Jamming of Static Quasi-2D Emulsions at Various Surfactant Concentrations Open Access

Wu, Rui (2014)

Permanent URL: https://etd.library.emory.edu/concern/etds/0p096706r?locale=en
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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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