Abstract
We experimentally investigated the role of surfactants in
static oil-in-water emulsions at and above the jamming point. More
specifically, we varied the surfactant concentration in the
emulsions and studied how the variations impact the critical
scaling behaviors of emulsions. The emulsion consists of
bidispersed droplets that are ~140 μm in radius on
average. The droplets are confined between two parallel glass
plates in order to construct a quasi-2D system, which is analogous
to other 2D packing systems such as granular disks,
except for the following two distinctions. First, our
emulsion system is frictionless at static equilibrium, thus the
inter-droplet forces are strictly perpendicular to the contact
length; second, the droplets in emulsions are highly deformable
when compressed, which allows us to easily reach a high area
fraction up to Φ~0.95, which is
impossible to achieve for disks. We found that increasing
surfactant concentration lowers the jamming point
Φc, which we ascribed to
the attractive depletion forces induced by micelles formed by
surfactants in the emulsion. We studied the critical scaling of
coordination number z, and found that the impact of
depletion forces is reflected in the behavior of z near the
jamming point, but not significant enough over the entire range
of Φ. We also attempted to study
inter-droplet forces and bulk pressure of the system, but the
empirical law we used to compute forces was developed for a fixed
surfactant concentration, therefore unsuitable for our data.
Instead, we examined how the average contact length L
changes with Φ, and concluded that
in the context of droplet interactions, the influence of depletion
forces may be extended to above the jamming point. Our next step
will be to re-calibrate the empirical force law and to investigate
how it changes with surfactant concentration, which may yield
results that are helpful in understanding the role of surfactants
in emulsion at a microscopic level.
Table of Contents
Introduction
......................................1
Procedures ........................................4
1. Producing droplets ...........................4
2. Varying surfactant concentration .......5
3. Sample chamber and microscopy ........7
Results .............................................8
1. Identifying the jamming point ...........10
2. Critical scaling ..............................13
3. Effect of variations in radius ratio .....18
Conclusion .......................................20
References .......................................22
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