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| High-throughput (HT) tools to measure properties of complex fluids
will greatly facilitate formulations science. Industrial members of
the NIST Combinatorial Methods Center viewed a new HT method to measure
interfacial tension as a primary need for product development. We
have addressed this challenge with a novel microfluidic, high-throughput
strategy to materials research and development. |
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| Steven D. Hudson, João
T. Cabral, and Kathryn L.
Beers |
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| By developing microfluidic (mF) instrumentation to measure the behavior
of polymers in complex fluids, we aim to demonstrate exciting new
approaches to formulation science. With fast, flexible methods of
designing and assembling milli-scale models of emulsion or colloid
processing tools, high-throughput screening and optimization of new
materials and discovery of new applications can be achieved across
multiple stages of process development. Chemical suppliers and their
customers will be able to obtain more and higher quality data relevant
to the polymer additive properties critical to their business. |
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| Most current mF technology is directed toward life science applications
with devices designed primarily for low viscosity, aqueous solutions.
We have developed a soft lithographic technique, which imparts solvent
resistance to our devices and facilitates the integration of both
aqueous and organic media. Multiple compositions can be evaluated
by varying the input of either phase. Combinatorial mF systems can
then be developed to measure viscosity, interfacial tension, and stability
of emulsions in an automated and high-throughput manner. Our systems
allow multiple feedstocks to be mixed in a range of compositions and
have their above-mentioned properties measured over a range of temperatures.
We envision a combinatorial factory, in which materials synthesis,
characterization, and processing are integrated. |
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| As a first step towards this goal, and in consultation with industrial
sponsors from ICI National Starch and Procter and Gamble, we are developing
an instrument to measure interfacial tension of water/oil systems
that is accurate within a few percent over a wide range of interfacial
tension (from 0.1 to 50 mN/m). Moreover, microfluidic technology is
well-suited to kinetic analysis, since the properties of the fluid
can be analyzed at different points along the channel. In this way,
dynamic interfacial properties associated with interface age, as well
as quasi-equilibrium properties, are accessible. |
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| We have implemented the HT instrument using a microfluidic device
connected to fluid pumps and mounted on a microscope equipped with
a fast camera. Experiment and data handling is computer controlled,
featuring a real-time image analysis program. The device is fabricated
using a novel rapid prototyping (RP) technique reported earlier and
takes advantage of frontal photopolymerization (FPP) to generate 3-dimensional
structures. This contact photolithography technique allows for the
control of both lateral and vertical dimensions of patterns imaged
on a negative polymer resist, which is then replicated on a transparent
elastomer (PDMS). Assisted by flow simulations, we have devised a
mF device geometry capable of drop formation, mixing of components,
and drop acceleration and deformation (Fig. 1). |
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| Figure 1: Microfluidic prototype for measuring interfacial
tension in immiscible fluids, displaying a combinatorial droplet mixer,
acceleration junctions, and a series of extensional flow constrictions.
This 3-dimensional device is fabricated using a novel rapid prototyping
technique based on frontal photopolymerization and is sealed against
a microscope glass slide. Inset: Schematic microchannel design. Fluids
1 a and b are mixed together and injected as drops into immiscible
stream 2. These drops are then fed by 3a and 3b into channel 4 for
analysis and measurement. The constrictions in channel 4 accelerate
and therefore stretch the drops. Multiple constrictions permit measurement
at different interface age. |
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| A custom-built LabVIEW program drives fluid pumps, analyzes images
in real time (Fig. 3) and computes drop shape, velocity, and fluid
extension rate. Following Taylor's classical theory for drop deformation
in extensional flow fields, we compute the material rate of change
of the drop deformation D (defined in Fig. 2): |
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where 
This rate is directly proportional to interfacial tension  .
Specifically, the drop relaxation time  is:
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where ao is the undeformed drop radius, continuous
phase viscosityis the continuous phase viscosity, is
the shear rate, and
is the relative viscosity of the drop. |
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| Figure 2: Freeze-frame image of drops flowing left to
right in an extensional flow gradient. When the drops leave the constriction
(the channel walls appear as slanted lines in the left half of the
image), the flow decelerates in proportion to the change in cross-sectional
area. The drops, which are generated periodically in time, therefore,
become closer together. This deceleration corresponds to a stretching
in the transverse direction; note that the drops at the left side
are stretched vertically, and their deformation decays as they pass
to the right. |
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| The current measurement rate may exceed 100 data points/s and is
dependent on drop formation and image acquisition rates. |
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| Figure 3: LabVIEW interface used to control the instrument
and record data. |
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| Figure 4: a.) Simulated and b.) experimental analysis
of drop deformation upon departure from the channel constriction.
The radius ao, deformation D and velocity u of the drops are measured
directly by image analysis, at a rate of > 100 data points / s.
The extension rate e is simply the gradient in the velocity du/dx.
When the data is plotted in this way, the slope is directly proportional
to the interfacial tension. For water in oil, the tension is (31.8
± 0.8) mN/m, which is in accord with pendant drop measurement
(32.0 mN/m). For the surfactant (EO19PO30EO19) aqueous solution in
oil, the tension is 7.2 mN/m. In this experiment, the interface age
is approximately 1 s. |
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| For More Information on this Topic |
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| E. J. Amis,
P. R. Start (Polymers Division, NIST) Focus Project Members: ICI
National Starch; Procter and Gamble |
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| J.T. Cabral, S.
D. Hudson, C. Harrison, and J.
F. Douglas "Frontal Photopolymerization for Microfluidic
Applications," Langmuir, in press. |
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