(535d) Marangoni Phenomena during Droplet Formation in Microfluidics

Droplet formation is a common
process in industrial [1] and research applications [2]. Microfluidics are used
to study emulsification, coalescence and foaming  [3, 4, 5]. Droplet based microfluidics
show remarkable control over the experimental conditions, excellent observation
capabilities and high throughput of experimental results [6]. Droplets in
microfluidics have been used as micro-reactors [7] and
as vessels to study cell-cell interactions [8].

Surfactants are
used in order to stabilise the produced droplets against coalescence and
facilitate break-up by reducing interfacial tension [9]. The droplet formation
process involves the formation and shearing of interfaces. This can lead to
surfactant concentration gradients along the interface that create interfacial
tension gradients that ultimately manifest as Marangoni stresses that might affect
the formation process [2]. Furthermore, since the time scales of droplet
formation are comparable, and sometimes smaller, than the equilibration time
scales of surfactants, the whole process evolves under non-equilibrium
conditions [10]. There have been recent attempts to elucidate any dynamic
surfactant effects but they have not been resolved completely yet [4, 11].

In this work, we study single
droplet formation in a flow focusing microfluidic device, made from polydimethylsiloxane
(PDMS) using soft lithography. Channels are rectangular with a uniform height
of 150 μm. The dispersed
phase was a water/glycerol mixture while the continuous phase was silicone oil.
During the experiments, we observed the formation process in the flow-focusing
junction. Three types of solutions were studied,
containing quantities of C10TAB, C12TAB or no surfactant. Surfactant
concentrations were lower than the critical micelle concentration and the
surfactant solutions were chosen to have the same
equilibrium interfacial tensions.

We focused our study on the
liquid bridge, the region of fluid that connects the expanding drop to the bulk
of the solution. By using Ghost Particle Velocimetry (GPV) [3, 12, 13, 14], a
non-intrusive optical technique that allows us to extract velocity profiles, we
are able to identify Marangoni phenomena manifesting close to the interface. Droplet
formation was recorded using a high-speed camera
connected to an inverted microscope at 25,000 fps. Images were
processed using ImageJ and cross correlation,
of the produced speckle patterns for GPV, was carried out using PIVlab, an open -source Matlab
toolbox.

The droplet formation process is
a competition between the interfacial tension force, which reduces interfacial
area, and the drag force exerted by the continuous phase [4]. By examining the
velocity profiles along the interface, we are able to detect differences
between similar systems. Marangoni stresses act from regions of low surfactant
concentration to regions of high surfactant concentration [9]. We have
demonstrated that systems that have the same equilibrium interfacial tensions
do not display the same fluid velocities along the interface during the droplet
formation process under the same conditions. Fluid flow in
the final stages of break-up, is driven by the excessive capillary pressure,
expelling fluid from the thinning neck region [2]. Any acting stresses
due to dynamic surfactant effect should affect the local velocity fields close
to the interface. At small surfactant concentrations, the manifestation of
Marangoni phenomena leads to smaller velocities close to the interface as
displayed in Figure 1.

This is a direct result of
interfacial phenomena acting against the break-up. If we examine the droplet
sizes and the droplet detachment time scales for the case of the acting
Marangoni stresses, we see that they are larger than in the case where there
are no acting Marangoni stresses. If we examine the droplet formation process
of more concentrated solutions, under the same flow rates, the velocities along
the interface display no difference. Furthermore, in this case, droplet
detachment times and droplet sizes are the same for all surfactant solutions, suggesting,
there are no acting Marangoni phenomena.

We have successfully demonstrated
the presence of Marangoni phenomena acting on the thinning neck during droplet
formation in microfluidics. We were able to quantify their direct result and
monitor their effect on the droplet formation process. Furthermore, we have
demonstrated that GPV is a suitable technique for studying phenomena developing
on submillimetre scales extracting valuable information in order to quantify micro-scale
Marangoni phenomena.

References

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