Spinodal Decomposition in a Model Colloid-Polymer Mixture in Microgravity

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Original Entry by Michelle Borkin, AP225 Fall 2009

Overview

"Spinodal Decomposition in a Model Colloid-Polymer Mixture in Microgravity."

A. E. Bailey, W. C. K. Poon, R. J. Christianson, A. B. Schofield, U. Gasser, V. Prasad, S. Manley, P. N. Segre, L. Cipelletti, W. V. Meyer, M. P. Doherty, S. Sankaran, A. L. Jankovsky, W. L. Shiley, J. P. Bowen, J. C. Eggers, C. Kurta, T. Lorik, Jr., P. N. Pusey, and D. A. Weitz, Physical Review Letters 99, 205701 (2007).

Keywords

Colloidal Dispersion, Polymer, Spinodal decomposition, Depletion interactions, quench, light scattering, colloid-polymer mixture

Summary

A quenched colloid-polymer mixture was prepared in a microgravity environment to study the mixtures evolution from spinodal decomposition to interfacial tension driven coarsening. The experiment was conducted on the International Space Station (ISS) as part of NASA's "Physics of Colloids in Space" project. The sample was imaged with the project's light scattering instrumentation and direct digital imaging cameras. The mixture was composed of polymethylmethacrylate particles and polystyrene. The liquid after total separation was 45%:55% by volume liquid and gas phases. On Earth, this separation took ~2 hours, whereas in the microgravity environment it took ~30 times longer and was able to be studied is far greater detail. Microgravity also resulted in larger density differences between the final phases as well as an ultralow interfacial tension. Also, it was observed that the evolution over time of the deep quenching follows the same evolutionary function as in binary liquids. This implies variations in the characteristic length scale of colloidal gels depend on the rate of coarsening.

For more information about this experiment and others from the "Physics of Colloids in Space" project, go to:


Soft Matter

Figure 1.

This paper focuses on studying the properties of a colloid-polymer mixture. These mixtures are composed of colloidal particles with non-absorbing polymers. The colloidal particles will attract and push away the polymers (such depletion interactions are driven by osmotic pressure) and the unbalanced system will result in the coarsening of the sample so the colloidal particles will bunch together.

These colloid-polymer mixtures are commonly studied to investigate phase change behavior since as the colloids attract they will form a "colloid rich liquid" while the rest of the solution will become a "colloid poor gas" exhibiting spinodal decomposition (i.e. when a fluid is quenched and separates into gas and liquid phases). As this process occurs, the size of the domain <math>l</math> as a function of time <math>t</math> changes from

<math>l(t)\,\sim\, t^{1/3}</math>

at early times to

<math>l(t)\,\sim\, t</math>

after the interfacial area begins to drive viscous flows. This behaviour in the observed coarsening is also observed in other molecular fluids and polymer blend solutions. This phase transition and coarsening can be observed in Figure 1.

A key motivation for this experiment being conducted in outer space is due to the microgravity environment. Here on Earth, measuring the <math>l(t)</math> scaling laws described above are extremely difficult. This is because the <math>l</math> scales according to the capillary length,

<math>l_{capillary}\, = \, (\gamma / \delta\rho g)^{1/2}</math>

where <math>\gamma</math> is the interfacial tension, <math>\delta\rho</math> is the difference in density between the phases, and <math>g</math> is the gravitational constant. Forces due to buoyancy ultimately destroy the structure when <math>l\, > \, l_{capillary}</math>. However, one does not have to worry about this limitation in a microgravity environment thus the phase separation and coarsening behaviors are able to be studied in far greater detail.