sinceconfinement of the surfacetreatmentisachievedbyphysical
pattern the wettability of a double emulsion device. We make the first
drop maker hydrophobic and the second hydrophilic (A). To form
O/W/O double emulsions, we invert the pattern (B). To confirm that the
double emulsions are formed properly, we image samples collected from
both devices, lower panels. The scale bars denote 100 mm.
To form W/O/W double emulsions we use flow confinement to
1776 | Lab Chip, 2010, 10, 1774–1776This journal is ª The Royal Society of Chemistry 2010