The kinetics of emulsion coalescence determine emulsion stability and also emulsion rheology, which depends on the distribution of droplet sizes in the system. In a concentrated emulsion, which is opaque, conventional optical methods for monitoring the droplet size distribution are often impractical, and cannot distinguish between flocculation and coalescence. Nuclear magnetic resonance (NMR) is uniquely suited to measuring droplet size distributions by observing restricted diffusion, which is unaffected by flocculation. In addition, NMR imaging provides the capability of measuring droplet concentration and velocity profiles in a flowing material.
We wish to quantify the effect of shear flow on coalescence of a concentrated
emulsion. We have constructed a combined annular Couette flow cell and magnetic
resonance imaging probe. The wide-gap Couette geometry, where the emulsion is
sheared between concentric cylinders, produces a planar, axisymmetric shear
flow. The imaging probe, consisting of a 300 MHz birdcage coil, is combined with
an imaging system utilizing maximum gradients of 95 G/cm. The emulsion is
composed of 20-40% volume fraction iso-octane in water, stabilized by Tween 20
surfactant. Blends of the iso-octane with a denser oil can be used to achieve
density-matched emulsions, which would simulate systems in microgravity during
earth-based experiments.
We have demonstrated the capability of extracting the droplet size distribution
in a quiescent emulsion from restricted diffusion data obtained by using a pulsed gradient
stimulated echo sequence. Figure 1 shows, for the first time, that this NMR method produces
nearly quantitative agreement with light scattering measurements. Also, we have
employed NMR imaging to visualize and measure the velocity profile of water in
the Couette flow cell, by the time-of-flight method, displayed in figure 2. Data
from monitoring the droplet size distribution, droplet concentration and
velocity profiles over time in the flowing emulsion system will be presented.
d'Avila, M., Shapley, N., Walton, J., Dungan, S., Phillips, R., Powell, R., Development of a Magnetic Resonance Imaging Technique for Measuring Emulsion Coalescence, Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 930-945, August 9, 2000.